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GNAT Reference Manual
GNAT, The GNU Ada 95 Compiler
Version 3.15p
Date: 2002/05/14 10:15:10
Ada Core Technologies, Inc.
Copyright (C) 1995-2001, Free Software Foundation
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU Free Documentation License", with the Front-Cover Texts being "GNAT Reference Manual", and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".
-- The Detailed Node Listing ---
About This Guide
What This Reference Manual Contains Related Information
The Implementation of Standard I/O
The GNAT Library
Text_IO
Wide_Text_IO
8.6.1 Stream Pointer Positioning 8.6.2 Reading and Writing Non-Regular Files
Interfacing to Other Languages
10.1 Interfacing to C 10.2 Interfacing to C++ 10.3 Interfacing to COBOL 10.4 Interfacing to Fortran 10.5 Interfacing to non-GNAT Ada code
GNAT Implementation of Tasking
12.1 Mapping Ada Tasks onto the Underlying Kernel Threads 12.2 Ensuring Compliance with the Real-Time Annex
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This manual contains useful information in writing programs using the GNAT compiler. It includes information on implementation dependent characteristics of GNAT, including all the information required by Annex M of the standard.
Ada 95 is designed to be highly portable,and guarantees that, for most programs, Ada 95 compilers behave in exactly the same manner on different machines. However, since Ada 95 is designed to be used in a wide variety of applications, it also contains a number of system dependent features to Functbe used in interfacing to the external world.
Note: Any program that makes use of implementation-dependent features may be non-portable. You should follow good programming practice and isolate and clearly document any sections of your program that make use of these features in a non-portable manner.
What This Reference Manual Contains Conventions Related Information
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This reference manual contains the following chapters:
This reference manual assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.
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Following are examples of the typographical and graphic conventions used in this guide:
Functions, utility program names, standard names,
and classes.
Option flags
Variables.
and then shown this way. |
Commands that are entered by the user are preceded in this manual by the characters `$ ' (dollar sign followed by space). If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual. If your system uses some other prompt, then the command will appear with the `$' replaced by whatever prompt character you are using.
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Ada 95 defines a set of pragmas that can be used to supply additional information to the compiler. These language defined pragmas are implemented in GNAT and work as described in the Ada 95 Reference Manual.
In addition, Ada 95 allows implementations to define additional pragmas whose meaning is defined by the implementation. GNAT provides a number of these implementation-dependent pragmas which can be used to extend and enhance the functionality of the compiler. This section of the GNAT Reference Manual describes these additional pragmas.
Note that any program using these pragmas may not be portable to other compilers (although GNAT implements this set of pragmas on all platforms). Therefore if portability to other compilers is an important consideration, the use of these pragmas should be minimized.
pragma Abort_Defer
pragma Abort_Defer; |
This pragma must appear at the start of the statement sequence of a
handled sequence of statements (right after the begin). It has
the effect of deferring aborts for the sequence of statements (but not
for the declarations or handlers, if any, associated with this statement
sequence).
pragma Ada_83
pragma Ada_83; |
A configuration pragma that establishes Ada 83 mode for the unit to
which it applies, regardless of the mode set by the command line
switches. In Ada 83 mode, GNAT attempts to be as compatible with
the syntax and semantics of Ada 83, as defined in the original Ada
83 Reference Manual as possible. In particular, the new Ada 95
keywords are not recognized, optional package bodies are allowed,
and generics may name types with unknown discriminants without using
the (<>) notation. In addition, some but not all of the additional
restrictions of Ada 83 are enforced.
Ada 83 mode is intended for two purposes. Firstly, it allows existing legacy Ada 83 code to be compiled and adapted to GNAT with less effort. Secondly, it aids in keeping code backwards compatible with Ada 83. However, there is no guarantee that code that is processed correctly by GNAT in Ada 83 mode will in fact compile and execute with an Ada 83 compiler, since GNAT does not enforce all the additional checks required by Ada 83.
pragma Ada_95
pragma Ada_95; |
A configuration pragma that establishes Ada 95 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the Ada and System
packages and their children, so you need not specify it in these
contexts. This pragma is useful when writing a reusable component that
itself uses Ada 95 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.
pragma Annotate
pragma Annotate (IDENTIFIER {, ARG});
ARG ::= NAME | EXPRESSION
|
This pragma is used to annotate programs. identifier identifies
the type of annotation. GNAT verifies this is an identifier, but does
not otherwise analyze it. The arg argument
can be either a string literal or an
expression. String literals are assumed to be of type
Standard.String. Names of entities are simply analyzed as entity
names. All other expressions are analyzed as expressions, and must be
unambiguous.
The analyzed pragma is retained in the tree, but not otherwise processed by any part of the GNAT compiler. This pragma is intended for use by external tools, including ASIS.
pragma Assert
pragma Assert ( boolean_EXPRESSION [, static_string_EXPRESSION]) |
The effect of this pragma depends on whether the corresponding command line switch is set to activate assertions. The pragma expands into code equivalent to the following:
if assertions-enabled then
if not boolean_EXPRESSION then
System.Assertions.Raise_Assert_Failure
(string_EXPRESSION);
end if;
end if;
|
The string argument, if given, is the message that will be associated with the exception occurrence if the exception is raised. If no second argument is given, the default message is `file:nnn', where file is the name of the source file containing the assert, and nnn is the line number of the assert. A pragma is not a statement, so if a statement sequence contains nothing but a pragma assert, then a null statement is required in addition, as in:
... if J > 3 then pragma Assert (K > 3, "Bad value for K"); null; end if; |
Note that, as with the if statement to which it is equivalent, the
type of the expression is either Standard.Boolean, or any type derived
from this standard type.
If assertions are disabled (switch -gnata not used), then there
is no effect (and in particular, any side effects from the expression
are suppressed). More precisely it is not quite true that the pragma
has no effect, since the expression is analyzed, and may cause types
to be frozen if they are mentioned here for the first time.
If assertions are enabled, then the given expression is tested, and if
it is False then System.Assertions.Raise_Assert_Failure is called
which results in the raising of Assert_Failure with the given message.
If the boolean expression has side effects, these side effects will turn on and off with the setting of the assertions mode, resulting in assertions that have an effect on the program. You should generally avoid side effects in the expression arguments of this pragma. However, the expressions are analyzed for semantic correctness whether or not assertions are enabled, so turning assertions on and off cannot affect the legality of a program.
pragma Ast_Entry
pragma AST_Entry (entry_IDENTIFIER); |
This pragma is implemented only in the OpenVMS implementation of GNAT. The
argument is the simple name of a single entry; at most one AST_Entry
pragma is allowed for any given entry. This pragma must be used in
conjunction with the AST_Entry attribute, and is only allowed after
the entry declaration and in the same task type specification or single task
as the entry to which it applies. This pragma specifies that the given entry
may be used to handle an OpenVMS asynchronous system trap (AST)
resulting from an OpenVMS system service call. The pragma does not affect
normal use of the entry. For further details on this pragma, see the
DEC Ada Language Reference Manual, section 9.12a.
pragma C_Pass_By_Copy
pragma C_Pass_By_Copy ([Max_Size =>] static_integer_EXPRESSION); |
Normally the default mechanism for passing C convention records to C
convention subprograms is to pass them by reference, as suggested by RM
B.3(69). Use the configuration pragma C_Pass_By_Copy to change
this default, by requiring that record formal parameters be passed by
copy if all of the following conditions are met:
Convention C.
If these conditions are met the argument is passed by copy, i.e. in a manner consistent with what C expects if the corresponding formal in the C prototype is a struct (rather than a pointer to a struct).
You can also pass records by copy by specifying the convention
C_Pass_By_Copy for the record type, or by using the extended
Import and Export pragmas, which allow specification of
passing mechanisms on a parameter by parameter basis.
pragma Comment
pragma Comment (static_string_EXPRESSION); |
This is almost identical in effect to pragma Ident. It allows the
placement of a comment into the object file and hence into the
executable file if the operating system permits such usage. The
difference is that Comment, unlike Ident, has no limit on the
length of the string argument, and no limitations on placement
of the pragma (it can be placed anywhere in the main source unit).
pragma Common_Object
pragma Common_Object (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Size =>] EXTERNAL_SYMBOL] )
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma enables the shared use of variables stored in overlaid
linker areas corresponding to the use of COMMON
in Fortran. The single
object local_name is assigned to the area designated by
the External argument.
You may define a record to correspond to a series
of fields. The size argument
is syntax checked in GNAT, but otherwise ignored.
Common_Object is not supported on all platforms. If no
support is available, then the code generator will issue a message
indicating that the necessary attribute for implementation of this
pragma is not available.
pragma Complex_Representation
pragma Complex_Representation
([Entity =>] LOCAL_NAME);
|
The Entity argument must be the name of a record type which has two fields of the same floating-point type. The effect of this pragma is to force gcc to use the special internal complex representation form for this record, which may be more efficient. Note that this may result in the code for this type not conforming to standard ABI (application binary interface) requirements for the handling of record types. For example, in some environments, there is a requirement for passing records by pointer, and the use of this pragma may result in passing this type in floating-point registers.
pragma Component_Alignment
pragma Component_Alignment (
[Form =>] ALIGNMENT_CHOICE
[, [Name =>] type_LOCAL_NAME]);
ALIGNMENT_CHOICE ::=
Component_Size
| Component_Size_4
| Storage_Unit
| Default
|
Specifies the alignment of components in array or record types. The meaning of the Form argument is as follows:
Component_Size
Component_Size_4
Storage_Unit
System.Storage_Unit.
Default
Default choice is the same as
the Storage_Unit choice (byte alignment). For all other systems,
the Default choice is the same as Component_Size (natural
alignment).
If the Name parameter is present, type_local_name must
refer to a local record or array type, and the specified alignment
choice applies to the specified type. The use of
Component_Alignment together with a pragma Pack causes the
Component_Alignment pragma to be ignored. The use of
Component_Alignment together with a record representation clause
is only effective for fields not specified by the representation clause.
If the Name parameter is absent, the pragma can be used as either
a configuration pragma, in which case it applies to one or more units in
accordance with the normal rules for configuration pragmas, or it can be
used within a declarative part, in which case it applies to types that
are declared within this declarative part, or within any nested scope
within this declarative part. In either case it specifies the alignment
to be applied to any record or array type which has otherwise standard
representation.
If the alignment for a record or array type is not specified (using
pragma Pack, pragma Component_Alignment, or a record rep
clause), the GNAT uses the default alignment as described previously.
pragma CPP_Class
pragma CPP_Class ([Entity =>] LOCAL_NAME); |
The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.
If (and only if) the type is tagged, at least one component in the
record must be of type Interfaces.CPP.Vtable_Ptr, corresponding
to the C++ Vtable (or Vtables in the case of multiple inheritance) used
for dispatching.
Types for which CPP_Class is specified do not have assignment or
equality operators defined (such operations can be imported or declared
as subprograms as required). Initialization is allowed only by
constructor functions (see pragma CPP_Constructor).
Pragma CPP_Class is intended primarily for automatic generation
using an automatic binding generator tool.
See 10.2 Interfacing to C++ for related information.
pragma CPP_Constructor
pragma CPP_Constructor ([Entity =>] LOCAL_NAME); |
This pragma identifies an imported function (imported in the usual way
with pragma Import) as corresponding to a C++
constructor. The argument is a name that must have been
previously mentioned in a pragma Import
with Convention = CPP, and must be of one of the following
forms:
function Fname return T'Class
function Fname (...) return T'Class
where T is a tagged type to which the pragma CPP_Class applies.
The first form is the default constructor, used when an object of type T is created on the Ada side with no explicit constructor. Other constructors (including the copy constructor, which is simply a special case of the second form in which the one and only argument is of type T), can only appear in two contexts:
Although the constructor is described as a function that returns a value on the Ada side, it is typically a procedure with an extra implicit argument (the object being initialized) at the implementation level. GNAT issues the appropriate call, whatever it is, to get the object properly initialized.
In the case of derived objects, you may use one of two possible forms for declaring and creating an object:
New_Object : Derived_T
New_Object : Derived_T := (constructor-function-call with ...)
In the first case the default constructor is called and extension fields if any are initialized according to the default initialization expressions in the Ada declaration. In the second case, the given constructor is called and the extension aggregate indicates the explicit values of the extension fields.
If no constructors are imported, it is impossible to create any objects on the Ada side. If no default constructor is imported, only the initialization forms using an explicit call to a constructor are permitted.
Pragma CPP_Constructor is intended primarily for automatic generation
using an automatic binding generator tool.
See 10.2 Interfacing to C++ for more related information.
pragma CPP_Virtual
pragma CPP_Virtual
[Entity =>] ENTITY,
[, [Vtable_Ptr =>] vtable_ENTITY,]
[, [Position =>] static_integer_EXPRESSION])
|
This pragma serves the same function as pragma Import in that
case of a virtual function imported from C++. The Entity argument
must be a
primitive subprogram of a tagged type to which pragma CPP_Class
applies. The Vtable_Ptr argument specifies
the Vtable_Ptr component which contains the
entry for this virtual function. The Position argument
is the sequential number
counting virtual functions for this Vtable starting at 1.
The Vtable_Ptr and Position arguments may be omitted if
there is one Vtable_Ptr present (single inheritance case) and all
virtual functions are imported. In that case the compiler can deduce both
these values.
No External_Name or Link_Name arguments are required for a
virtual function, since it is always accessed indirectly via the
appropriate Vtable entry.
Pragma CPP_Virtual is intended primarily for automatic generation
using an automatic binding generator tool.
See 10.2 Interfacing to C++ for related information.
pragma CPP_Vtable
pragma CPP_Vtable ( [Entity =>] ENTITY, [Vtable_Ptr =>] vtable_ENTITY, [Entry_Count =>] static_integer_EXPRESSION); |
Given a record to which the pragma CPP_Class applies,
this pragma can be specified for each component of type
CPP.Interfaces.Vtable_Ptr.
Entity is the tagged type, Vtable_Ptr
is the record field of type Vtable_Ptr, and Entry_Count is
the number of virtual functions on the C++ side. Not all of these
functions need to be imported on the Ada side.
You may omit the CPP_Vtable pragma if there is only one
Vtable_Ptr component in the record and all virtual functions are
imported on the Ada side (the default value for the entry count in this
case is simply the total number of virtual functions).
Pragma CPP_Vtable is intended primarily for automatic generation
using an automatic binding generator tool.
See 10.2 Interfacing to C++ for related information.
pragma Debug
pragma Debug (PROCEDURE_CALL_WITHOUT_SEMICOLON); PROCEDURE_CALL_WITHOUT_SEMICOLON ::= PROCEDURE_NAME | PROCEDURE_PREFIX ACTUAL_PARAMETER_PART |
The argument has the syntactic form of an expression, meeting the syntactic requirements for pragmas.
If assertions are not enabled on the command line, this pragma has no
effect. If asserts are enabled, the semantics of the pragma is exactly
equivalent to the procedure call statement corresponding to the argument
with a terminating semicolon. Pragmas are permitted in sequences of
declarations, so you can use pragma Debug to intersperse calls to
debug procedures in the middle of declarations.
pragma Elaboration_Checks
pragma Elaboration_Checks (RM | Static); |
This is a configuration pragma that provides control over the
elaboration model used by the compilation affected by the
pragma. If the parameter is RM, then the dynamic elaboration
model described in the Ada Reference Manual is used, as though
the -gnatE switch had been specified on the command
line. If the parameter is Static, then the default GNAT static
model is used. This configuration pragma overrides the setting
of the command line. For full details on the elaboration models
used by the GNAT compiler, see section "Elaboration Order
Handling in GNAT" in the GNAT User's Guide.
pragma Eliminate
pragma Eliminate (
[Unit_Name =>] IDENTIFIER |
SELECTED_COMPONENT);
pragma Eliminate (
[Unit_Name =>] IDENTIFIER |
SELECTED_COMPONENT,
[Entity =>] IDENTIFIER |
SELECTED_COMPONENT |
STRING_LITERAL
[,[Parameter_Types =>] PARAMETER_TYPES]
[,[Result_Type =>] result_SUBTYPE_NAME]
[,[Homonym_Number =>] INTEGER_LITERAL]);
PARAMETER_TYPES ::= (SUBTYPE_NAME {, SUBTYPE_NAME})
SUBTYPE_NAME ::= STRING_LITERAL
|
This pragma indicates that the given entity is not used outside the compilation unit it is defined in. The entity may be either a subprogram or a variable.
If the entity to be eliminated is a library level subprogram, then
the first form of pragma Eliminate is used with only a single argument.
In this form, the Unit_Name argument specifies the name of the
library level unit to be eliminated.
In all other cases, both Unit_Name and Entity arguments
are required. item is an entity of a library package, then the first
argument specifies the unit name, and the second argument specifies
the particular entity. If the second argument is in string form, it must
correspond to the internal manner in which GNAT stores entity names (see
compilation unit Namet in the compiler sources for details).
The remaining parameters are optionally used to distinguish between overloaded subprograms. There are two ways of doing this.
Use Parameter_Types and Result_Type to specify the
profile of the subprogram to be eliminated in a manner similar to that
used for
the extended Import and Export pragmas, except that the
subtype names are always given as string literals, again corresponding
to the internal manner in which GNAT stores entity names.
Alternatively, the Homonym_Number parameter is used to specify
which overloaded alternative is to be eliminated. A value of 1 indicates
the first subprogram (in lexical order), 2 indicates the second etc.
The effect of the pragma is to allow the compiler to eliminate the code or data associated with the named entity. Any reference to an eliminated entity outside the compilation unit it is defined in, causes a compile time or link time error.
The parameters of this pragma may be given in any order, as long as the usual rules for use of named parameters and position parameters are used.
The intention of pragma Eliminate is to allow a program to be compiled
in a system independent manner, with unused entities eliminated, without
the requirement of modifying the source text. Normally the required set
of Eliminate pragmas is constructed automatically using the gnatelim tool.
Elimination of unused entities local to a compilation unit is automatic,
without requiring the use of pragma Eliminate.
Note that the reason this pragma takes string literals where names might
be expected is that a pragma Eliminate can appear in a context where the
relevant names are not visible.
pragma Export_Exception
pragma Export_Exception (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL,]
[, [Form =>] Ada | VMS]
[, [Code =>] static_integer_EXPRESSION]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma is implemented only in the OpenVMS implementation of GNAT. It causes the specified exception to be propagated outside of the Ada program, so that it can be handled by programs written in other OpenVMS languages. This pragma establishes an external name for an Ada exception and makes the name available to the OpenVMS Linker as a global symbol. For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a3.2.
pragma Export_Function ...
Syntax:
pragma Export_Function (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Result_Type =>] result_SUBTYPE_MARK]
[, [Mechanism =>] MECHANISM]
[, [Result_Mechanism =>] MECHANISM_NAME]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
Use this pragma to make a function externally callable and optionally
provide information on mechanisms to be used for passing parameter and
result values. We recommend, for the purposes of improving portability,
this pragma always be used in conjunction with a separate pragma
Export, which must precede the pragma Export_Function.
GNAT does not require a separate pragma Export, but if none is
present, Convention Ada is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a Export or Convention
pragma that specifies the desired foreign convention.
Pragma Export_Function
(and Export, if present) must appear in the same declarative
region as the function to which they apply.
internal_name must uniquely designate the function to which the
pragma applies. If more than one function name exists of this name in
the declarative part you must use the Parameter_Types and
Result_Type parameters is mandatory to achieve the required
unique designation. subtype_ marks in these parameters must
exactly match the subtypes in the corresponding function specification,
using positional notation to match parameters with subtype marks.
Passing by descriptor is supported only on the OpenVMS ports of GNAT.
pragma Export_Object ...
pragma Export_Object
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Size =>] EXTERNAL_SYMBOL]
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma designates an object as exported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal Export pragma applied to an object. You may use a
separate Export pragma (and you probably should from the point of view
of portability), but it is not required. Size is syntax checked,
but otherwise ignored by GNAT.
pragma Export_Procedure ...
pragma Export_Procedure (
[Internal =>] LOCAL_NAME
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Mechanism =>] MECHANISM]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
This pragma is identical to Export_Function except that it
applies to a procedure rather than a function and the parameters
Result_Type and Result_Mechanism are not permitted.
GNAT does not require a separate pragma Export, but if none is
present, Convention Ada is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a Export or Convention
pragma that specifies the desired foreign convention.
pragma Export_Valued_Procedure
pragma Export_Valued_Procedure (
[Internal =>] LOCAL_NAME
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Mechanism =>] MECHANISM]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
This pragma is identical to Export_Procedure except that the
first parameter of local_name, which must be present, must be of
mode OUT, and externally the subprogram is treated as a function
with this parameter as the result of the function. GNAT provides for
this capability to allow the use of OUT and IN OUT
parameters in interfacing to external functions (which are not permitted
in Ada functions).
GNAT does not require a separate pragma Export, but if none is
present, Convention Ada is assumed, which is almost certainly
not what is wanted since the whole point of this pragma is to interface
with foreign language functions, so it is usually appropriate to use this
pragma in conjunction with a Export or Convention
pragma that specifies the desired foreign convention.
pragma Extend_System
pragma Extend_System ([Name =>] IDENTIFIER); |
This pragma is used to provide backwards compatibility with other
implementations that extend the facilities of package System. In
GNAT, System contains only the definitions that are present in
the Ada 95 RM. However, other implementations, notably the DEC Ada 83
implementation, provide many extensions to package System.
For each such implementation accommodated by this pragma, GNAT provides a
package Aux_xxx, e.g. Aux_DEC for the DEC Ada 83
implementation, which provides the required additional definitions. You
can use this package in two ways. You can with it in the normal
way and access entities either by selection or using a use
clause. In this case no special processing is required.
However, if existing code contains references such as
System.xxx where xxx is an entity in the extended
definitions provided in package System, you may use this pragma
to extend visibility in System in a non-standard way that
provides greater compatibility with the existing code. Pragma
Extend_System is a configuration pragma whose single argument is
the name of the package containing the extended definition
(e.g. Aux_DEC for the DEC Ada case). A unit compiled under
control of this pragma will be processed using special visibility
processing that looks in package System.Aux_xxx where
Aux_xxx is the pragma argument for any entity referenced in
package System, but not found in package System.
You can use this pragma either to access a predefined System
extension supplied with the compiler, for example Aux_DEC or
you can construct your own extension unit following the above
definition. Note that such a package is a child of System
and thus is considered part of the implementation. To compile
it you will have to use the appropriate switch for compiling
system units. See the GNAT User's Guide for details.
pragma External
pragma External ( [ Convention =>] convention_IDENTIFIER, [ Entity =>] local_NAME [, [External_Name =>] static_string_EXPRESSION ] [, [Link_Name =>] static_string_EXPRESSION ]); |
This pragma is identical in syntax and semantics to pragma
Export as defined in the Ada Reference Manual. It is
provided for compatibility with some Ada 83 compilers that
used this pragma for exactly the same purposes as pragma
Export before the latter was standardized.
pragma External_Name_Casing
pragma External_Name_Casing ( Uppercase | Lowercase [, Uppercase | Lowercase | As_Is]); |
This pragma provides control over the casing of external names associated with Import and Export pragmas. There are two cases to consider:
pragma Import (C, C_Routine); |
Since Ada is a case insensitive language, the spelling of the identifier in
the Ada source program does not provide any information on the desired
casing of the external name, and so a convention is needed. In GNAT the
default treatment is that such names are converted to all lower case
letters. This corresponds to the normal C style in many environments.
The first argument of pragma External_Name_Casing can be used to
control this treatment. If Uppercase is specified, then the name
will be forced to all uppercase letters. If Lowercase is specified,
then the normal default of all lower case letters will be used.
This same implicit treatment is also used in the case of extended DEC Ada 83 compatible Import and Export pragmas where an external name is explicitly specified using an identifier rather than a string.
pragma Import (C, C_Routine, "C_routine"); |
In this case, the string literal normally provides the exact casing required
for the external name. The second argument of pragma
External_Name_Casing may be used to modify this behavior.
If Uppercase is specified, then the name
will be forced to all uppercase letters. If Lowercase is specified,
then the name will be forced to all lowercase letters. A specification of
As_Is provides the normal default behavior in which the casing is
taken from the string provided.
This pragma may appear anywhere that a pragma is valid. In particular, it can be used as a configuration pragma in the `gnat.adc' file, in which case it applies to all subsequent compilations, or it can be used as a program unit pragma, in which case it only applies to the current unit, or it can be used more locally to control individual Import/Export pragmas.
It is primarily intended for use with OpenVMS systems, where many compilers convert all symbols to upper case by default. For interfacing to such compilers (e.g. the DEC C compiler), it may be convenient to use the pragma:
pragma External_Name_Casing (Uppercase, Uppercase); |
to enforce the upper casing of all external symbols.
pragma Finalize_Storage_Only
pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME); |
This pragma allows the compiler not to emit a Finalize call for objects defined at the library level. This is mostly useful for types where finalization is only used to deal with storage reclamation since in most environments it is not necessary to reclaim memory just before terminating execution, hence the name.
pragma Float_Representation
pragma Float_Representation (FLOAT_REP); FLOAT_REP ::= VAX_Float | IEEE_Float |
This pragma is implemented only in the OpenVMS implementation of GNAT.
It allows control over the internal representation chosen for the predefined
floating point types declared in the packages Standard and
System. For further details on this pragma, see the
DEC Ada Language Reference Manual, section 3.5.7a. Note that to use this
pragma, the standard runtime libraries must be recompiled. See the
description of the GNAT LIBRARY command in the OpenVMS version
of the GNAT Users Guide for details on the use of this command.
pragma Ident
pragma Ident (static_string_EXPRESSION); |
This pragma provides a string identification in the generated object file, if the system supports the concept of this kind of identification string. The maximum permitted length of the string literal is 31 characters. This pragma is allowed only in the outermost declarative part or declarative items of a compilation unit. On OpenVMS systems, the effect of the pragma is identical to the effect of the DEC Ada 83 pragma of the same name.
pragma Import_Exception
pragma Import_Exception (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL,]
[, [Form =>] Ada | VMS]
[, [Code =>] static_integer_EXPRESSION]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma is implemented only in the OpenVMS implementation of GNAT. It allows OpenVMS conditions (for example, from OpenVMS system services or other OpenVMS languages) to be propagated to Ada programs as Ada exceptions. The pragma specifies that the exception associated with an exception declaration in an Ada program be defined externally (in non-Ada code). For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a.3.1.
pragma Import_Function ...
pragma Import_Function (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Result_Type =>] SUBTYPE_MARK]
[, [Mechanism =>] MECHANISM]
[, [Result_Mechanism =>] MECHANISM_NAME]
[, [First_Optional_Parameter =>] IDENTIFIER]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
This pragma is used in conjunction with a pragma Import to
specify additional information for an imported function. The pragma
Import (or equivalent pragma Interface) must precede the
Import_Function pragma and both must appear in the same
declarative part as the function specification.
The Internal_Name argument must uniquely designate
the function to which the
pragma applies. If more than one function name exists of this name in
the declarative part you must use the Parameter_Types and
Result_Type parameters to achieve the required unique
designation. Subtype marks in these parameters must exactly match the
subtypes in the corresponding function specification, using positional
notation to match parameters with subtype marks.
You may optionally use the Mechanism and Result_Mechanism parameters to specify passing mechanisms for the parameters and result. If you specify a single mechanism name, it applies to all parameters. Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation. If the mechanism is not specified, the default mechanism is used.
Passing by descriptor is supported only on the to OpenVMS ports of GNAT.
First_Optional_Parameter applies only to OpenVMS ports of GNAT.
It specifies that the designated parameter and all following parameters
are optional, meaning that they are not passed at the generated code
level (this is distinct from the notion of optional parameters in Ada
where the parameters are passed anyway with the designated optional
parameters). All optional parameters must be of mode IN and have
default parameter values that are either known at compile time
expressions, or uses of the 'Null_Parameter attribute.
pragma Import_Object
pragma Import_Object
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL],
[, [Size =>] EXTERNAL_SYMBOL])
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma designates an object as imported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal Import pragma applied to an object. Unlike the
subprogram case, you need not use a separate Import pragma,
although you may do so (and probably should do so from a portability
point of view). size is syntax checked, but otherwise ignored by
GNAT.
pragma Import_Procedure
pragma Import_Procedure (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Mechanism =>] MECHANISM]
[, [First_Optional_Parameter =>] IDENTIFIER]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
This pragma is identical to Import_Function except that it
applies to a procedure rather than a function and the parameters
Result_Type and Result_Mechanism are not permitted.
pragma Import_Valued_Procedure ...
pragma Import_Valued_Procedure (
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Parameter_Types =>] PARAMETER_TYPES]
[, [Mechanism =>] MECHANISM]
[, [First_Optional_Parameter =>] IDENTIFIER]);
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
PARAMETER_TYPES ::=
null
| SUBTYPE_MARK {, SUBTYPE_MARK}
MECHANISM ::=
MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})
MECHANISM_ASSOCIATION ::=
[formal_parameter_NAME =>] MECHANISM_NAME
MECHANISM_NAME ::=
Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]
CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca
|
This pragma is identical to Import_Procedure except that the
first parameter of local_name, which must be present, must be of
mode OUT, and externally the subprogram is treated as a function
with this parameter as the result of the function. The purpose of this
capability is to allow the use of OUT and IN OUT
parameters in interfacing to external functions (which are not permitted
in Ada functions). You may optionally use the Mechanism
parameters to specify passing mechanisms for the parameters.
If you specify a single mechanism name, it applies to all parameters.
Otherwise you may specify a mechanism on a parameter by parameter
basis using either positional or named notation. If the mechanism is not
specified, the default mechanism is used.
Note that it is important to use this pragma in conjunction with a separate pragma Import that specifies the desired convention, since otherwise the default convention is Ada, which is almost certainly not what is required.
pragma Initialize_Scalars
pragma Initialize_Scalars; |
This pragma is similar to Normalize_Scalars conceptually but has
two important differences. First, there is no requirement for the pragma
to be used uniformly in all units of a partition, in particular, it is fine
to use this just for some or all of the application units of a partition,
without needing to recompile the run-time library.
In the case where some units are compiled with the pragma, and some without, then a declaration of a variable where the type is defined in package Standard or is locally declared will always be subject to initialization, as will any declaration of a scalar variable. For composite variables, whether the variable is initialized may also depend on whether the package in which the type of the variable is declared is compiled with the pragma.
The other important difference is that there is control over the value used for initializing scalar objects. At bind time, you can select whether to initialize with invalid values (like Normalize_Scalars), or with high or low values, or with a specified bit pattern. See the users guide for binder options for specifying these cases.
This means that you can compile a program, and then without having to recompile the program, you can run it with different values being used for initializing otherwise uninitialized values, to test if your program behavior depends on the choice. Of course the behavior should not change, and if it does, then most likely you have an erroneous reference to an uninitialized value.
Note that pragma Initialize_Scalars is particularly useful in
conjunction with the enhanced validity checking that is now provided
in GNAT, which checks for invalid values under more conditions.
Using this feature (see description of the -gnatv flag in the
users guide) in conjunction with pragma Initialize_Scalars
provides a powerful new tool to assist in the detection of problems
caused by uninitialized variables.
pragma Inline_Always
pragma Inline_Always (NAME [, NAME]); |
Similar to pragma Inline except that inlining is not subject to
the use of option -gnatn for inter-unit inlining.
pragma Inline_Generic
pragma Inline_Generic (generic_package_NAME) |
This is implemented for compatibility with DEC Ada 83 and is recognized, but otherwise ignored, by GNAT. All generic instantiations are inlined by default when using GNAT.
pragma Interface
pragma Interface (
[Convention =>] convention_identifier,
[Entity =>] local_name
[, [External_Name =>] static_string_expression],
[, [Link_Name =>] static_string_expression]);
|
This pragma is identical in syntax and semantics to
the standard Ada 95 pragma Import. It is provided for compatibility
with Ada 83. The definition is upwards compatible both with pragma
Interface as defined in the Ada 83 Reference Manual, and also
with some extended implementations of this pragma in certain Ada 83
implementations.
pragma Interface_Name
pragma Interface_Name (
[Entity =>] LOCAL_NAME
[, [External_Name =>] static_string_EXPRESSION]
[, [Link_Name =>] static_string_EXPRESSION]);
|
This pragma provides an alternative way of specifying the interface name for an interfaced subprogram, and is provided for compatibility with Ada 83 compilers that use the pragma for this purpose. You must provide at least one of External_Name or Link_Name.
pragma License
pragma License (Unrestricted | GPL | Modified_GPL | Restricted); |
This pragma is provided to allow automated checking for appropriate license
conditions with respect to the standard and modified GPL. A pragma License,
which is a configuration pragma that typically appears at the start of a
source file or in a separate `gnat.adc' file, specifies the licensing
conditions of a unit as follows:
with'ed by a restricted unit.
with units
which are licensed under the modified GPL (this is the whole point of the
modified GPL).
Normally a unit with no License pragma is considered to have an
unknown license, and no checking is done. However, standard GNAT headers
are recognized, and license information is derived from them as follows.
These default actions means that a program with a restricted license pragma
will automatically get warnings if a GPL unit is inappropriately
with'ed. For example, the program:
with Sem_Ch3; with GNAT.Sockets; procedure Secret_Stuff is ... end Secret_Stuff |
if compiled with pragma License (Restricted) in a
`gnat.adc' file will generate the warning:
1. with Sem_Ch3;
|
>>> license of withed unit "Sem_Ch3" is incompatible
2. with GNAT.Sockets;
3. procedure Secret_Stuff is
|
Sem_Ch3 since it is part of the GNAT
compiler and is licensed under the
GPL, but no warning for GNAT.Sockets which is part of the GNAT
run time, and is therefore licensed under the modified GPL.
pragma Link_With
pragma Link_With (static_string_EXPRESSION {,static_string_EXPRESSION});
|
This pragma is provided for compatibility with certain Ada 83 compilers.
It has exactly the same effect as pragma Linker_Options except
that spaces occurring within one of the string expressions are treated
as separators. For example, in the following case:
pragma Link_With ("-labc -ldef");
|
results in passing the strings -labc and -ldef as two
separate arguments to the linker. In addition pragma Link_With allows
multiple arguments, with the same effect as successive pragmas.
pragma Linker_Alias
pragma Linker_Alias ( [Entity =>] LOCAL_NAME [Alias =>] static_string_EXPRESSION); |
This pragma establishes a linker alias for the given named entity. For further details on the exact effect, consult the GCC manual.
pragma Linker_Section
pragma Linker_Section ( [Entity =>] LOCAL_NAME [Section =>] static_string_EXPRESSION); |
This pragma specifies the name of the linker section for the given entity. For further details on the exact effect, consult the GCC manual.
pragma No_Run_Time
pragma No_Run_Time; |
This is a configuration pragma that makes sure the user code does not use nor need anything from the GNAT run time. This is mostly useful in context where code certification is required. Please consult the GNAT Pro High-Integrity Edition User's Guide for additional information.
pragma Normalize_Scalars
pragma Normalize_Scalars; |
This is a language defined pragma which is fully implemented in GNAT. The effect is to cause all scalar objects that are not otherwise initialized to be initialized. The initial values are implementation dependent and are as follows:
Standard.Character
Standard.Wide_Character
Integer types
subtype Ityp is integer range 1 .. 10; |
then objects of type x will be initialized to Integer'First, a negative
number that is certainly outside the range of subtype Ityp.
Real types
Modular types
Enumeration types
2 ** typ'Size - 1. This will be out of range of the enumeration
subtype in all cases except where the subtype contains exactly
2**8, 2**16, or 2**32 elements.
pragma Long_Float
pragma Long_Float (FLOAT_FORMAT); FLOAT_FORMAT ::= D_Float | G_Float |
This pragma is implemented only in the OpenVMS implementation of GNAT.
It allows control over the internal representation chosen for the predefined
type Long_Float and for floating point type representations with
digits specified in the range 7 through 15.
For further details on this pragma, see the
DEC Ada Language Reference Manual, section 3.5.7b. Note that to use this
pragma, the standard runtime libraries must be recompiled. See the
description of the GNAT LIBRARY command in the OpenVMS version
of the GNAT User's Guide for details on the use of this command.
pragma Machine_Attribute ...
pragma Machine_Attribute ( [Attribute_Name =>] string_EXPRESSION, [Entity =>] LOCAL_NAME); |
Machine dependent attributes can be specified for types and/or
declarations. Currently only subprogram entities are supported. This
pragma is semantically equivalent to
__attribute__((string_expression)) in GNU C,
where string_expression is
recognized by the GNU C macros VALID_MACHINE_TYPE_ATTRIBUTE and
VALID_MACHINE_DECL_ATTRIBUTE which are defined in the
configuration header file `tm.h' for each machine. See the GCC
manual for further information.
pragma Main_Storage
pragma Main_Storage (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]); MAIN_STORAGE_OPTION ::= [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION | [TOP_GUARD =>] static_SIMPLE_EXPRESSION |
This pragma is provided for compatibility with OpenVMS Vax Systems. It has no effect in GNAT, other than being syntax checked. Note that the pragma also has no effect in DEC Ada 83 for OpenVMS Alpha Systems.
pragma No_Return
pragma No_Return (procedure_LOCAL_NAME); |
procedure_local_NAME must refer to one or more procedure
declarations in the current declarative part. A procedure to which this
pragma is applied may not contain any explicit return statements,
and also may not contain any implicit return statements from falling off
the end of a statement sequence. One use of this pragma is to identify
procedures whose only purpose is to raise an exception.
Another use of this pragma is to suppress incorrect warnings about missing returns in functions, where the last statement of a function statement sequence is a call to such a procedure.
pragma Passive
pragma Passive ([Semaphore | No]); |
Syntax checked, but otherwise ignored by GNAT. This is recognized for
compatibility with DEC Ada 83 implementations, where it is used within a
task definition to request that a task be made passive. If the argument
Semaphore is present, or no argument is omitted, then DEC Ada 83
treats the pragma as an assertion that the containing task is passive
and that optimization of context switch with this task is permitted and
desired. If the argument No is present, the task must not be
optimized. GNAT does not attempt to optimize any tasks in this manner
(since protected objects are available in place of passive tasks).
pragma Polling
pragma Polling (ON | OFF); |
This pragma controls the generation of polling code. This is normally off.
If pragma Polling (ON) is used then periodic calls are generated to
the routine Ada.Exceptions.Poll. This routine is a separate unit in the
runtime library, and can be found in file `a-excpol.adb'.
Pragma Polling can appear as a configuration pragma (for example it can be
placed in the `gnat.adc' file) to enable polling globally, or it can be used
in the statement or declaration sequence to control polling more locally.
A call to the polling routine is generated at the start of every loop and
at the start of every subprogram call. This guarantees that the Poll
routine is called frequently, and places an upper bound (determined by
the complexity of the code) on the period between two Poll calls.
The primary purpose of the polling interface is to enable asynchronous
aborts on targets that cannot otherwise support it (for example Windows
NT), but it may be used for any other purpose requiring periodic polling.
The standard version is null, and can be replaced by a user program. This
will require re-compilation of the Ada.Exceptions package that can be found
in files `a-except.ads' and `a-except.adb'.
A standard alternative unit (in file `4wexcpol.adb' in the standard GNAT
distribution) is used to enable the asynchronous abort capability on
targets that do not normally support the capability. The version of Poll
in this file makes a call to the appropriate runtime routine to test for
an abort condition.
Note that polling can also be enabled by use of the -gnatP switch. See
the GNAT User's Guide for details.
pragma Propagate_Exceptions
pragma Propagate_Exceptions (subprogram_LOCAL_NAME); |
This pragma indicates that the given entity, which is the name of an
imported foreign-language subprogram may receive an Ada exception,
and that the exception should be propagated. It is relevant only if
zero cost exception handling is in use, and is thus never needed if
the alternative longjmp / setjmp implementation of exceptions is used
(although it is harmless to use it in such cases).
The implementation of fast exceptions always properly propagates
exceptions through Ada code, as described in the Ada Reference Manual.
However, this manual is silent about the propagation of exceptions
through foreign code. For example, consider the
situation where P1 calls
P2, and P2 calls P3, where
P1 and P3 are in Ada, but P2 is in C.
P3 raises an Ada exception. The question is whether or not
it will be propagated through P2 and can be handled in
P1.
For the longjmp / setjmp implementation of exceptions, the answer is
always yes. For some targets on which zero cost exception handling
is implemented, the answer is also always yes. However, there are
some targets, notably in the current version all x86 architecture
targets, in which the answer is that such propagation does not
happen automatically. If such propagation is required on these
targets, it is mandatory to use Propagate_Exceptions to
name all foreign language routines through which Ada exceptions
may be propagated.
pragma Psect_Object
pragma Psect_Object
[Internal =>] LOCAL_NAME,
[, [External =>] EXTERNAL_SYMBOL]
[, [Size =>] EXTERNAL_SYMBOL]
EXTERNAL_SYMBOL ::=
IDENTIFIER
| static_string_EXPRESSION
|
This pragma is identical in effect to pragma Common_Object.
pragma Pure_Function
pragma Pure_Function ([Entity =>] function_LOCAL_NAME); |
This pragma appears in the same declarative part as a function
declaration (or a set of function declarations if more than one
overloaded declaration exists, in which case the pragma applies
to all entities). If specifies that the function Entity is
to be considered pure for the purposes of code generation. This means
that the compiler can assume that there are no side effects, and
in particular that two calls with identical arguments produce the
same result. It also means that the function can be used in an
address clause.
Note that, quite deliberately, there are no static checks to try
to ensure that this promise is met, so Pure_Function can be used
with functions that are conceptually pure, even if they do modify
global variables. For example, a square root function that is
instrumented to count the number of times it is called is still
conceptually pure, and can still be optimized, even though it
modifies a global variable (the count). Memo functions are another
example (where a table of previous calls is kept and consulted to
avoid re-computation).
Note: Most functions in a Pure package are automatically pure, and
there is no need to use pragma Pure_Function for such functions. An
exception is any function that has at least one formal of type
System.Address or a type derived from it. Such functions are not
considered pure by default, since the compiler assumes that the
Address parameter may be functioning as a pointer and that the
referenced data may change even if the address value does not. The use
of pragma Pure_Function for such a function will override this default
assumption, and cause the compiler to treat such a function as pure.
Note: If pragma Pure_Function is applied to a renamed function, it
applies to the underlying renamed function. This can be used to
disambiguate cases of overloading where some but not all functions
in a set of overloaded functions are to be designated as pure.
pragma Ravenscar
pragma Ravenscar |
A configuration pragma that establishes the following set of restrictions:
No_Abort_Statements
No_Select_Statements
No_Task_Hierarchy
No_Task_Allocators
No_Dynamic_Priorities
No_Terminate_Alternatives
No_Dynamic_Interrupts
No_Protected_Type_Allocators
No_Local_Protected_Objects
No_Requeue
No_Calendar
No_Relative_Delay
No_Task_Attributes
Static_Storage_Size
Boolean_Entry_Barriers
Max_Asynchronous_Select_Nesting = 0
Max_Task_Entries = 0
Max_Protected_Entries = 1
Max_Select_Alternatives = 0
No_Task_Termination
No_Entry_Queue
This set of restrictions corresponds to the definition of the "Ravenscar Profile" for limited tasking, devised and published by the International Real-Time Ada Workshop, 1997.
The above set is a superset of the restrictions provided by pragma
Restricted_Run_Time, it includes six additional restrictions
(Boolean_Entry_Barriers, No_Select_Statements,
No_Calendar, Static_Storage_Size,
No_Relative_Delay and No_Task_Termination). This means
that pragma Ravenscar, like the pragma Restricted_Run_Time, automatically
causes the use of a simplified, more efficient version of the tasking
run-time system.
pragma Restricted_Run_Time
pragma Restricted_Run_Time |
A configuration pragma that establishes the following set of restrictions:
This set of restrictions causes the automatic selection of a simplified version of the run time that provides improved performance for the limited set of tasking functionality permitted by this set of restrictions.
pragma Share_Generic
pragma Share_Generic (NAME {, NAME});
|
This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT. GNAT does not provide the capability for sharing of generic code. All generic instantiations result in making an inlined copy of the template with appropriate substitutions.
pragma Source_File_Name
pragma Source_File_Name ( [Unit_Name =>] unit_NAME, Spec_File_Name => STRING_LITERAL); pragma Source_File_Name ( [Unit_Name =>] unit_NAME, Body_File_Name => STRING_LITERAL); |
Use this to override the normal naming convention. It is a configuration pragma, and so has the usual applicability of configuration pragmas (i.e. it applies to either an entire partition, or to all units in a compilation, or to a single unit, depending on how it is used. unit_name is mapped to file_name_literal. The identifier for the second argument is required, and indicates whether this is the file name for the spec or for the body.
Another form of the Source_File_Name pragma allows
the specification of patterns defining alternative file naming schemes
to apply to all files.
pragma Source_File_Name (Spec_File_Name => STRING_LITERAL [,Casing => CASING_SPEC] [,Dot_Replacement => STRING_LITERAL]); pragma Source_File_Name (Body_File_Name => STRING_LITERAL [,Casing => CASING_SPEC] [,Dot_Replacement => STRING_LITERAL]); pragma Source_File_Name (Subunit_File_Name => STRING_LITERAL [,Casing => CASING_SPEC] [,Dot_Replacement => STRING_LITERAL]); CASING_SPEC ::= Lowercase | Uppercase | Mixedcase |
The first argument is a pattern that contains a single asterisk indicating the point at which the unit name is to be inserted in the pattern string to form the file name. The second argument is optional. If present it specifies the casing of the unit name in the resulting file name string. The default is lower case. Finally the third argument allows for systematic replacement of any dots in the unit name by the specified string literal.
For more details on the use of the Source_File_Name pragma,
see the sections "Using Other File Names" and
"Alternative File Naming Schemes" in the GNAT User's Guide.
pragma Source_Reference
pragma Source_Reference (INTEGER_LITERAL,
STRING_LITERAL);
|
This pragma must appear as the first line of a source file.
integer_literal is the logical line number of the line following
the pragma line (for use in error messages and debugging
information). string_literal is a static string constant that
specifies the file name to be used in error messages and debugging
information. This is most notably used for the output of gnatchop
with the -r switch, to make sure that the original unchopped
source file is the one referred to.
The second argument must be a string literal, it cannot be a static string expression other than a string literal. This is because its value is needed for error messages issued by all phases of the compiler.
pragma Stream_Convert
pragma Stream_Convert ( [Entity =>] type_LOCAL_NAME, [Read =>] function_NAME, [Write =>] function NAME); |
This pragma provides an efficient way of providing stream functions for types defined in packages. Not only is it simpler to use than declaring the necessary functions with attribute representation clauses, but more significantly, it allows the declaration to made in such a way that the stream packages are not loaded unless they are needed. The use of the Stream_Convert pragma adds no overhead at all, unless the stream attributes are actually used on the designated type.
The first argument specifies the type for which stream functions are provided. The second parameter provides a function used to read values of this type. It must name a function whose argument type may be any subtype, and whose returned type must be the type given as the first argument to the pragma.
The meaning of the Read parameter is that if a stream attribute directly or indirectly specifies reading of the type given as the first parameter, then a value of the type given as the argument to the Read function is read from the stream, and then the Read function is used to convert this to the required target type.
Similarly the Write parameter specifies how to treat write attributes that directly or indirectly apply to the type given as the first parameter. It must have an input parameter of the type specified by the first parameter, and the return type must be the same as the input type of the Read function. The effect is to first call the Write function to convert to the given stream type, and then write the result type to the stream.
The Read and Write functions must not be overloaded subprograms. If necessary renamings can be supplied to meet this requirement. The usage of this attribute is best illustrated by a simple example, taken from the GNAT implementation of package Ada.Strings.Unbounded:
function To_Unbounded (S : String)
return Unbounded_String
renames To_Unbounded_String;
pragma Stream_Convert
(Unbounded_String, To_Unbounded, To_String);
|
The specifications of the referenced functions, as given in the Ada 95 Reference Manual are:
function To_Unbounded_String (Source : String) return Unbounded_String; function To_String (Source : Unbounded_String) return String; |
The effect is that if the value of an unbounded string is written to a
stream, then the representation of the item in the stream is in the same
format used for Standard.String, and this same representation is
expected when a value of this type is read from the stream.
pragma Style_Checks
pragma Style_Checks (string_LITERAL | ALL_CHECKS |
On | Off [, LOCAL_NAME]);
|
This pragma is used in conjunction with compiler switches to control the built in style checking provided by GNAT. The compiler switches, if set provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma. This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the `gnat.adc' file).
The form with a string literal specifies which style options are to be
activated. These are additive, so they apply in addition to any previously
set style check options. The codes for the options are the same as those
used in the -gnaty switch to gcc or gnatmake.
For example the following two methods can be used to enable
layout checking:
pragma Style_Checks ("l");
gcc -c -gnatyl ...
|
The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the gnaty switch with no options. See GNAT User's
Guide for details.
The forms with Off and On
can be used to temporarily disable style checks
as shown in the following example:
pragma Style_Checks ("k"); -- requires keywords in lower case
pragma Style_Checks (Off); -- turn off style checks
NULL; -- this will not generate an error message
pragma Style_Checks (On); -- turn style checks back on
NULL; -- this will generate an error message
|
Finally the two argument form is allowed only if the first argument is
On or Off. The effect is to turn of semantic style checks
for the specified entity, as shown in the following example:
pragma Style_Checks ("r"); -- require consistency of identifier casing
Arg : Integer;
Rf1 : Integer := ARG; -- incorrect, wrong case
pragma Style_Checks (Off, Arg);
Rf2 : Integer := ARG; -- OK, no error
|
pragma Subtitle
pragma Subtitle ([Subtitle =>] STRING_LITERAL); |
This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT.
pragma Suppress_All
pragma Suppress_All; |
This pragma can only appear immediately following a compilation
unit. The effect is to apply Suppress (All_Checks) to the unit
which it follows. This pragma is implemented for compatibility with DEC
Ada 83 usage. The use of pragma Suppress (All_Checks) as a normal
configuration pragma is the preferred usage in GNAT.
pragma Suppress_Initialization
pragma Suppress_Initialization ([Entity =>] type_Name); |
This pragma suppresses any implicit or explicit initialization associated with the given type name for all variables of this type.
pragma Task_Info
pragma Task_Info (EXPRESSION); |
This pragma appears within a task definition (like pragma
Priority) and applies to the task in which it appears. The
argument must be of type System.Task_Info.Task_Info_Type.
The Task_Info pragma provides system dependent control over
aspect of tasking implementation, for example, the ability to map
tasks to specific processors. For details on the facilities available
for the version of GNAT that you are using, see the documentation
in the specification of package System.Task_Info in the runtime
library.
pragma Task_Name
pragma Task_Name (string_EXPRESSION); |
This pragma appears within a task definition (like pragma
Priority) and applies to the task in which it appears. The
argument must be of type String, and provides a name to be used for
the task instance when the task is created. Note that this expression
is not required to be static, and in particular, it can contain
references to task discriminants. This facility can be used to
provide different names for different tasks as they are created,
as illustrated in the example below.
The task name is recorded internally in the run-time structures
and is accessible to tools like the debugger. In addition the
routine Ada.Task_Identification.Image will return this
string, with a unique task address appended.
-- Example of the use of pragma Task_Name
with Ada.Task_Identification;
use Ada.Task_Identification;
with Text_IO; use Text_IO;
procedure t3 is
type Astring is access String;
task type Task_Typ (Name : access String) is
pragma Task_Name (Name.all);
end Task_Typ;
task body Task_Typ is
Nam : constant String := Image (Current_Task);
begin
Put_Line ("-->" & Nam (1 .. 14) & "<--");
end Task_Typ;
type Ptr_Task is access Task_Typ;
Task_Var : Ptr_Task;
begin
Task_Var :=
new Task_Typ (new String'("This is task 1"));
Task_Var :=
new Task_Typ (new String'("This is task 2"));
end;
|
pragma Task_Storage
pragma Task_Storage [Task_Type =>] LOCAL_NAME, [Top_Guard =>] static_integer_EXPRESSION); |
This pragma specifies the length of the guard area for tasks. The guard
area is an additional storage area allocated to a task. A value of zero
means that either no guard area is created or a minimal guard area is
created, depending on the target. This pragma can appear anywhere a
Storage_Size attribute definition clause is allowed for a task
type.
pragma Time_Slice
pragma Time_Slice (static_duration_EXPRESSION); |
For implementations of GNAT on operating systems where it is possible to supply a time slice value, this pragma may be used for this purpose. It is ignored if it is used in a system that does not allow this control, or if it appears in other than the main program unit. Note that the effect of this pragma is identical to the effect of the DEC Ada 83 pragma of the same name when operating under OpenVMS systems.
pragma Title
pragma Title (TITLING_OPTION [, TITLING OPTION]); TITLING_OPTION ::= [Title =>] STRING_LITERAL, | [Subtitle =>] STRING_LITERAL |
Syntax checked but otherwise ignored by GNAT. This is a listing control pragma used in DEC Ada 83 implementations to provide a title and/or subtitle for the program listing. The program listing generated by GNAT does not have titles or subtitles.
Unlike other pragmas, the full flexibility of named notation is allowed for this pragma, i.e. the parameters may be given in any order if named notation is used, and named and positional notation can be mixed following the normal rules for procedure calls in Ada.
pragma Unchecked_Union
pragma Unchecked_Union (first_subtype_LOCAL_NAME) |
This pragma is used to declare that the specified type should be represented in a manner equivalent to a C union type, and is intended only for use in interfacing with C code that uses union types. In Ada terms, the named type must obey the following rules:
In addition, given a type that meets the above requirements, the following restrictions apply to its use throughout the program:
Equality and inequality operations on unchecked_unions are not
available, since there is no discriminant to compare and the compiler
does not even know how many bits to compare. It is implementation
dependent whether this is detected at compile time as an illegality or
whether it is undetected and considered to be an erroneous construct. In
GNAT, a direct comparison is illegal, but GNAT does not attempt to catch
the composite case (where two composites are compared that contain an
unchecked union component), so such comparisons are simply considered
erroneous.
The layout of the resulting type corresponds exactly to a C union, where
each branch of the union corresponds to a single variant in the Ada
record. The semantics of the Ada program is not changed in any way by
the pragma, i.e. provided the above restrictions are followed, and no
erroneous incorrect references to fields or erroneous comparisons occur,
the semantics is exactly as described by the Ada reference manual.
Pragma Suppress (Discriminant_Check) applies implicitly to the
type and the default convention is C
pragma Unimplemented_Unit
pragma Unimplemented_Unit; |
If this pragma occurs in a unit that is processed by the compiler, GNAT aborts with the message `xxx not implemented', where xxx is the name of the current compilation unit. This pragma is intended to allow the compiler to handle unimplemented library units in a clean manner.
The abort only happens if code is being generated. Thus you can use specs of unimplemented packages in syntax or semantic checking mode.
pragma Unreferenced
pragma Unreferenced (local_Name {, local_Name});
|
This pragma signals that the entities whose names are listed are deliberately not referenced. This suppresses warnings about the entities being unreferenced, and in addition a warning will be generated if one of these entities is in fact referenced.
This is particularly useful for clearly signalling that a particular parameter is not referenced in some particular subprogram implementation and that this is deliberate. It can also be useful in the case of objects declared only for their initialization or finalization side effects.
If local_Name identifies more than one matching homonym in the
current scope, then the entity most recently declared is the one to which
the pragma applies.
pragma Unreserve_All_Interrupts
pragma Unreserve_All_Interrupts; |
Normally certain interrupts are reserved to the implementation. Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22). A typical example is the SIGINT interrupt used in
many systems for an Ctrl-C interrupt. Normally this interrupt is
reserved to the implementation, so that Ctrl-C can be used to
interrupt execution.
If the pragma Unreserve_All_Interrupts appears anywhere in any unit in
a program, then all such interrupts are unreserved. This allows the
program to handle these interrupts, but disables their standard
functions. For example, if this pragma is used, then pressing
Ctrl-C will not automatically interrupt execution. However,
a program can then handle the SIGINT interrupt as it chooses.
For a full list of the interrupts handled in a specific implementation,
see the source code for the specification of Ada.Interrupts.Names in
file `a-intnam.ads'. This is a target dependent file that contains the
list of interrupts recognized for a given target. The documentation in
this file also specifies what interrupts are affected by the use of
the Unreserve_All_Interrupts pragma.
pragma Unsuppress
pragma Unsuppress (IDENTIFIER [, [On =>] NAME]); |
This pragma undoes the effect of a previous pragma Suppress. If
there is no corresponding pragma Suppress in effect, it has no
effect. The range of the effect is the same as for pragma
Suppress. The meaning of the arguments is identical to that used
in pragma Suppress.
One important application is to ensure that checks are on in cases where code depends on the checks for its correct functioning, so that the code will compile correctly even if the compiler switches are set to suppress checks.
pragma Use_VADS_Size
pragma Use_VADS_Size; |
This is a configuration pragma. In a unit to which it applies, any use of the 'Size attribute is automatically interpreted as a use of the 'VADS_Size attribute. Note that this may result in incorrect semantic processing of valid Ada 95 programs. This is intended to aid in the handling of legacy code which depends on the interpretation of Size as implemented in the VADS compiler. See description of the VADS_Size attribute for further details.
pragma Validity_Checks
pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off); |
This pragma is used in conjunction with compiler switches to control the built in validity checking provided by GNAT. The compiler switches, if set provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma. This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the `gnat.adc' file).
The form with a string literal specifies which validity options are to be
activated. The validity checks are first set to include only the default
reference manual settings, and then a string of letters in the string
specifies the exact set of options required. The form of this string
is exactly as described for the -gnatVx compiler switch (see the
GNAT users guide for details). For example the following two methods
can be used to enable validity checking for mode in and
in out subprogram parameters:
pragma Validity_Checks ("im");
gcc -c -gnatVim ...
|
The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the gnatva switch.
The forms with Off and On
can be used to temporarily disable validity checks
as shown in the following example:
pragma Validity_Checks ("c"); -- validity checks for copies
pragma Validity_Checks (Off); -- turn off validity checks
A := B; -- B will not be validity checked
pragma Validity_Checks (On); -- turn validity checks back on
A := C; -- C will be validity checked
|
pragma Volatile
pragma Volatile (local_NAME) |
This pragma is defined by the Ada 95 Reference Manual, and the GNAT implementation is fully conformant with this definition. The reason it is mentioned in this section is that a pragma of the same name was supplied in some Ada 83 compilers, including DEC Ada 83. The Ada 95 implementation of pragma Volatile is upwards compatible with the implementation in Dec Ada 83.
pragma Warnings
pragma Warnings (On | Off [, LOCAL_NAME]); |
Normally warnings are enabled, with the output being controlled by
the command line switch. Warnings (Off) turns off generation of
warnings until a Warnings (On) is encountered or the end of the
current unit. If generation of warnings is turned off using this
pragma, then no warning messages are output, regardless of the
setting of the command line switches.
The form with a single argument is a configuration pragma.
If the local_name parameter is present, warnings are suppressed for
the specified entity. This suppression is effective from the point where
it occurs till the end of the extended scope of the variable (similar to
the scope of Suppress).
pragma Weak_External
pragma Weak_External ([Entity =>] LOCAL_NAME); |
This pragma specifies that the given entity should be marked as a weak external (one that does not have to be resolved) for the linker. For further details, consult the GCC manual.
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In addition, Ada 95 allows implementations to define additional attributes whose meaning is defined by the implementation. GNAT provides a number of these implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler. This section of the GNAT reference manual describes these additional attributes.
Note that any program using these attributes may not be portable to other compilers (although GNAT implements this set of attributes on all platforms). Therefore if portability to other compilers is an important consideration, you should minimize the use of these attributes.
Abort_Signal
Standard'Abort_Signal (Standard is the only allowed
prefix) provides the entity for the special exception used to signal
task abort or asynchronous transfer of control. Normally this attribute
should only be used in the tasking runtime (it is highly peculiar, and
completely outside the normal semantics of Ada, for a user program to
intercept the abort exception).
Address_Size
Standard'Address_Size (Standard is the only allowed
prefix) is a static constant giving the number of bits in an
Address. It is used primarily for constructing the definition of
Memory_Size in package Standard, but may be freely used in user
programs and has the advantage of being static, while a direct
reference to System.Address'Size is non-static because Address
is a private type.
Asm_Input
Asm_Input attribute denotes a function that takes two
parameters. The first is a string, the second is an expression of the
type designated by the prefix. The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g. what kind of register is required). The second argument is the
value to be used as the input argument. The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
11. Machine Code Insertions
Asm_Output
Asm_Output attribute denotes a function that takes two
parameters. The first is a string, the second is the name of a variable
of the type designated by the attribute prefix. The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g. what kind of register is
required). The second argument is the variable to be updated with the
result. The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end. If there are no output operands, then this argument may
either be omitted, or explicitly given as No_Output_Operands.
11. Machine Code Insertions
AST_Entry
Extend_System (Aux_DEC)). This value enables the given entry to
be called when an AST occurs. For further details, refer to the DEC Ada
Language Reference Manual, section 9.12a.
Bit
obj'Bit, where obj is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object. The value of this attribute is of the
type Universal_Integer, and is always a non-negative number not
exceeding the value of System.Storage_Unit.
For an object that is a variable or a constant allocated in a register, the value is zero. (The use of this attribute does not force the allocation of a variable to memory).
For an object that is a formal parameter, this attribute applies to either the matching actual parameter or to a copy of the matching actual parameter.
For an access object the value is zero. Note that
obj.all'Bit is subject to an Access_Check for the
designated object. Similarly for a record component
X.C'Bit is subject to a discriminant check and
X(I).Bit and X(I1..I2)'Bit
are subject to index checks.
This attribute is designed to be compatible with the DEC Ada 83 definition
and implementation of the Bit attribute.
Bit_Position
R.C'Bit, where R is a record object and C is one
of the fields of the record type, yields the bit
offset within the record contains the first bit of
storage allocated for the object. The value of this attribute is of the
type Universal_Integer. The value depends only on the field
C and is independent of the alignment of
the containing record R.
Code_Address
'Address
attribute may be applied to subprograms in Ada 95, but the
intended effect from the Ada 95 reference manual seems to be to provide
an address value which can be used to call the subprogram by means of
an address clause as in the following example:
procedure K is ... procedure L; for L'Address use K'Address; pragma Import (Ada, L); |
A call to L is then expected to result in a call to K. In Ada 83, where
there were no access-to-subprogram values, this was a common work around
for getting the effect of an indirect call.
GNAT implements the above use of Address and the technique illustrated
by the example code works correctly.
However, for some purposes, it is useful to have the address of the start
of the generated code for the subprogram. On some architectures, this is
not necessarily the same as the Address value described above. For example,
the Address value may reference a subprogram descriptor rather than the
subprogram itself.
The 'Code_Address attribute, which can only be applied to
subprogram entities, always returns the address of the start of the
generated code of the specified subprogram, which may or may not be
the same value as is returned by the corresponding 'Address
attribute.
Default_Bit_Order
Standard'Default_Bit_Order (Standard is the only
permissible prefix), provides the value System.Default_Bit_Order
as a Pos value (0 for High_Order_First, 1 for
Low_Order_First). This is used to construct the definition of
Default_Bit_Order in package System.
Elaborated
'Elaborated attribute must be a unit name. The
value is a Boolean which indicates whether or not the given unit has been
elaborated. This attribute is primarily intended for internal use by the
generated code for dynamic elaboration checking, but it can also be used
in user programs. The value will always be True once elaboration of all
units has been completed.
Elab_Body
Elab_Spec
Emax
Emax attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Enum_Rep
S'Enum_Rep denotes a
function with the following specification:
function S'Enum_Rep (Arg : S'Base) return Universal_Integer; |
It is also allowable to apply Enum_Rep directly to an object of an
enumeration type or to a non-overloaded enumeration
literal. In this case S'Enum_Rep is equivalent to
typ'Enum_Rep(S) where typ is the type of the
enumeration literal or object.
The function returns the representation value for the given enumeration
value. This will be equal to value of the Pos attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e. the result is static if the argument is static).
S'Enum_Rep can also be used with integer types and objects, in which
case it simply returns the integer value. The reason for this is to allow
it to be used for (<>) discrete formal arguments in a generic unit that
can be instantiated with either enumeration types or integer types. Note
that if Enum_Rep is used on a modular type whose upper bound exceeds the
upper bound of the largest signed integer type, and the argument is a
variable, so that the universal integer calculation is done at run-time,
then the call to Enum_Rep may raise Constraint_Error.
Epsilon
Epsilon attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Fixed_Value
S'Fixed_Value denotes a
function with the following specification:
function S'Fixed_Value (Arg : Universal_Integer) return S; |
The value returned is the fixed-point value V such that
V = Arg * S'Small |
The effect is thus equivalent to first converting the argument to the integer type used to represent S, and then doing an unchecked conversion to the fixed-point type. This attribute is primarily intended for use in implementation of the input-output functions for fixed-point values.
Has_Discriminants
Has_Discriminants attribute is a type. The result
is a Boolean value which is True if the type has discriminants, and False
otherwise. The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has discriminants.
Img
Img attribute differs from Image in that it may be
applied to objects as well as types, in which case it gives the
Image for the subtype of the object. This is convenient for
debugging:
Put_Line ("X = " & X'Img);
|
has the same meaning as the more verbose:
Put_Line ("X = " & type'Image (X));
|
where type is the subtype of the object X.
Integer_Value
S'Integer_Value denotes a
function with the following specification:
function S'Integer_Value (Arg : Universal_Fixed) return S; |
The value returned is the integer value V, such that
Arg = V * type'Small |
The effect is thus equivalent to first doing an unchecked convert from the fixed-point type to its corresponding implementation type, and then converting the result to the target integer type. This attribute is primarily intended for use in implementation of the standard input-output functions for fixed-point values.
Large
Large attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Machine_Size
Object_Size attribute. It is
provided for compatibility with the DEC Ada 83 attribute of this name.
Mantissa
Mantissa attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Max_Interrupt_Priority
Standard'Max_Interrupt_Priority (Standard is the only
permissible prefix), provides the value
System.Max_Interrupt_Priority and is intended primarily for
constructing this definition in package System.
Max_Priority
Standard'Max_Priority (Standard is the only permissible
prefix) provides the value System.Max_Priority and is intended
primarily for constructing this definition in package System.
Maximum_Alignment
Standard'Maximum_Alignment (Standard is the only
permissible prefix) provides the maximum useful alignment value for the
target. This is a static value that can be used to specify the alignment
for an object, guaranteeing that it is properly aligned in all
cases. This is useful when an external object is imported and its
alignment requirements are unknown.
Mechanism_Code
function'Mechanism_Code yields an integer code for the
mechanism used for the result of function, and
subprogram'Mechanism_Code (n) yields the mechanism
used for formal parameter number n (a static integer value with 1
meaning the first parameter) of subprogram. The code returned is:
Values from 3 through 10 are only relevant to Digital OpenVMS implementations.
Null_Parameter
T'Null_Parameter denotes an imaginary object of
type or subtype T allocated at machine address zero. The attribute
is allowed only as the default expression of a formal parameter, or as
an actual expression of a subprogram call. In either case, the
subprogram must be imported.
The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default).
This capability is needed to specify that a zero address should be
passed for a record or other composite object passed by reference.
There is no way of indicating this without the Null_Parameter
attribute.
Object_Size
Natural'Size is
31, but by default objects of type Natural will have a size of 32 bits.
Similarly, a record containing an integer and a character:
type Rec is record I : Integer; C : Character; end record; |
will have a size of 40 (that is Rec'Size will be 40. The
alignment will be 4, because of the
integer field, and so the default size of record objects for this type
will be 64 (8 bytes).
The type'Object_Size attribute
has been added to GNAT to allow the
default object size of a type to be easily determined. For example,
Natural'Object_Size is 32, and
Rec'Object_Size (for the record type in the above example) will be
64. Note also that, unlike the situation with the
Size attribute as defined in the Ada RM, the
Object_Size attribute can be specified individually
for different subtypes. For example:
type R is new Integer; subtype R1 is R range 1 .. 10; subtype R2 is R range 1 .. 10; for R2'Object_Size use 8; |
In this example, R'Object_Size and R1'Object_Size are both
32 since the default object size for a subtype is the same as the object size
for the parent subtype. This means that objects of type R
or R1 will
by default be 32 bits (four bytes). But objects of type
R2 will be only
8 bits (one byte), since R2'Object_Size has been set to 8.
Passed_By_Reference
type'Passed_By_Reference for any subtype type returns
a value of type Boolean value that is True if the type is
normally passed by reference and False if the type is normally
passed by copy in calls. For scalar types, the result is always False
and is static. For non-scalar types, the result is non-static.
Range_Length
type'Range_Length for any discrete type type yields
the number of values represented by the subtype (zero for a null
range). The result is static for static subtypes. Range_Length
applied to the index subtype of a one dimensional array always gives the
same result as Range applied to the array itself.
Safe_Emax
Safe_Emax attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Safe_Large
Safe_Large attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Safe_Large
Safe_Large attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
Small
Small attribute is defined in Ada 95 only for fixed-point types.
GNAT also allows this attribute to be applied to floating-point types
for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute when applied to floating-point types.
Storage_Unit
Standard'Storage_Unit (Standard is the only permissible
prefix) provides the value System.Storage_Unit and is intended
primarily for constructing this definition in package System.
Tick
Standard'Tick (Standard is the only permissible prefix)
provides the value of System.Tick and is intended primarily for
constructing this definition in package System.
To_Address
System'To_Address
(System is the only permissible prefix)
denotes a function identical to
System.Storage_Elements.To_Address except that
it is a static attribute. This means that if its argument is
a static expression, then the result of the attribute is a
static expression. The result is that such an expression can be
used in contexts (e.g. preelaborable packages) which require a
static expression and where the function call could not be used
(since the function call is always non-static, even if its
argument is static).
Type_Class
type'Type_Class for any type or subtype type yields
the value of the type class for the full type of type. If
type is a generic formal type, the value is the value for the
corresponding actual subtype. The value of this attribute is of type
System.Aux_DEC.Type_Class, which has the following definition:
type Type_Class is
(Type_Class_Enumeration,
Type_Class_Integer,
Type_Class_Fixed_Point,
Type_Class_Floating_Point,
Type_Class_Array,
Type_Class_Record,
Type_Class_Access,
Type_Class_Task,
Type_Class_Address);
|
Protected types yield the value Type_Class_Task, which thus
applies to all concurrent types. This attribute is designed to
be compatible with the DEC Ada 83 attribute of the same name.
UET_Address
UET_Address attribute can only be used for a prefix which
denotes a library package. It yields the address of the unit exception
table when zero cost exception handling is used. This attribute is
intended only for use within the GNAT implementation. See the unit
Ada.Exceptions in files `a-except.ads' and `a-except.adb'
for details on how this attribute is used in the implementation.
Universal_Literal_String
Universal_Literal_String must be a named
number. The static result is the string consisting of the characters of
the number as defined in the original source. This allows the user
program to access the actual text of named numbers without intermediate
conversions and without the need to enclose the strings in quotes (which
would preclude their use as numbers). This is used internally for the
construction of values of the floating-point attributes from the file
`ttypef.ads', but may also be used by user programs.
Unrestricted_Access
Unrestricted_Access attribute is similar to Access
except that all accessibility and aliased view checks are omitted. This
is a user-beware attribute. It is similar to
Address, for which it is a desirable replacement where the value
desired is an access type. In other words, its effect is identical to
first applying the Address attribute and then doing an unchecked
conversion to a desired access type. In GNAT, but not necessarily in
other implementations, the use of static chains for inner level
subprograms means that Unrestricted_Access applied to a
subprogram yields a value that can be called as long as the subprogram
is in scope (normal Ada 95 accessibility rules restrict this usage).
VADS_Size
'VADS_Size attribute is intended to make it easier to port
legacy code which relies on the semantics of 'Size as implemented
by the VADS Ada 83 compiler. GNAT makes a best effort at duplicating the
same semantic interpretation. In particular, 'VADS_Size applied
to a predefined or other primitive type with no Size clause yields the
Object_Size (for example, Natural'Size is 32 rather than 31 on
typical machines). In addition 'VADS_Size applied to an object
gives the result that would be obtained by applying the attribute to
the corresponding type.
Value_Size
type'Value_Size is the number of bits required to represent
a value of the given subtype. It is the same as type'Size,
but, unlike Size, may be set for non-first subtypes.
Wchar_T_Size
Standard'Wchar_T_Size (Standard is the only permissible
prefix) provides the size in bits of the C wchar_t type
primarily for constructing the definition of this type in
package Interfaces.C.
Word_Size
Standard'Word_Size (Standard is the only permissible
prefix) provides the value System.Word_Size and is intended
primarily for constructing this definition in package System.
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In addition, there are sections throughout the Ada 95 reference manual headed by the phrase "implementation advice". These sections are not normative, i.e. they do not specify requirements that all compilers must follow. Rather they provide advice on generally desirable behavior. You may wonder why they are not requirements. The most typical answer is that they describe behavior that seems generally desirable, but cannot be provided on all systems, or which may be undesirable on some systems.
As far as practical, GNAT follows the implementation advice sections in the Ada 95 Reference Manual. This chapter contains a table giving the reference manual section number, paragraph number and several keywords for each advice. Each entry consists of the text of the advice followed by the GNAT interpretation of this advice. Most often, this simply says "followed", which means that GNAT follows the advice. However, in a number of cases, GNAT deliberately deviates from this advice, in which case the text describes what GNAT does and why.
Program_Error if
feasible.
Not relevant. All specialized needs annex features are either supported,
or diagnosed at compile time.
Program_Error.
Followed in all cases in which the implementation detects a bounded
error or erroneous execution. Not all such situations are detected at
runtime.
Abort_Defer
Ada_83
Assert
CPP_Class
CPP_Constructor
CPP_Virtual
CPP_Vtable
Debug
Interface_Name
Machine_Attribute
Unimplemented_Unit
Unchecked_Union
In each of the above cases, it is essential to the purpose of the pragma that this advice not be followed. For details see the separate section on implementation defined pragmas.
Import;
A pragma used to configure the environment by adding, removing, or
replacing library_items.
See response to paragraph 16 of this same section.
Character and Wide_Character, the set of graphic
characters of Character should nevertheless remain a proper
subset of the set of graphic characters of Wide_Character. Any
character set "localizations" should be reflected in the results of
the subprograms defined in the language-defined package
Characters.Handling (see A.3) available in such a mode. In a mode with
an alternative interpretation of Character, the implementation should
also support a corresponding change in what is a legal
identifier_letter.
Not all wide character modes follow this advice, in particular the JIS
and IEC modes reflect standard usage in Japan, and in these encoding,
the upper half of the Latin-1 set is not part of the wide-character
subset, since the most significant bit is used for wide character
encoding. However, this only applies to the external forms. Internally
there is no such restriction.
An implementation should support Long_Integer in addition to
Integer if the target machine supports 32-bit (or longer)
arithmetic. No other named integer subtypes are recommended for package
Standard. Instead, appropriate named integer subtypes should be
provided in the library package Interfaces (see B.2).
Long_Integer is supported. Other standard integer types are supported
so this advice is not fully followed. These types
are supported for convenient interface to C, and so that all hardware
types of the machine are easily available.
An implementation for a two's complement machine should support
modular types with a binary modulus up to System.Max_Int*2+2. An
implementation should support a non-binary modules up to Integer'Last.
Followed.
S'Pos for an enumeration
subtype, if the value of the operand does not correspond to the internal
code for any enumeration literal of its type (perhaps due to an
un-initialized variable), then the implementation should raise
Program_Error. This is particularly important for enumeration
types with noncontiguous internal codes specified by an
enumeration_representation_clause.
Followed.
Long_Float in addition to
Float if the target machine supports 11 or more digits of
precision. No other named floating point subtypes are recommended for
package Standard. Instead, appropriate named floating point subtypes
should be provided in the library package Interfaces (see B.2).
Short_Float and Long_Long_Float are also provided. The
former provides improved compatibility with other implementations
supporting this type. The latter corresponds to the highest precision
floating-point type supported by the hardware. On most machines, this
will be the same as Long_Float, but on some machines, it will
correspond to the IEEE extended form. The notable case is all ia32
(x86) implementations, where Long_Long_Float corresponds to
the 80-bit extended precision format supported in hardware on this
processor. Note that the 128-bit format on SPARC is not supported,
since this is a software rather than a hardware format.
Convention
(Fortran, ...) applies to a multidimensional array type, then
column-major order should be used instead (see B.5, "Interfacing with
Fortran").
Followed.
Duration'Small
should be no greater than 100 microseconds.
Followed. (Duration'Small = 10**(-9)).
The time base for delay_relative_statements should be monotonic;
it need not be the same time base as used for Calendar.Clock.
Followed.
Exception_Message by default and Exception_Information
should produce information useful for
debugging. Exception_Message should be short, about one
line. Exception_Information can be long. Exception_Message
should not include the
Exception_Name. Exception_Information should include both
the Exception_Name and the Exception_Message.
Followed. For each exception that doesn't have a specified
Exception_Message, the compiler generates one containing the location
of the raise statement. This location has the form "file:line", where
file is the short file name (without path information) and line is the line
number in the file. Note that in the case of the Zero Cost Exception
mechanism, these messages become redundant with the Exception_Information that
contains a full backtrace of the calling sequence, so they are disabled.
To disable explicitly the generation of the source location message, use the
Pragma Discard_Names.
X : typ; for X'Address use To_address (16#2000#); |
will be rejected, since the To_Address expression is non-static. Instead write:
X_Address : constant Address : = To_Address ((16#2000#); X : typ; for X'Address use X_Address; |
An implementation need not support a specification for the Size
for a given composite subtype, nor the size or storage place for an
object (including a component) of a given composite subtype, unless the
constraints on the subtype and its composite subcomponents (if any) are
all static constraints.
Followed. Size Clauses are not permitted on non-static components, as
described above.
An aliased component, or a component whose type is by-reference, should always be allocated at an addressable location. Followed.
Pack is:
For a packed record type, the components should be packed as tightly as
possible subject to the Sizes of the component subtypes, and subject to
any record_representation_clause that applies to the type; the
implementation may, but need not, reorder components or cross aligned
word boundaries to improve the packing. A component whose Size is
greater than the word size may be allocated an integral number of words.
Followed. Tight packing of arrays is supported for all component sizes
up to 64-bits.
An implementation should support Address clauses for imported subprograms. Followed.
For an array X, X'Address should point at the first
component of the array, and not at the array bounds.
Followed.
The recommended level of support for the Address attribute is:
X'Address should produce a useful result if X is an
object that is aliased or of a by-reference type, or is an entity whose
Address has been specified.
Followed. A valid address will be produced even if none of those
conditions have been met. If necessary, the object is forced into
memory to ensure the address is valid.
An implementation should support Address clauses for imported
subprograms.
Followed.
Objects (including subcomponents) that are aliased or of a by-reference
type should be allocated on storage element boundaries.
Followed.
If the Address of an object is specified, or it is imported or exported,
then the implementation should not perform optimizations based on
assumptions of no aliases.
Followed.
Alignment attribute for
subtypes is:
An implementation should support specified Alignments that are factors
and multiples of the number of storage elements per word, subject to the
following:
Followed.
An implementation need not support specified Alignments for
combinations of Sizes and Alignments that cannot be easily
loaded and stored by available machine instructions.
Followed.
An implementation need not support specified Alignments that are
greater than the maximum Alignment the implementation ever returns by
default.
Followed.
The recommended level of support for the Alignment attribute for
objects is:
Same as above, for subtypes, but in addition:
Followed.
For stand-alone library-level objects of statically constrained
subtypes, the implementation should support all Alignments
supported by the target linker. For example, page alignment is likely to
be supported for such objects, but not for subtypes.
Followed.
Size attribute of
objects is:
A Size clause should be supported for an object if the specified
Size is at least as large as its subtype's Size, and
corresponds to a size in storage elements that is a multiple of the
object's Alignment (if the Alignment is nonzero).
Followed.
Size of a subtype is specified, and allows for efficient
independent addressability (see 9.10) on the target architecture, then
the Size of the following objects of the subtype should equal the
Size of the subtype:
Aliased objects (including components). Followed.
Size clause on a composite subtype should not affect the
internal layout of components.
Followed.
The recommended level of support for the Size attribute of subtypes is:
The Size (if not specified) of a static discrete or fixed point
subtype should be the number of bits needed to represent each value
belonging to the subtype using an unbiased representation, leaving space
for a sign bit only if the subtype contains negative values. If such a
subtype is a first subtype, then an implementation should support a
specified Size for it that reflects this representation.
Followed.
For a subtype implemented with levels of indirection, the Size
should include the size of the pointers, but not the size of what they
point at.
Followed.
Component_Size
attribute is:
An implementation need not support specified Component_Sizes that are
less than the Size of the component subtype.
Followed.
An implementation should support specified Component_Sizes that
are factors and multiples of the word size. For such
Component_Sizes, the array should contain no gaps between
components. For other Component_Sizes (if supported), the array
should contain no gaps between components when packing is also
specified; the implementation should forbid this combination in cases
where it cannot support a no-gaps representation.
Followed.
An implementation need not support enumeration representation clauses
for boolean types, but should at minimum support the internal codes in
the range System.Min_Int.System.Max_Int.
Followed.
record_representation_clauses is:
An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model. Followed.
A storage place should be supported if its size is equal to the
Size of the component subtype, and it starts and ends on a
boundary that obeys the Alignment of the component subtype.
Followed.
If the default bit ordering applies to the declaration of a given type,
then for a component whose subtype's Size is less than the word
size, any storage place that does not cross an aligned word boundary
should be supported.
Followed.
An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. Followed. The storage place for the tag field is the beginning of the tagged record, and its size is Address'Size. GNAT will reject an explicit component clause for the tag field.
An implementation need not support a component_clause for a
component of an extension part if the storage place is not after the
storage places of all components of the parent type, whether or not
those storage places had been specified.
Followed. The above advice on record representation clauses is followed,
and all mentioned features are implemented.
Word_Size = Storage_Unit, then the implementation
should support the non-default bit ordering in addition to the default
bit ordering.
Followed. Word size does not equal storage size in this implementation.
Thus non-default bit ordering is not supported.
Address should be of a private type.
Followed.
System and its children should reflect the target
environment semantics as closely as is reasonable. For example, on most
machines, it makes sense for address arithmetic to "wrap around".
Operations that do not make sense should raise Program_Error.
Followed. Address arithmetic is modular arithmetic that wraps around. No
operation raises Program_Error, since all operations make sense.
Size of an array object should not include its bounds; hence,
the bounds should not be part of the converted data.
Followed.
The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type. It should take advantage of the permission to return by reference when possible. Restrictions on unchecked conversions should be avoided unless required by the target environment. Followed. There are no restrictions on unchecked conversion. A warning is generated if the source and target types do not have the same size since the semantics in this case may be target dependent.
The recommended level of support for unchecked conversions is: Unchecked conversions should be supported and should be reversible in the cases where this clause defines the result. To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph. Followed.
A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects. Followed.
A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible. Followed.
Free should actually reclaim the
storage.
Followed.
Read and
Write for scalar objects. Otherwise, Read and Write
should use the smallest number of stream elements needed to represent
all values in the base range of the scalar type.
Followed. In particular, the interpretation chosen is that of AI-195,
which specifies that the size to be used is that of the first subtype.
Ada.Characters.Handling
Character
or Wide_Character, then the effects of the subprograms in
Characters.Handling should reflect the localizations. See also
3.5.2.
Followed. GNAT provides no such localized definitions.
Generator should be
reclaimed on exit from the scope of the object.
Followed.
If the generator period is sufficiently long in relation to the number
of distinct initiator values, then each possible value of
Initiator passed to Reset should initiate a sequence of
random numbers that does not, in a practical sense, overlap the sequence
initiated by any other value. If this is not possible, then the mapping
between initiator values and generator states should be a rapidly
varying function of the initiator value.
Followed. The generator period is sufficiently long for the first
condition here to hold true.
Get_Immediate
Get_Immediate procedures should be implemented with
unbuffered input. For a device such as a keyboard, input should be
available if a key has already been typed, whereas for a disk
file, input should always be available except at end of file. For a file
associated with a keyboard-like device, any line-editing features of the
underlying operating system should be disabled during the execution of
Get_Immediate.
Followed.
Export
Export to a given language,
then it should also allow the main subprogram to be written in that
language. It should support some mechanism for invoking the elaboration
of the Ada library units included in the system, and for invoking the
finalization of the environment task. On typical systems, the
recommended mechanism is to provide two subprograms whose link names are
adainit and adafinal. adainit should contain the
elaboration code for library units. adafinal should contain the
finalization code. These subprograms should have no effect the second
and subsequent time they are called.
Followed.
Automatic elaboration of pre-elaborated packages should be
provided when pragma Export is supported.
Followed when the main program is in Ada. If the main program is in a
foreign language, then
adainit must be called to elaborate pre-elaborated
packages.
For each supported convention L other than Intrinsic, an
implementation should support Import and Export pragmas
for objects of L-compatible types and for subprograms, and pragma
Convention for L-eligible types and for subprograms,
presuming the other language has corresponding features. Pragma
Convention need not be supported for scalar types.
Followed.
Interfaces
Interfaces.
Followed. An additional package not defined
in the Ada 95 Reference Manual is Interfaces.CPP, used
for interfacing to C++.
An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses. Followed. GNAT provides all the packages described in this section.
An Ada procedure corresponds to a void-returning C function. Followed.
An Ada function corresponds to a non-void C function. Followed.
An Ada in scalar parameter is passed as a scalar argument to a C
function.
Followed.
An Ada in parameter of an access-to-object type with designated
type T is passed as a t* argument to a C function,
where t is the C type corresponding to the Ada type T.
Followed.
An Ada access T parameter, or an Ada out or in out
parameter of an elementary type T, is passed as a t*
argument to a C function, where t is the C type corresponding to
the Ada type T. In the case of an elementary out or
in out parameter, a pointer to a temporary copy is used to
preserve by-copy semantics.
Followed.
An Ada parameter of a record type T, of any mode, is passed as a
t* argument to a C function, where t is the C
structure corresponding to the Ada type T.
Followed. This convention may be overridden by the use of the C_Pass_By_Copy
pragma, or Convention, or by explicitly specifying the mechanism for a given
call using an extended import or export pragma.
An Ada parameter of an array type with component type T, of any
mode, is passed as a t* argument to a C function, where
t is the C type corresponding to the Ada type T.
Followed.
An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification. Followed.
An Ada access T parameter is passed as a `BY REFERENCE' data item of the COBOL type corresponding to T. Followed.
An Ada in scalar parameter is passed as a `BY CONTENT' data item of the corresponding COBOL type. Followed.
Any other Ada parameter is passed as a `BY REFERENCE' data item of the COBOL type corresponding to the Ada parameter type; for scalars, a local copy is used if necessary to ensure by-copy semantics. Followed.
An Ada procedure corresponds to a Fortran subroutine. Followed.
An Ada function corresponds to a Fortran function. Followed.
An Ada parameter of an elementary, array, or record type T is passed as a T argument to a Fortran procedure, where T is the Fortran type corresponding to the Ada type T, and where the INTENT attribute of the corresponding dummy argument matches the Ada formal parameter mode; the Fortran implementation's parameter passing conventions are used. For elementary types, a local copy is used if necessary to ensure by-copy semantics. Followed.
An Ada parameter of an access-to-subprogram type is passed as a reference to a Fortran procedure whose interface corresponds to the designated subprogram's specification. Followed.
The interfacing pragmas (see Annex B) should support interface to
assembler; the default assembler should be associated with the
convention identifier Assembler.
Followed.
If an entity is exported to assembly language, then the implementation should allocate it at an addressable location, and should ensure that it is retained by the linking process, even if not otherwise referenced from the Ada code. The implementation should assume that any call to a machine code or assembler subprogram is allowed to read or update every object that is specified as exported. Followed.
It is recommended that intrinsic subprograms be provided for convenient access to any machine operations that provide special capabilities or efficiency and that are not otherwise available through the language constructs. Followed. A full set of machine operation intrinsic subprograms is provided.
Atomic read-modify-write operations--e.g., test and set, compare and swap, decrement and test, enqueue/dequeue. Followed on any target supporting such operations.
Standard numeric functions--e.g., sin, log. Followed on any target supporting such operations.
String manipulation operations--e.g., translate and test. Followed on any target supporting such operations.
Vector operations--e.g., compare vector against thresholds. Followed on any target supporting such operations.
Direct operations on I/O ports. Followed on any target supporting such operations.
Ceiling_Locking policy is not in effect, the
implementation should provide means for the application to specify which
interrupts are to be blocked during protected actions, if the underlying
system allows for a finer-grain control of interrupt blocking.
Followed. The underlying system does not allow for finer-grain control
of interrupt blocking.
Whenever practical, violations of any implementation-defined restrictions should be detected before run time. Followed. Compile time warnings are given when possible.
Interrupts
If implementation-defined forms of interrupt handler procedures are
supported, such as protected procedures with parameters, then for each
such form of a handler, a type analogous to Parameterless_Handler
should be specified in a child package of Interrupts, with the
same operations as in the predefined package Interrupts.
Followed.
Discard_Names
If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity. Followed.
The implementation should use names that end with `_Locking' for
locking policies defined by the implementation.
Followed. A single implementation-defined locking policy is defined,
whose name (Inheritance_Locking) follows this suggestion.
abort_statement is included in the list of
potentially blocking operations (see 9.5.1), it is recommended that this
statement be implemented in a way that never requires the task executing
the abort_statement to block.
Followed.
On a multi-processor, the delay associated with aborting a task on another processor should be bounded; the implementation should use periodic polling, if necessary, to achieve this. Followed.
Ravenscar and pragma
Restricted_Run_Time for more details.
Tick.
Such configuration mechanisms are not appropriate to this implementation
and are thus not supported.
It is recommended that Calendar.Clock and Real_Time.Clock
be implemented as transformations of the same time base.
Followed.
It is recommended that the best time base which exists in
the underlying system be available to the application through
Clock. Best may mean highest accuracy or largest range.
Followed.
The Write operation on a stream of type Params_Stream_Type
should raise Storage_Error if it runs out of space trying to
write the Item into the stream.
Followed by GLADE, a separately supplied PCS that can be used with
GNAT.
Interfaces.COBOL (respectively,
Interfaces.C) specified in Annex B and should support a
convention_identifier of COBOL (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.
Followed.
Interfaces.Fortran (respectively,
Interfaces.C) specified in Annex B and should support a
convention_identifier of Fortran (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.
Followed.
Similarly, because the usual mathematical meaning of addition of a
complex operand and a real operand is that the imaginary operand remains
unchanged, an implementation should not perform this operation by first
promoting the real operand to complex type and then performing a full
complex addition. In implementations in which the Signed_Zeros
attribute of the component type is True (and which therefore
conform to IEC 559:1989 in regard to the handling of the sign of zero in
predefined arithmetic operations), the latter technique will not
generate the required result when the imaginary component of the complex
operand is a negatively signed zero. (Explicit addition of the negative
zero to the zero obtained during promotion yields a positive zero.)
Analogous advice applies in the case of addition of a complex operand
and a pure-imaginary operand, and in the case of subtraction of a
complex operand and a real or pure-imaginary operand.
Not followed.
Implementations in which Real'Signed_Zeros is True should
attempt to provide a rational treatment of the signs of zero results and
result components. As one example, the result of the Argument
function should have the sign of the imaginary component of the
parameter X when the point represented by that parameter lies on
the positive real axis; as another, the sign of the imaginary component
of the Compose_From_Polar function should be the same as
(respectively, the opposite of) that of the Argument parameter when that
parameter has a value of zero and the Modulus parameter has a
nonnegative (respectively, negative) value.
Followed.
Complex_Types.Real'Signed_Zeros is
True should attempt to provide a rational treatment of the signs
of zero results and result components. For example, many of the complex
elementary functions have components that are odd functions of one of
the parameter components; in these cases, the result component should
have the sign of the parameter component at the origin. Other complex
elementary functions have zero components whose sign is opposite that of
a parameter component at the origin, or is always positive or always
negative.
Followed.
Cycle parameter should not be implemented by calling the
corresponding version with a Cycle parameter of
2.0*Numerics.Pi, since this will not provide the required
accuracy in some portions of the domain. For the same reason, the
version of Log without a Base parameter should not be
implemented by calling the corresponding version with a Base
parameter of Numerics.e.
Followed.
The version of the Compose_From_Polar function without a
Cycle parameter should not be implemented by calling the
corresponding version with a Cycle parameter of
2.0*Numerics.Pi, since this will not provide the required
accuracy in some portions of the domain.
Followed.
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A requirement for conforming Ada compilers is that they provide documentation describing how the implementation deals with each of these issues. In this chapter, you will find each point in annex M listed followed by a description in italic font of how GNAT handles the implementation dependence.
You can use this chapter as a guide to minimizing implementation dependent features in your programs if portability to other compilers and other operating systems is an important consideration. The numbers in each section below correspond to the paragraph number in the Ada 95 Reference Manual.
2. Whether or not each recommendation given in Implementation Advice is followed. See 1.1.2(37). See section 3. Implementation Advice.
3. Capacity limitations of the implementation. See 1.1.3(3). The complexity of programs that can be processed is limited only by the total amount of available virtual memory, and disk space for the generated object files.
4. Variations from the standard that are impractical to avoid given the implementation's execution environment. See 1.1.3(6). There are no variations from the standard.
5. Which code_statements cause external
interactions. See 1.1.3(10).
Any code_statement can potentially cause external interactions.
6. The coded representation for the text of an Ada program. See 2.1(4). See separate section on source representation.
7. The control functions allowed in comments. See 2.1(14). See separate section on source representation.
8. The representation for an end of line. See 2.2(2). See separate section on source representation.
9. Maximum supported line length and lexical element length. See 2.2(15). The maximum line length is 255 characters an the maximum length of a lexical element is also 255 characters.
10. Implementation defined pragmas. See 2.8(14).
See section 1. Implementation Defined Pragmas.
11. Effect of pragma Optimize. See 2.8(27).
Pragma Optimize, if given with a Time or Space
parameter, checks that the optimization flag is set, and aborts if it is
not.
12. The sequence of characters of the value returned by
S'Image when some of the graphic characters of
S'Wide_Image are not defined in Character. See
3.5(37).
The sequence of characters is as defined by the wide character encoding
method used for the source. See section on source representation for
further details.
13. The predefined integer types declared in
Standard. See 3.5.4(25).
Short_Short_Integer
Short_Integer
Integer
Long_Integer
Long_Long_Integer
14. Any nonstandard integer types and the operators defined for them. See 3.5.4(26). There are no nonstandard integer types.
15. Any nonstandard real types and the operators defined for them. See 3.5.6(8). There are no nonstandard real types.
16. What combinations of requested decimal precision and range are supported for floating point types. See 3.5.7(7). The precision and range is as defined by the IEEE standard.
17. The predefined floating point types declared in
Standard. See 3.5.7(16).
Short_Float
Float
Long_Float
Long_Long_Float
18. The small of an ordinary fixed point type. See 3.5.9(8).
Fine_Delta is 2**(-63)
19. What combinations of small, range, and digits are
supported for fixed point types. See 3.5.9(10).
Any combinations are permitted that do not result in a small less than
Fine_Delta and do not result in a mantissa larger than 63 bits.
If the mantissa is larger than 53 bits on machines where Long_Long_Float
is 64 bits (true of all architectures except ia32), then the output from
Text_IO is accurate to only 53 bits, rather than the full mantissa. This
is because floating-point conversions are used to convert fixed point.
20. The result of Tags.Expanded_Name for types declared
within an unnamed block_statement. See 3.9(10).
Block numbers of the form Bnnn, where nnn is a
decimal integer are allocated.
21. Implementation-defined attributes. See 4.1.4(12). See section 2. Implementation Defined Attributes.
22. Any implementation-defined time types. See 9.6(6). There are no implementation-defined time types.
23. The time base associated with relative delays.
See 9.6(20). The time base used is that provided by the C library
function gettimeofday.
24. The time base of the type Calendar.Time. See
9.6(23).
The time base used is that provided by the C library function
gettimeofday.
25. The time zone used for package Calendar
operations. See 9.6(24).
The time zone used by package Calendar is the current system time zone
setting for local time, as accessed by the C library function
localtime.
26. Any limit on delay_until_statements of
select_statements. See 9.6(29).
There are no such limits.
27. Whether or not two non overlapping parts of a composite
object are independently addressable, in the case where packing, record
layout, or Component_Size is specified for the object. See
9.10(1).
Separate components are independently addressable if they do not share
overlapping storage units.
28. The representation for a compilation. See 10.1(2).
A compilation is represented by a sequence of files presented to the
compiler in a single invocation of the gcc command.
29. Any restrictions on compilations that contain multiple compilation_units. See 10.1(4). No single file can contain more than one compilation unit, but any sequence of files can be presented to the compiler as a single compilation.
30. The mechanisms for creating an environment and for adding and replacing compilation units. See 10.1.4(3). See separate section on compilation model.
31. The manner of explicitly assigning library units to a partition. See 10.2(2). If a unit contains an Ada main program, then the Ada units for the partition are determined by recursive application of the rules in the Ada Reference Manual section 10.2(2-6). In other words, the Ada units will be those that are needed by the main program, and then this definition of need is applied recursively to those units, and the partition contains the transitive closure determined by this relationship. In short, all the necessary units are included, with no need to explicitly specify the list. If additional units are required, e.g. by foreign language units, then all units must be mentioned in the context clause of one of the needed Ada units.
If the partition contains no main program, or if the main program is in
a language other than Ada, then GNAT
provides the binder options -z and -n respectively, and in this case a
list of units can be explicitly supplied to the binder for inclusion in
the partition (all units needed by these units will also be included
automatically). For full details on the use of these options, refer to
the GNAT User's Guide sections on Binding and Linking.
32. The implementation-defined means, if any, of specifying which compilation units are needed by a given compilation unit. See 10.2(2). The units needed by a given compilation unit are as defined in the Ada Reference Manual section 10.2(2-6). There are no implementation-defined pragmas or other implementation-defined means for specifying needed units.
33. The manner of designating the main subprogram of a partition. See 10.2(7). The main program is designated by providing the name of the corresponding `ALI' file as the input parameter to the binder.
34. The order of elaboration of library_items. See
10.2(18).
The first constraint on ordering is that it meets the requirements of
chapter 10 of the Ada 95 Reference Manual. This still leaves some
implementation dependent choices, which are resolved by first
elaborating bodies as early as possible (i.e. in preference to specs
where there is a choice), and second by evaluating the immediate with
clauses of a unit to determine the probably best choice, and
third by elaborating in alphabetical order of unit names
where a choice still remains.
35. Parameter passing and function return for the main subprogram. See 10.2(21). The main program has no parameters. It may be a procedure, or a function returning an integer type. In the latter case, the returned integer value is the return code of the program.
36. The mechanisms for building and running partitions. See 10.2(24). GNAT itself supports programs with only a single partition. The GNATDIST tool provided with the GLADE package (which also includes an implementation of the PCS) provides a completely flexible method for building and running programs consisting of multiple partitions. See the separate GLADE manual for details.
37. The details of program execution, including program termination. See 10.2(25). See separate section on compilation model.
38. The semantics of any non-active partitions supported by the implementation. See 10.2(28). Passive partitions are supported on targets where shared memory is provided by the operating system. See the GLADE reference manual for further details.
39. The information returned by Exception_Message. See
11.4.1(10).
Exception message returns the null string unless a specific message has
been passed by the program.
40. The result of Exceptions.Exception_Name for types
declared within an unnamed block_statement. See 11.4.1(12).
Blocks have implementation defined names of the form Bnnn
where nnn is an integer.
41. The information returned by
Exception_Information. See 11.4.1(13).
Exception_Information returns a string in the following format:
Exception_Name: nnnnn Message: mmmmm PID: ppp Call stack traceback locations: 0xhhhh 0xhhhh 0xhhhh ... 0xhhh |
where
nnnn is the fully qualified name of the exception in all upper
case letters. This line is always present.
mmmm is the message (this line present only if message is non-null)
ppp is the Process Id value as a decimal integer (this line is
present only if the Process Id is non-zero). Currently we are
not making use of this field.
The line terminator sequence at the end of each line, including
the last line is a single LF character (16#0A#).
42. Implementation-defined check names. See 11.5(27). No implementation-defined check names are supported.
43. The interpretation of each aspect of representation. See 13.1(20). See separate section on data representations.
44. Any restrictions placed upon representation items. See 13.1(20). See separate section on data representations.
45. The meaning of Size for indefinite subtypes. See
13.3(48).
Size for an indefinite subtype is the maximum possible size, except that
for the case of a subprogram parameter, the size of the parameter object
is the actual size.
46. The default external representation for a type tag. See 13.3(75). The default external representation for a type tag is the fully expanded name of the type in upper case letters.
47. What determines whether a compilation unit is the same in two different partitions. See 13.3(76). A compilation unit is the same in two different partitions if and only if it derives from the same source file.
48. Implementation-defined components. See 13.5.1(15). The only implementation defined component is the tag for a tagged type, which contains a pointer to the dispatching table.
49. If Word_Size = Storage_Unit, the default bit
ordering. See 13.5.3(5).
Word_Size (32) is not the same as Storage_Unit (8) for this
implementation, so no non-default bit ordering is supported. The default
bit ordering corresponds to the natural endianness of the target architecture.
50. The contents of the visible part of package System
and its language-defined children. See 13.7(2).
See the definition of these packages in files `system.ads' and
`s-stoele.ads'.
51. The contents of the visible part of package
System.Machine_Code, and the meaning of
code_statements. See 13.8(7).
See the definition and documentation in file `s-maccod.ads'.
52. The effect of unchecked conversion. See 13.9(11). Unchecked conversion between types of the same size and results in an uninterpreted transmission of the bits from one type to the other. If the types are of unequal sizes, then in the case of discrete types, a shorter source is first zero or sign extended as necessary, and a shorter target is simply truncated on the left. For all non-discrete types, the source is first copied if necessary to ensure that the alignment requirements of the target are met, then a pointer is constructed to the source value, and the result is obtained by dereferencing this pointer after converting it to be a pointer to the target type.
53. The manner of choosing a storage pool for an access type
when Storage_Pool is not specified for the type. See 13.11(17).
There are 3 different standard pools used by the compiler when
Storage_Pool is not specified depending whether the type is local
to a subprogram or defined at the library level and whether
Storage_Sizeis specified or not. See documentation in the runtime
library units System.Pool_Global, System.Pool_Size and
System.Pool_Local in files `s-poosiz.ads',
`s-pooglo.ads' and `s-pooloc.ads' for full details on the
default pools used.
54. Whether or not the implementation provides user-accessible names for the standard pool type(s). See 13.11(17).
See documentation in the sources of the run time mentioned in paragraph
53 . All these pools are accessible by means of with'ing
these units.
55. The meaning of Storage_Size. See 13.11(18).
Storage_Size is measured in storage units, and refers to the
total space available for an access type collection, or to the primary
stack space for a task.
56. Implementation-defined aspects of storage pools. See 13.11(22). See documentation in the sources of the run time mentioned in paragraph 53 for details on GNAT-defined aspects of storage pools.
57. The set of restrictions allowed in a pragma
Restrictions. See 13.12(7).
All RM defined Restriction identifiers are implemented. The following
additional restriction identifiers are provided. There are two separate
lists of implementation dependent restriction identifiers. The first
set requires consistency throughout a partition (in other words, if the
restriction identifier is used for any compilation unit in the partition,
then all compilation units in the partition must obey the restriction.
Boolean_Entry_Barriers
Ravenscar).
Max_Entry_Queue_Depth => Expr
No_Calendar
Ada.Calendar.
No_Dynamic_Interrupts
No_Enumeration_Maps
No_Entry_Calls_In_Elaboration_Code
No_Exception_Handlers
No_Implicit_Conditionals
No_Implicit_Loops
for loops, either by modifying
the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
for loop. The details and use of this restriction are described in
more detail in the High Integrity product documentation.
No_Local_Protected_Objects
No_Protected_Type_Allocators
No_Secondary_Stack
No_Select_Statements
select may not appear.
This is one of the restrictions of the Ravenscar
profile for limited tasking (see also pragma Ravenscar).
No_Standard_Storage_Pools
No_Streams
Ada.Streams.
No_Task_Attributes
Ada.Task_Attributes.
No_Task_Termination
No_Tasking
Max_Tasks => 0
except that violations are caught at compile time and cause in illegality
to be signalled either by the compiler or binder.
No_Wide_Characters
Wide_Character or Wide_String
appear, and that no wide character literals
appear in the program (that is literals representing characters not in
type Character.
Static_Priorities
Ada.Dynamic_Priorities.
Static_Storage_Size
The second set of implementation dependent restriction identifiers does not require partition-wide consistency. The restriction may be enforced for a single compilation unit without any effect on any of the other compilation units in the partition.
No_Elaboration_Code
Preelaborate. There are cases in which pragma Preelaborate
still permits code to be generated (e.g. code to initialize a large
array to all zeroes), and there are cases of units which do not meet
the requirements for pragma Preelaborate, but for which no elaboration
code is generated. Generally, it is the case that preelaborable units
will meet the restrictions, with the exception of large aggregates
initialized with an others_clause, and exception declarations (which
generate calls to a run-time registry procedure). Note that this restriction
is enforced on a unit by unit basis, it need not be obeyed consistently
throughout a partition.
No_Entry_Queue
No_Implementation_Attributes
No_Implementation_Pragmas
No_Implementation_Restrictions
No_Implementation_Restrictions itself)
are present. With this restriction, the only other restriction identifiers
that can be used are those defined in the Ada 95 Reference Manual.
58. The consequences of violating limitations on
Restrictions pragmas. See 13.12(9).
Restrictions that can be checked at compile time result in illegalities
if violated. Currently there are no other consequences of violating
restrictions.
59. The representation used by the Read and
Write attributes of elementary types in terms of stream
elements. See 13.13.2(9).
The representation is the in-memory representation of the base type of
the type, using the number of bits corresponding to the
type'Size value, and the natural ordering of the machine.
60. The names and characteristics of the numeric subtypes
declared in the visible part of package Standard. See A.1(3).
See items describing the integer and floating-point types supported.
61. The accuracy actually achieved by the elementary functions. See A.5.1(1). The elementary functions correspond to the functions available in the C library. Only fast math mode is implemented.
62. The sign of a zero result from some of the operators or
functions in Numerics.Generic_Elementary_Functions, when
Float_Type'Signed_Zeros is True. See A.5.1(46).
The sign of zeroes follows the requirements of the IEEE 754 standard on
floating-point.
63. The value of
Numerics.Float_Random.Max_Image_Width. See A.5.2(27).
Maximum image width is 649, see library file `a-numran.ads'.
64. The value of
Numerics.Discrete_Random.Max_Image_Width. See A.5.2(27).
Maximum image width is 80, see library file `a-nudira.ads'.
65. The algorithms for random number generation. See A.5.2(32). The algorithm is documented in the source files `a-numran.ads' and `a-numran.adb'.
66. The string representation of a random number generator's state. See A.5.2(38). See the documentation contained in the file `a-numran.adb'.
67. The minimum time interval between calls to the time-dependent Reset procedure that are guaranteed to initiate different random number sequences. See A.5.2(45). The minimum period between reset calls to guarantee distinct series of random numbers is one microsecond.
68. The values of the Model_Mantissa,
Model_Emin, Model_Epsilon, Model,
Safe_First, and Safe_Last attributes, if the Numerics
Annex is not supported. See A.5.3(72).
See the source file `ttypef.ads' for the values of all numeric
attributes.
69. Any implementation-defined characteristics of the input-output packages. See A.7(14). There are no special implementation defined characteristics for these packages.
70. The value of Buffer_Size in Storage_IO. See
A.9(10).
All type representations are contiguous, and the Buffer_Size is
the value of type'Size rounded up to the next storage unit
boundary.
71. External files for standard input, standard output, and standard error See A.10(5). These files are mapped onto the files provided by the C streams libraries. See source file `i-cstrea.ads' for further details.
72. The accuracy of the value produced by Put. See
A.10.9(36).
If more digits are requested in the output than are represented by the
precision of the value, zeroes are output in the corresponding least
significant digit positions.
73. The meaning of Argument_Count, Argument, and
Command_Name. See A.15(1).
These are mapped onto the argv and argc parameters of the
main program in the natural manner.
74. Implementation-defined convention names. See B.1(11). The following convention names are supported
Ada
Asm
Assembler
C
C_Pass_By_Copy
COBOL
CPP
Default
DLL
Win32
External
Fortran
Intrinsic
Import with convention Intrinsic, see
separate section on Intrinsic Subprograms.
Stdcall
Stubbed
Program_Error exception. If a
pragma Import specifies convention stubbed then no body need
be present at all. This convention is useful during development for the
inclusion of subprograms whose body has not yet been written.
75. The meaning of link names. See B.1(36). Link names are the actual names used by the linker.
76. The manner of choosing link names when neither the link name nor the address of an imported or exported entity is specified. See B.1(36). The default linker name is that which would be assigned by the relevant external language, interpreting the Ada name as being in all lower case letters.
77. The effect of pragma Linker_Options. See B.1(37).
The string passed to Linker_Options is presented uninterpreted as
an argument to the link command, unless it contains Ascii.NUL characters.
NUL characters if they appear act as argument separators, so for example
pragma Linker_Options ("-labc" & ASCII.Nul & "-ldef");
|
causes two separate arguments -labc and -ldef to be passed to the
linker. The order of linker options is preserved for a given unit. The final
list of options passed to the linker is in reverse order of the elaboration
order. For example, linker options fo a body always appear before the options
from the corresponding package spec.
78. The contents of the visible part of package
Interfaces and its language-defined descendants. See B.2(1).
See files with prefix `i-' in the distributed library.
79. Implementation-defined children of package
Interfaces. The contents of the visible part of package
Interfaces. See B.2(11).
See files with prefix `i-' in the distributed library.
80. The types Floating, Long_Floating,
Binary, Long_Binary, Decimal_ Element, and
COBOL_Character; and the initialization of the variables
Ada_To_COBOL and COBOL_To_Ada, in
Interfaces.COBOL. See B.4(50).
Floating
Long_Floating
Binary
Long_Binary
Decimal_Element
COBOL_Character
For initialization, see the file `i-cobol.ads' in the distributed library.
81. Support for access to machine instructions. See C.1(1). See documentation in file `s-maccod.ads' in the distributed library.
82. Implementation-defined aspects of access to machine operations. See C.1(9). See documentation in file `s-maccod.ads' in the distributed library.
83. Implementation-defined aspects of interrupts. See C.3(2).
Interrupts are mapped to signals or conditions as appropriate. See
definition of unit
Ada.Interrupt_Names in source file `a-intnam.ads' for details
on the interrupts supported on a particular target.
84. Implementation-defined aspects of pre-elaboration. See C.4(13). GNAT does not permit a partition to be restarted without reloading, except under control of the debugger.
85. The semantics of pragma Discard_Names. See C.5(7).
Pragma Discard_Names causes names of enumeration literals to
be suppressed. In the presence of this pragma, the Image attribute
provides the image of the Pos of the literal, and Value accepts
Pos values.
86. The result of the Task_Identification.Image
attribute. See C.7.1(7).
The result of this attribute is an 8-digit hexadecimal string
representing the virtual address of the task control block.
87. The value of Current_Task when in a protected entry
or interrupt handler. See C.7.1(17).
Protected entries or interrupt handlers can be executed by any
convenient thread, so the value of Current_Task is undefined.
88. The effect of calling Current_Task from an entry
body or interrupt handler. See C.7.1(19).
The effect of calling Current_Task from an entry body or
interrupt handler is to return the identification of the task currently
executing the code.
89. Implementation-defined aspects of
Task_Attributes. See C.7.2(19).
There are no implementation-defined aspects of Task_Attributes.
90. Values of all Metrics. See D(2).
The metrics information for GNAT depends on the performance of the
underlying operating system. The sources of the run-time for tasking
implementation, together with the output from -gnatG can be
used to determine the exact sequence of operating systems calls made
to implement various tasking constructs. Together with appropriate
information on the performance of the underlying operating system,
on the exact target in use, this information can be used to determine
the required metrics.
91. The declarations of Any_Priority and
Priority. See D.1(11).
See declarations in file `system.ads'.
92. Implementation-defined execution resources. See D.1(15). There are no implementation-defined execution resources.
93. Whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy. See D.2.1(3). On a multi-processor, a task that is waiting for access to a protected object does not keep its processor busy.
94. The affect of implementation defined execution resources on task dispatching. See D.2.1(9). Tasks map to threads in the threads package used by GNAT. Where possible and appropriate, these threads correspond to native threads of the underlying operating system.
95. Implementation-defined policy_identifiers allowed
in a pragma Task_Dispatching_Policy. See D.2.2(3).
There are no implementation-defined policy-identifiers allowed in this
pragma.
96. Implementation-defined aspects of priority inversion. See D.2.2(16). Execution of a task cannot be preempted by the implementation processing of delay expirations for lower priority tasks.
97. Implementation defined task dispatching. See D.2.2(18). The policy is the same as that of the underlying threads implementation.
98. Implementation-defined policy_identifiers allowed
in a pragma Locking_Policy. See D.3(4).
The only implementation defined policy permitted in GNAT is
Inheritance_Locking. On targets that support this policy, locking
is implemented by inheritance, i.e. the task owning the lock operates
at a priority equal to the highest priority of any task currently
requesting the lock.
99. Default ceiling priorities. See D.3(10).
The ceiling priority of protected objects of the type
System.Interrupt_Priority'Last as described in the Ada 95
Reference Manual D.3(10),
100. The ceiling of any protected object used internally by
the implementation. See D.3(16).
The ceiling priority of internal protected objects is
System.Priority'Last.
101. Implementation-defined queuing policies. See D.4(1). There are no implementation-defined queueing policies.
102. On a multiprocessor, any conditions that cause the completion of an aborted construct to be delayed later than what is specified for a single processor. See D.6(3). The semantics for abort on a multi-processor is the same as on a single processor, there are no further delays.
103. Any operations that implicitly require heap storage allocation. See D.7(8). The only operation that implicitly requires heap storage allocation is task creation.
104. Implementation-defined aspects of pragma
Restrictions. See D.7(20).
There are no such implementation-defined aspects.
105. Implementation-defined aspects of package
Real_Time. See D.8(17).
There are no implementation defined aspects of package Real_Time.
106. Implementation-defined aspects of
delay_statements. See D.9(8).
Any difference greater than one microsecond will cause the task to be
delayed (see D.9(7)).
107. The upper bound on the duration of interrupt blocking caused by the implementation. See D.12(5). The upper bound is determined by the underlying operating system. In no cases is it more than 10 milliseconds.
108. The means for creating and executing distributed programs. See E(5). The GLADE package provides a utility GNATDIST for creating and executing distributed programs. See the GLADE reference manual for further details.
109. Any events that can result in a partition becoming inaccessible. See E.1(7). See the GLADE reference manual for full details on such events.
110. The scheduling policies, treatment of priorities, and management of shared resources between partitions in certain cases. See E.1(11). See the GLADE reference manual for full details on these aspects of multi-partition execution.
111. Events that cause the version of a compilation unit to change. See E.3(5). Editing the source file of a compilation unit, or the source files of any units on which it is dependent in a significant way cause the version to change. No other actions cause the version number to change. All changes are significant except those which affect only layout, capitalization or comments.
112. Whether the execution of the remote subprogram is immediately aborted as a result of cancellation. See E.4(13). See the GLADE reference manual for details on the effect of abort in a distributed application.
113. Implementation-defined aspects of the PCS. See E.5(25). See the GLADE reference manual for a full description of all implementation defined aspects of the PCS.
114. Implementation-defined interfaces in the PCS. See E.5(26). See the GLADE reference manual for a full description of all implementation defined interfaces.
115. The values of named numbers in the package
Decimal. See F.2(7).
Max_Scale
Min_Scale
Min_Delta
Max_Delta
Max_Decimal_Digits
116. The value of Max_Picture_Length in the package
Text_IO.Editing. See F.3.3(16).
64
117. The value of Max_Picture_Length in the package
Wide_Text_IO.Editing. See F.3.4(5).
64
118. The accuracy actually achieved by the complex elementary functions and by other complex arithmetic operations. See G.1(1). Standard library functions are used for the complex arithmetic operations. Only fast math mode is currently supported.
119. The sign of a zero result (or a component thereof) from
any operator or function in Numerics.Generic_Complex_Types, when
Real'Signed_Zeros is True. See G.1.1(53).
The signs of zero values are as recommended by the relevant
implementation advice.
120. The sign of a zero result (or a component thereof) from
any operator or function in
Numerics.Generic_Complex_Elementary_Functions, when
Real'Signed_Zeros is True. See G.1.2(45).
The signs of zero values are as recommended by the relevant
implementation advice.
121. Whether the strict mode or the relaxed mode is the default. See G.2(2). The strict mode is the default. There is no separate relaxed mode. GNAT provides a highly efficient implementation of strict mode.
122. The result interval in certain cases of fixed-to-float conversion. See G.2.1(10). For cases where the result interval is implementation dependent, the accuracy is that provided by performing all operations in 64-bit IEEE floating-point format.
123. The result of a floating point arithmetic operation in
overflow situations, when the Machine_Overflows attribute of the
result type is False. See G.2.1(13).
Infinite and Nan values are produced as dictated by the IEEE
floating-point standard.
124. The result interval for division (or exponentiation by a negative exponent), when the floating point hardware implements division as multiplication by a reciprocal. See G.2.1(16). Not relevant, division is IEEE exact.
125. The definition of close result set, which determines the accuracy of certain fixed point multiplications and divisions. See G.2.3(5). Operations in the close result set are performed using IEEE long format floating-point arithmetic. The input operands are converted to floating-point, the operation is done in floating-point, and the result is converted to the target type.
126. Conditions on a universal_real operand of a fixed
point multiplication or division for which the result shall be in the
perfect result set. See G.2.3(22).
The result is only defined to be in the perfect result set if the result
can be computed by a single scaling operation involving a scale factor
representable in 64-bits.
127. The result of a fixed point arithmetic operation in
overflow situations, when the Machine_Overflows attribute of the
result type is False. See G.2.3(27).
Not relevant, Machine_Overflows is True for fixed-point
types.
128. The result of an elementary function reference in
overflow situations, when the Machine_Overflows attribute of the
result type is False. See G.2.4(4).
IEEE infinite and Nan values are produced as appropriate.
129. The value of the angle threshold, within which certain elementary functions, complex arithmetic operations, and complex elementary functions yield results conforming to a maximum relative error bound. See G.2.4(10). Information on this subject is not yet available.
130. The accuracy of certain elementary functions for parameters beyond the angle threshold. See G.2.4(10). Information on this subject is not yet available.
131. The result of a complex arithmetic operation or complex
elementary function reference in overflow situations, when the
Machine_Overflows attribute of the corresponding real type is
False. See G.2.6(5).
IEEE infinite and Nan values are produced as appropriate.
132. The accuracy of certain complex arithmetic operations and certain complex elementary functions for parameters (or components thereof) beyond the angle threshold. See G.2.6(8). Information on those subjects is not yet available.
133. Information regarding bounded errors and erroneous execution. See H.2(1). Information on this subject is not yet available.
134. Implementation-defined aspects of pragma
Inspection_Point. See H.3.2(8).
Pragma Inspection_Point ensures that the variable is live and can
be examined by the debugger at the inspection point.
135. Implementation-defined aspects of pragma
Restrictions. See H.4(25).
There are no implementation-defined aspects of pragma Restrictions. The
use of pragma Restrictions [No_Exceptions] has no effect on the
generated code. Checks must suppressed by use of pragma Suppress.
136. Any restrictions on pragma Restrictions. See
H.4(27).
There are no restrictions on pragma Restrictions.
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GNAT allows a user application program to write the declaration:
pragma Import (Intrinsic, name); |
providing that the name corresponds to one of the implemented intrinsic subprograms in GNAT, and that the parameter profile of the referenced subprogram meets the requirements. This chapter describes the set of implemented intrinsic subprograms, and the requirements on parameter profiles. Note that no body is supplied; as with other uses of pragma Import, the body is supplied elsewhere (in this case by the compiler itself). Note that any use of this feature is potentially non-portable, since the Ada standard does not require Ada compilers to implement this feature.
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All the predefined numeric operators in package Standard
in pragma Import (Intrinsic,..)
declarations. In the binary operator case, the operands must have the same
size. The operand or operands must also be appropriate for
the operator. For example, for addition, the operands must
both be floating-point or both be fixed-point, and the
right operand for "**" must have a root type of
Standard.Integer'Base.
You can use an intrinsic operator declaration as in the following example:
type Int1 is new Integer; type Int2 is new Integer; function "+" (X1 : Int1; X2 : Int2) return Int1; function "+" (X1 : Int1; X2 : Int2) return Int2; pragma Import (Intrinsic, "+"); |
This declaration would permit "mixed mode" arithmetic on items
of the differing types Int1 and Int2.
It is also possible to specify such operators for private types, if the
full views are appropriate arithmetic types.
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GNAT.Source_Info. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
GNAT.Source_Info.Enclosing_Entity to obtain the name of
the current subprogram, package, task, entry, or protected subprogram.
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GNAT.Current_Exception. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
GNAT.Current_Exception.Exception_Information to obtain
the exception information associated with the current exception.
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GNAT.Current_Exception. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
GNAT.Current_Exception.Exception_Message to obtain
the message associated with the current exception.
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GNAT.Current_Exception. The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
GNAT.Current_Exception.Exception_Name to obtain
the name of the current exception.
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GNAT.Source_Info. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
GNAT.Source_Info.File to obtain the name of the current
file.
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GNAT.Source_Info. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
GNAT.Source_Info.Line to obtain the number of the current
source line.
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Rotate_Left function is available only
for the predefined modular types in package Interfaces. However, in
GNAT it is possible to define a Rotate_Left function for a user
defined modular type or any signed integer type as in this example:
function Shift_Left
(Value : My_Modular_Type;
Amount : Natural)
return My_Modular_Type;
|
The requirements are that the profile be exactly as in the example
above. The only modifications allowed are in the formal parameter
names, and in the type of Value and the return type, which
must be the same, and must be either a signed integer type, or
a modular integer type with a binary modulus, and the size must
be 8. 16, 32 or 64 bits.
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Rotate_Right function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for Rotate_Left.
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Shift_Left function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for Rotate_Left.
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Shift_Right function can be defined for any user defined
binary modular integer type, or signed integer type, as described
above for Rotate_Left.
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Shift_Right_Arithmetic function can be defined for any user
defined binary modular integer type, or signed integer type, as described
above for Rotate_Left.
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GNAT.Source_Info. The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
GNAT.Source_Info.Source_Location to obtain the current
source file location.
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This section describes the representation clauses accepted by GNAT, and their effect on the representation of corresponding data objects.
GNAT fully implements Annex C (Systems Programming). This means that all the implementation advice sections in chapter 13 are fully implemented. However, these sections only require a minimal level of support for representation clauses. GNAT provides much more extensive capabilities, and this section describes the additional capabilities provided.
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GNAT requires that all alignment clauses specify a power of 2, and all default alignments are always a power of 2. The default alignment values are as follows:
Pack is
used and all fields are packable (see separate section on pragma Pack),
then the resulting alignment is 1.
A special case is when the size of the record is given explicitly, or a full record representation clause is given, and the size of the record is 2, 4, or 8 bytes. In this case, an alignment is chosen to match the size of the record. For example, if we have:
type Small is record
A, B : Character;
end record;
|
then the default alignment of the record type Small is 2, not 1. This
leads to more efficient code when the record is treated as a unit, and also
allows the type to specified as Atomic on architectures requiring
strict alignment.
An alignment clause may always specify a larger alignment than the default value, up to some maximum value dependent on the target (obtainable by using the attribute reference System'Maximum_Alignment). The only case in which it is permissible to specify a smaller alignment than the default value is in the case of a record for which a record representation clause is given. In this case, packable fields for which a component clause is given still result in a default alignment corresponding to the original type, but this may be overridden, since these components in fact only require an alignment of one byte. For example, given
type v is record
a : integer;
end record;
for v use record
a at 0 range 0 .. 31;
end record;
for v'alignment use 1;
|
The default alignment for the type v is 4, as a result of the
integer field in the record, but since this field is placed with a
component clause, it is permissible, as shown, to override the default
alignment of the record to a smaller value.
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The default size of types is as specified in the reference manual. For objects, GNAT will generally increase the type size so that the object size is a multiple of storage units, and also a multiple of the alignment. For example
type Smallint is range 1 .. 6;
type Rec is record
y1 : integer;
y2 : boolean;
end record;
|
In this example, Smallint
has a size of 3, as specified by the RM rules,
but objects of this type will have a size of 8,
since objects by default occupy an integral number
of storage units. On some targets, notably older
versions of the Digital Alpha, the size of stand
alone objects of this type may be 32, reflecting
the inability of the hardware to do byte load/stores.
Similarly, the size of type Rec is 40 bits, but
the alignment is 4, so objects of this type will have
their size increased to 64 bits so that it is a multiple
of the alignment. The reason for this decision, which is
in accordance with the specific note in RM 13.3(43):
A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero). |
An explicit size clause may be used to override the default size by increasing it. For example, if we have:
type My_Boolean is new Boolean; for My_Boolean'Size use 32; |
then objects of this type will always be 32 bits long. In the case of discrete types, the size can be increased up to 64 bits, with the effect that the entire specified field is used to hold the value, sign- or zero-extended as appropriate. If more than 64 bits is specified, then padding space is allocated after the value, and a warning is issued that there are unused bits.
Similarly the size of records and arrays may be increased, and the effect is to add padding bits after the value. This also causes a warning message to be generated.
The largest Size value permitted in GNAT is 2**32-1. Since this is a Size in bits, this corresponds to an object of size 256 megabytes (minus one). This limitation is true on all targets. The reason for this limitation is that it improves the quality of the code in many cases if it is known that a Size value can be accommodated in an object of type Integer.
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For tasks, the Storage_Size clause specifies the amount of space
to be allocated for the task stack. This cannot be extended, and if the
stack is exhausted, then Storage_Error will be raised if stack
checking is enabled. If the default size of 20K bytes is insufficient,
then you need to use a Storage_Size attribute definition clause,
or a Storage_Size pragma in the task definition to set the
appropriate required size. A useful technique is to include in every
task definition a pragma of the form:
pragma Storage_Size (Default_Stack_Size); |
Then Default_Stack_Size can be defined in a global package, and modified as required. Any tasks requiring different task stack sizes from the default can have an appropriate alternative reference in the pragma.
For access types, the Storage_Size clause specifies the maximum
space available for allocation of objects of the type. If this space is
exceeded then Storage_Error will be raised by an allocation attempt.
In the case where the access type is declared local to a subprogram, the
use of a Storage_Size clause triggers automatic use of a special
predefined storage pool (System.Pool_Size) that ensures that all
space for the pool is automatically reclaimed on exit from the scope in
which the type is declared.
A special case recognized by the compiler is the specification of a
Storage_Size of zero for an access type. This means that no
items can be allocated from the pool, and this is recognized at compile
time, and all the overhead normally associated with maintaining a fixed
size storage pool is eliminated. Consider the following example:
procedure p is
type R is array (Natural) of Character;
type P is access all R;
for P'Storage_Size use 0;
-- Above access type intended only for interfacing purposes
y : P;
procedure g (m : P);
pragma Import (C, g);
-- ...
begin
-- ...
y := new R;
end;
|
As indicated in this example, these dummy storage pools are often useful in connection with interfacing where no object will ever be allocated. If you compile the above example, you get the warning:
p.adb:16:09: warning: allocation from empty storage pool p.adb:16:09: warning: Storage_Error will be raised at run time |
Of course in practice, there will not be any explicit allocators in the case of such an access declaration.
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An issue arises in the case of variant record objects of whether Size gives information about a particular variant, or the maximum size required for any variant. Consider the following program
with Text_IO; use Text_IO;
procedure q is
type R1 (A : Boolean := False) is record
case A is
when True => X : Character;
when False => null;
end case;
end record;
V1 : R1 (False);
V2 : R1;
begin
Put_Line (Integer'Image (V1'Size));
Put_Line (Integer'Image (V2'Size));
end q;
|
Here we are dealing with a variant record, where the True variant requires 16 bits, and the False variant requires 8 bits. In the above example, both V1 and V2 contain the False variant, which is only 8 bits long. However, the result of running the program is:
8 16 |
The reason for the difference here is that the discriminant value of V1 is fixed, and will always be False. It is not possible to assign a True variant value to V1, therefore 8 bits is sufficient. On the other hand, in the case of V2, the initial discriminant value is False (from the default), but it is possible to assign a True variant value to V2, therefore 16 bits must be allocated for V2 in the general case, even fewer bits may be needed at any particular point during the program execution.
As can be seen from the output of this program, the 'Size
attribute applied to such an object in GNAT gives the actual allocated
size of the variable, which is the largest size of any of the variants.
The Ada Reference Manual is not completely clear on what choice should
be made here, but the GNAT behavior seems most consistent with the
language in the RM.
In some cases, it may be desirable to obtain the size of the current variant, rather than the size of the largest variant. This can be achieved in GNAT by making use of the fact that in the case of a subprogram parameter, GNAT does indeed return the size of the current variant (because a subprogram has no way of knowing how much space is actually allocated for the actual).
Consider the following modified version of the above program:
with Text_IO; use Text_IO;
procedure q is
type R1 (A : Boolean := False) is record
case A is
when True => X : Character;
when False => null;
end case;
end record;
V2 : R1;
function Size (V : R1) return Integer is
begin
return V'Size;
end Size;
begin
Put_Line (Integer'Image (V2'Size));
Put_Line (Integer'IMage (Size (V2)));
V2 := (True, 'x');
Put_Line (Integer'Image (V2'Size));
Put_Line (Integer'IMage (Size (V2)));
end q;
|
The output from this program is
16 8 16 16 |
Here we see that while the 'Size attribute always returns
the maximum size, regardless of the current variant value, the
Size function does indeed return the size of the current
variant value.
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In the case of scalars with a range starting at other than zero, it is possible in some cases to specify a size smaller than the default minimum value, and in such cases, GNAT uses an unsigned biased representation, in which zero is used to represent the lower bound, and successive values represent successive values of the type.
For example, suppose we have the declaration:
type Small is range -7 .. -4; for Small'Size use 2; |
Although the default size of type Small is 4, the Size
clause is accepted by GNAT and results in the following representation
scheme:
-7 is represented as 2#00# -6 is represented as 2#01# -5 is represented as 2#10# -4 is represented as 2#11# |
Biased representation is only used if the specified Size clause
cannot be accepted in any other manner. These reduced sizes that force
biased representation can be used for all discrete types except for
enumeration types for which a representation clause is given.
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In Ada 95, the Size of a discrete type is the minimum number of bits
required to hold values of the type. Although this interpretation was
allowed in Ada 83, it was not required, and this requirement in practice
can cause some significant difficulties. For example, in most Ada 83
compilers, Natural'Size was 32. However, in Ada-95,
Natural'Size is
typically 31. This means that code may change in behavior when moving
from Ada 83 to Ada 95. For example, consider:
type Rec is record;
A : Natural;
B : Natural;
end record;
for Rec use record
for A use at 0 range 0 .. Natural'Size - 1;
for B use at 0 range Natural'Size .. 2 * Natural'Size - 1;
end record;
|
In the above code, since the typical size of Natural objects
is 32 bits and Natural'Size is 31, the above code can cause
unexpected inefficient packing in Ada 95, and in general there are
surprising cases where the fact that the object size can exceed the
size of the type causes surprises.
To help get around this problem GNAT provides two implementation
dependent attributes Value_Size and Object_Size. When
applied to a type, these attributes yield the size of the type
(corresponding to the RM defined size attribute), and the size of
objects of the type respectively.
The Object_Size is used for determining the default size of
objects and components. This size value can be referred to using the
Object_Size attribute. The phrase "is used" here means that it is
the basis of the determination of the size. The backend is free to
pad this up if necessary for efficiency, e.g. an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.
The default rules for the value of Object_Size for fixed-point and
discrete types are as follows:
Object_Size for base subtypes reflect the natural hardware
size in bits (run the utility gnatpsta to find those values for numeric types).
Enumeration types and fixed-point base subtypes have 8, 16, 32 or 64
bits for this size, depending on the range of values to be stored.
Object_Size of a subtype is the same as the
Object_Size of
the type from which it is obtained.
Object_Size of a derived base type is copied from the parent
base type, and the Object_Size of a derived first subtype is copied
from the parent first subtype.
The Value_Size attribute
is the number of bits required to store a value
of the type. This size can be referred to using the Value_Size
attribute. This value is used to determine how tightly to pack
records or arrays with components of this type, and also affects
the semantics of unchecked conversion (unchecked conversions where
the Value_Size values differ generate a warning, and are potentially
target dependent).
The default rules for the value of Value_Size are as follows:
Value_Size for a base subtype is the minimum number of bits
required to store all values of the type (including the sign bit
only if negative values are possible).
Value_Size as the first subtype. This is a
consequence of RM 13.1(14) ("if two subtypes statically match,
then their subtype-specific aspects are the same".)
Value_Size corresponding to the minimum
number of bits required to store all values of the subtype. For
dynamic bounds, it is assumed that the value can range down or up
to the corresponding bound of the ancestor
The RM defined attribute Size corresponds to the
Value_Size attribute.
The Size attribute may be defined for a first-named subtype. This sets
the Value_Size of
the first-named subtype to the given value, and the
Object_Size of this first-named subtype to the given value padded up
to an appropriate boundary. It is a consequence of the default rules
above that this Object_Size will apply to all further subtypes. On the
other hand, Value_Size is affected only for the first subtype, any
dynamic subtypes obtained from it directly, and any statically matching
subtypes. The Value_Size of any other static subtypes is not affected.
Value_Size and
Object_Size may be explicitly set for any subtype using
an attribute definition clause. Note that the use of these attributes
can cause the RM 13.1(14) rule to be violated. If two access types
reference aliased objects whose subtypes have differing Object_Size
values as a result of explicit attribute definition clauses, then it
is erroneous to convert from one access subtype to the other.
At the implementation level, Esize stores the Object_SIze and the
RM_Size field stores the Value_Size (and hence the value of the
Size attribute,
which, as noted above, is equivalent to Value_Size).
To get a feel for the difference, consider the following examples (note that in each case the base is short_short_integer with a size of 8):
Object_Size Value_Size type x1 is range 0 .. 5; 8 3 type x2 is range 0 .. 5; for x2'size use 12; 12 12 subtype x3 is x2 range 0 .. 3; 12 2 subtype x4 is x2'base range 0 .. 10; 8 4 subtype x5 is x2 range 0 .. dynamic; 12 (7) subtype x6 is x2'base range 0 .. dynamic; 8 (7) |
Note: the entries marked (7) are not actually specified by the Ada 95 RM, but it seems in the spirit of the RM rules to allocate the minimum number of bits known to be large enough to hold the given range of values.
So far, so good, but GNAT has to obey the RM rules, so the question is
under what conditions must the RM Size be used.
The following is a list
of the occasions on which the RM Size must be used:
Size for a type
For types other than discrete and fixed-point types, the Object_Size
and Value_Size are the same (and equivalent to the RM attribute Size).
Only Size may be specified for such types.
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Normally, the value specified in a component clause must be consistent with the subtype of the array component with regard to size and alignment. In other words, the value specified must be at least equal to the size of this subtype, and must be a multiple of the alignment value.
In addition, component size clauses are allowed which cause the array to be packed, by specifying a smaller value. The cases in which this is allowed are for component size values in the range 1 through 63. The value specified must not be smaller than the Size of the subtype. GNAT will accurately honor all packing requests in this range. For example, if we have:
type r is array (1 .. 8) of Natural; for r'Size use 31; |
then the resulting array has a length of 31 bytes (248 bits = 8 * 31). Of course access to the components of such an array is considerably less efficient than if the natural component size of 32 is used.
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For record subtypes, GNAT permits the specification of the Bit_Order
attribute. The specification may either correspond to the default bit
order for the target, in which case the specification has no effect and
places no additional restrictions, or it may be for the non-standard
setting (that is the opposite of the default).
In the case where the non-standard value is specified, the effect is to renumber bits within each byte, but the ordering of bytes is not affected. There are certain restrictions placed on component clauses as follows:
Low_Order_First
being the default, then the following two declarations have exactly
the same effect:
type R1 is record
A : Boolean;
B : Integer range 1 .. 120;
end record;
for R1 use record
A at 0 range 0 .. 0;
B at 0 range 1 .. 7;
end record;
type R2 is record
A : Boolean;
B : Integer range 1 .. 120;
end record;
for R2'Bit_Order use High_Order_First;
for R2 use record
A at 0 range 7 .. 7;
B at 0 range 0 .. 6;
end record;
|
The useful application here is to write the second declaration with the
Bit_Order attribute definition clause, and know that it will be treated
the same, regardless of whether the target is little-endian or big-endian.
Bit_Order specification does not affect the ordering of bytes.
In particular, the following attempt at getting an endian-independent integer
does not work:
type R2 is record
A : Integer;
end record;
for R2'Bit_Order use High_Order_First;
for R2 use record
A at 0 range 0 .. 31;
end record;
|
This declaration will result in a little-endian integer on a
little-endian machine, and a big-endian integer on a big-endian machine.
If byte flipping is required for interoperability between big- and
little-endian machines, this must be explicitly programmed. This capability
is not provided by Bit_Order.
Since the misconception that Bit_Order automatically deals with all
endian-related incompatibilities is a common one, the specification of
a component field that is an integral number of bytes will always
generate a warning. This warning may be suppressed using
pragma Suppress if desired. The following section contains additional
details regarding the issue of byte ordering.
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In this section we will review the effect of the Bit_Order attribute
definition clause on byte ordering. Briefly, it has no effect at all, but
a detailed example will be helpful. Before giving this
example, let us review the precise
definition of the effect of defining Bit_Order. The effect of a
non-standard bit order is described in section 15.5.3 of the Ada
Reference Manual:
2 A bit ordering is a method of interpreting the meaning of the storage place attributes. |
To understand the precise definition of storage place attributes in this context, we visit section 13.5.1 of the manual:
13 A record_representation_clause (without the mod_clause) specifies the layout. The storage place attributes (see 13.5.2) are taken from the values of the position, first_bit, and last_bit expressions after normalizing those values so that first_bit is less than Storage_Unit. |
The critical point here is that storage places are taken from
the values after normalization, not before. So the Bit_Order
interpretation applies to normalized values. The interpretation
is described in the later part of the 15.5.3 paragraph:
2 A bit ordering is a method of interpreting the meaning of the storage place attributes. High_Order_First (known in the vernacular as ``big endian'') means that the first bit of a storage element (bit 0) is the most significant bit (interpreting the sequence of bits that represent a component as an unsigned integer value). Low_Order_First (known in the vernacular as ``little endian'') means the opposite: the first bit is the least significant. |
Note that the numbering is with respect to the bits of a storage unit. In other words, the specification affects only the numbering of bits within a single storage unit.
We can make the effect clearer by giving an example.
Suppose that we have an external device which presents two bytes, the first byte presented, which is the first (low addressed byte) of the two byte record is called Master, and the second byte is called Slave.
The left most (most significant bit is called Control for each byte, and the remaining 7 bits are called V1, V2, ... V7, where V7 is the rightmost (least significant) bit.
On a big-endian machine, we can write the following representation clause
type Data is record
Master_Control : Bit;
Master_V1 : Bit;
Master_V2 : Bit;
Master_V3 : Bit;
Master_V4 : Bit;
Master_V5 : Bit;
Master_V6 : Bit;
Master_V7 : Bit;
Slave_Control : Bit;
Slave_V1 : Bit;
Slave_V2 : Bit;
Slave_V3 : Bit;
Slave_V4 : Bit;
Slave_V5 : Bit;
Slave_V6 : Bit;
Slave_V7 : Bit;
end record;
for Data use record
Master_Control at 0 range 0 .. 0;
Master_V1 at 0 range 1 .. 1;
Master_V2 at 0 range 2 .. 2;
Master_V3 at 0 range 3 .. 3;
Master_V4 at 0 range 4 .. 4;
Master_V5 at 0 range 5 .. 5;
Master_V6 at 0 range 6 .. 6;
Master_V7 at 0 range 7 .. 7;
Slave_Control at 1 range 0 .. 0;
Slave_V1 at 1 range 1 .. 1;
Slave_V2 at 1 range 2 .. 2;
Slave_V3 at 1 range 3 .. 3;
Slave_V4 at 1 range 4 .. 4;
Slave_V5 at 1 range 5 .. 5;
Slave_V6 at 1 range 6 .. 6;
Slave_V7 at 1 range 7 .. 7;
end record;
|
Now if we move this to a little endian machine, then the bit ordering within the byte is backwards, so we have to rewrite the record rep clause as:
for Data use record
Master_Control at 0 range 7 .. 7;
Master_V1 at 0 range 6 .. 6;
Master_V2 at 0 range 5 .. 5;
Master_V3 at 0 range 4 .. 4;
Master_V4 at 0 range 3 .. 3;
Master_V5 at 0 range 2 .. 2;
Master_V6 at 0 range 1 .. 1;
Master_V7 at 0 range 0 .. 0;
Slave_Control at 1 range 7 .. 7;
Slave_V1 at 1 range 6 .. 6;
Slave_V2 at 1 range 5 .. 5;
Slave_V3 at 1 range 4 .. 4;
Slave_V4 at 1 range 3 .. 3;
Slave_V5 at 1 range 2 .. 2;
Slave_V6 at 1 range 1 .. 1;
Slave_V7 at 1 range 0 .. 0;
end record;
|
It is a nuisance to have to rewrite the clause, especially if
the code has to be maintained on both machines. However,
this is a case that we can handle with the
Bit_Order attribute if it is implemented.
Note that the implementation is not required on byte addressed
machines, but it is indeed implemented in GNAT.
This means that we can simply use the
first record clause, together with the declaration
for Data'Bit_Order use High_Order_First; |
and the effect is what is desired, namely the layout is exactly the same, independent of whether the code is compiled on a big-endian or little-endian machine.
The important point to understand is that byte ordering is not affected.
A Bit_Order attribute definition never affects which byte a field
ends up in, only where it ends up in that byte.
To make this clear, let us rewrite the record rep clause of the previous
example as:
for Data'Bit_Order use High_Order_First;
for Data use record
Master_Control at 0 range 0 .. 0;
Master_V1 at 0 range 1 .. 1;
Master_V2 at 0 range 2 .. 2;
Master_V3 at 0 range 3 .. 3;
Master_V4 at 0 range 4 .. 4;
Master_V5 at 0 range 5 .. 5;
Master_V6 at 0 range 6 .. 6;
Master_V7 at 0 range 7 .. 7;
Slave_Control at 0 range 8 .. 8;
Slave_V1 at 0 range 9 .. 9;
Slave_V2 at 0 range 10 .. 10;
Slave_V3 at 0 range 11 .. 11;
Slave_V4 at 0 range 12 .. 12;
Slave_V5 at 0 range 13 .. 13;
Slave_V6 at 0 range 14 .. 14;
Slave_V7 at 0 range 15 .. 15;
end record;
|
This is exactly equivalent to saying (a repeat of the first example):
for Data'Bit_Order use High_Order_First;
for Data use record
Master_Control at 0 range 0 .. 0;
Master_V1 at 0 range 1 .. 1;
Master_V2 at 0 range 2 .. 2;
Master_V3 at 0 range 3 .. 3;
Master_V4 at 0 range 4 .. 4;
Master_V5 at 0 range 5 .. 5;
Master_V6 at 0 range 6 .. 6;
Master_V7 at 0 range 7 .. 7;
Slave_Control at 1 range 0 .. 0;
Slave_V1 at 1 range 1 .. 1;
Slave_V2 at 1 range 2 .. 2;
Slave_V3 at 1 range 3 .. 3;
Slave_V4 at 1 range 4 .. 4;
Slave_V5 at 1 range 5 .. 5;
Slave_V6 at 1 range 6 .. 6;
Slave_V7 at 1 range 7 .. 7;
end record;
|
Why are they equivalent? Well take a specific field, the Slave_V2
field. The storage place attributes are obtained by normalizing the
values given so that the First_Bit value is less than 8. After
nromalizing the values (0,10,10) we get (1,2,2) which is exactly what
we specified in the other case.
Now one might expect that the Bit_Order attribute might affect
bit numbering within the entire record component (two bytes in this
case, thus affecting which byte fields end up in), but that is not
the way this feature is defined, it only affects numbering of bits,
not which byte they end up in.
Consequently it never makes sense to specify a starting bit number
greater than 7 (for a byte addressable field) if an attribute
definition for Bit_Order has been given, and indeed it
may be actively confusing to specify such a value, so the compiler
generates a warning for such usage.
If you do need to control byte ordering then appropriate conditional values must be used. If in our example, the slave byte came first on some machines we might write:
Master_Byte_First constant Boolean := ...;
Master_Byte : constant Natural :=
1 - Boolean'Pos (Master_Byte_First);
Slave_Byte : constant Natural :=
Boolean'Pos (Master_Byte_First);
for Data'Bit_Order use High_Order_First;
for Data use record
Master_Control at Master_Byte range 0 .. 0;
Master_V1 at Master_Byte range 1 .. 1;
Master_V2 at Master_Byte range 2 .. 2;
Master_V3 at Master_Byte range 3 .. 3;
Master_V4 at Master_Byte range 4 .. 4;
Master_V5 at Master_Byte range 5 .. 5;
Master_V6 at Master_Byte range 6 .. 6;
Master_V7 at Master_Byte range 7 .. 7;
Slave_Control at Slave_Byte range 0 .. 0;
Slave_V1 at Slave_Byte range 1 .. 1;
Slave_V2 at Slave_Byte range 2 .. 2;
Slave_V3 at Slave_Byte range 3 .. 3;
Slave_V4 at Slave_Byte range 4 .. 4;
Slave_V5 at Slave_Byte range 5 .. 5;
Slave_V6 at Slave_Byte range 6 .. 6;
Slave_V7 at Slave_Byte range 7 .. 7;
end record;
|
Now to switch between machines, all that is necessary is
to set the boolean constant Master_Byte_First in
an appropriate manner.
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Pragma Pack applied to an array has no effect unless the component type
is packable. For a component type to be packable, it must be one of the
following cases:
For all these cases, if the component subtype size is in the range
1 through 63, then the effect of the pragma Pack is exactly as though a
component size were specified giving the component subtype size.
For example if we have:
type r is range 0 .. 17; type ar is array (1 .. 8) of r; pragma Pack (ar); |
Then the component size of ar will be set to 5 (i.e. to r'size,
and the size of the array ar will be exactly 40 bits.
Note that in some cases this rather fierce approach to packing can produce
unexpected effects. For example, in Ada 95, type Natural typically has a
size of 31, meaning that if you pack an array of Natural, you get 31-bit
close packing, which saves a few bits, but results in far less efficient
access. Since many other Ada compilers will ignore such a packing request,
GNAT will generate a warning on some uses of pragma Pack that it guesses
might not be what is intended. You can easily remove this warning by
using an explicit Component_Size setting instead, which never generates
a warning, since the intention of the programmer is clear in this case.
GNAT treats packed arrays in one of two ways. If the size of the array is known at compile time and is less than 64 bits, then internally the array is represented as a single modular type, of exactly the appropriate number of bits. If the length is greater than 63 bits, or is not known at compile time, then the packed array is represented as an array of bytes, and the length is always a multiple of 8 bits.
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Pragma Pack applied to a record will pack the components to reduce wasted
space from alignment gaps and by reducing the amount of space taken by
components. We distinguish between package components and non-packable
components. Components of the following types are considered packable:
All packable components occupy the exact number of bits corresponding to
their Size value, and are packed with no padding bits, i.e. they
can start on an arbitrary bit boundary.
All other types are non-packable, they occupy an integral number of storage units, and are placed at a boundary corresponding to their alignment requirements.
For example, consider the record
type Rb1 is array (1 .. 13) of Boolean;
pragma Pack (rb1);
type Rb2 is array (1 .. 65) of Boolean;
pragma Pack (rb2);
type x2 is record
l1 : Boolean;
l2 : Duration;
l3 : Float;
l4 : Boolean;
l5 : Rb1;
l6 : Rb2;
end record;
pragma Pack (x2);
|
The representation for the record x2 is as follows:
for x2'Size use 224; for x2 use record l1 at 0 range 0 .. 0; l2 at 0 range 1 .. 64; l3 at 12 range 0 .. 31; l4 at 16 range 0 .. 0; l5 at 16 range 1 .. 13; l6 at 18 range 0 .. 71; end record; |
Studying this example, we see that the packable fields l1
and l2 are
of length equal to their sizes, and placed at specific bit boundaries (and
not byte boundaries) to
eliminate padding. But l3 is of a non-packable float type, so
it is on the next appropriate alignment boundary.
The next two fields are fully packable, so l4 and l5 are
minimally packed with no gaps. However, type Rb2 is a packed
array that is longer than 64 bits, so it is itself non-packable. Thus
the l6 field is aligned to the next byte boundary, and takes an
integral number of bytes, i.e. 72 bits.
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Record representation clauses may be given for all record types, including types obtained by record extension. Component clauses are allowed for any static component. The restrictions on component clauses depend on the type of the component.
For all components of an elementary type, the only restriction on component clauses is that the size must be at least the 'Size value of the type (actually the Value_Size). There are no restrictions due to alignment, and such components may freely cross storage boundaries.
Packed arrays with a size up to and including 64 bits are represented internally using a modular type with the appropriate number of bits, and thus the same lack of restriction applies. For example, if you declare:
type R is array (1 .. 49) of Boolean; pragma Pack (R); for R'Size use 49; |
then a component clause for a component of type R may start on any specified bit boundary, and may specify a value of 49 bits or greater.
For non-primitive types, including packed arrays with a size greater than 64 bits, component clauses must respect the alignment requirement of the type, in particular, always starting on a byte boundary, and the length must be a multiple of the storage unit.
The tag field of a tagged type always occupies an address sized field at the start of the record. No component clause may attempt to overlay this tag.
In the case of a record extension T1, of a type T, no component clause applied to the type T1 can specify a storage location that would overlap the first T'Size bytes of the record.
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The only restriction on enumeration clauses is that the range of values must be representable. For the signed case, if one or more of the representation values are negative, all values must be in the range:
System.Min_Int .. System.Max_Int |
For the unsigned case, where all values are non negative, the values must be in the range:
0 .. System.Max_Binary_Modulus; |
A confirming representation clause is one in which the values range from 0 in sequence, i.e. a clause that confirms the default representation for an enumeration type. Such a confirming representation is permitted by these rules, and is specially recognized by the compiler so that no extra overhead results from the use of such a clause.
If an array has an index type which is an enumeration type to which an enumeration clause has been applied, then the array is stored in a compact manner. Consider the declarations:
type r is (A, B, C); for r use (A => 1, B => 5, C => 10); type t is array (r) of Character; |
The array type t corresponds to a vector with exactly three elements and
has a default size equal to 3*Character'Size. This ensures efficient
use of space, but means that accesses to elements of the array will incur
the overhead of converting representation values to the corresponding
positional values, (i.e. the value delivered by the Pos attribute).
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The reference manual allows a general restriction on representation clauses, as found in RM 13.1(22):
An implementation need not support representation items containing nonstatic expressions, except that an implementation should support a representation item for a given entity if each nonstatic expression in the representation item is a name that statically denotes a constant declared before the entity. |
In practice this is applicable only to address clauses, since this is the only case in which a non-static expression is permitted by the syntax. As the AARM notes in sections 13.1 (22.a-22.h):
22.a Reason: This is to avoid the following sort
of thing:
22.b X : Integer := F(...);
Y : Address := G(...);
for X'Address use Y;
22.c In the above, we have to evaluate the
initialization expression for X before we
know where to put the result. This seems
like an unreasonable implementation burden.
22.d The above code should instead be written
like this:
22.e Y : constant Address := G(...);
X : Integer := F(...);
for X'Address use Y;
22.f This allows the expression ``Y'' to be safely
evaluated before X is created.
22.g The constant could be a formal parameter of mode in.
22.h An implementation can support other nonstatic
expressions if it wants to. Expressions of type
Address are hardly ever static, but their value
might be known at compile time anyway in many
cases.
|
GNAT does indeed permit many additional cases of non-static expressions. In particular, if the type involved is elementary there are no restrictions (since in this case, holding a temporary copy of the initialization value, if one is present, is inexpensive). In addition, if there is no implicit or explicit initialization, then there are no restrictions. GNAT will reject only the case where all three of these conditions hold:
Anchor : Some_Initialized_Type;
Overlay : Some_Initialized_Type;
for Overlay'Address use Anchor'Address;
|
However, the prefix of the address clause cannot be an array component, or a component of a discriminated record.
As noted above in section 22.h, address values are typically non-static. In particular the To_Address function, even if applied to a literal value, is a non-static function call. To avoid this minor annoyance, GNAT provides the implementation defined attribute 'To_Address. The following two expressions have identical values:
Another issue with address clauses is the interaction with alignment requirements. When an address clause is given for an object, the address value must be consistent with the alignment of the object (which is usually the same as the alignment of the type of the object). If an address clause is given that specifies an inappropriately aligned address value, then the program execution is erroneous.
Since this source of erroneous behavior can have unfortunate effects, GNAT
checks (at compile time if possible, generating a warning, or at execution
time with a run-time check) that the alignment is appropriate. If the
run-time check fails, then Program_Error is raised. This run-time
check is suppressed if range checks are suppressed, or if
pragma Restrictions (No_Elaboration_Code) is in effect.
To_Address (16#1234_0000#) System'To_Address (16#1234_0000#); |
except that the second form is considered to be a static expression, and thus when used as an address clause value is always permitted.
Additionally, GNAT treats as static an address clause that is an unchecked_conversion of a static integer value. This simplifies the porting of legacy code, and provides a portable equivalent to the GNAT attribute To_Address.
An address clause cannot be given for an exported object. More understandably the real restriction is that objects with an address clause cannot be exported. This is because such variables are not defined by the Ada program, so there is no external object so export.
It is permissible to give an address clause and a pragma Import for the same object. In this case, the variable is not really defined by the Ada program, so there is no external symbol to be linked. The link name and the external name are ignored in this case. The reason that we allow this combination is that it provides a useful idiom to avoid unwanted initializations on objects with address clauses.
When an address clause is given for an object that has implicit or explicit initialization, then by default initialization takes place. This means that the effect of the object declaration is to overwrite the memory at the specified address. This is almost always not what the programmer wants, so GNAT will output a warning:
with System;
package G is
type R is record
M : Integer := 0;
end record;
Ext : R;
for Ext'Address use System'To_Address (16#1234_1234#);
|
>>> warning: implicit initialization of "Ext" may
modify overlaid storage
>>> warning: use pragma Import for "Ext" to suppress
initialization (RM B(24))
end G;
|
As indicated by the warning message, the solution is to use a (dummy) pragma Import to suppress this initialization. The pragma tell the compiler that the object is declared and initialized elsewhere. The following package compiles without warnings (and the initialization is suppressed):
with System;
package G is
type R is record
M : Integer := 0;
end record;
Ext : R;
for Ext'Address use System'To_Address (16#1234_1234#);
pragma Import (Ada, Ext);
end G;
|
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Normally the specification of a foreign language convention for a type or an object has no effect on the chosen representation. In particular, the representation chosen for data in GNAT generally meets the standard system conventions, and for example records are laid out in a manner that is consistent with C. This means that specifying convention C (for example) has no effect.
There are three exceptions to this general rule:
type Color is (Red, Green, Blue); |
8 bits is sufficient to store all values of the type, so by default, objects
of type Color will be represented using 8 bits. However, normal C
convention is to use 32 bits for all enum values in C, since enum values
are essentially of type int. If pragma Convention C is specified for an
Ada enumeration type, then the size is modified as necessary (usually to
32 bits) to be consistent with the C convention for enum values.
Fortran has a similar convention for LOGICAL values (any nonzero
value represents true).
To accommodate the Fortran and C conventions, if a pragma Convention specifies C or Fortran convention for a derived Boolean, as in the following example:
type C_Switch is new Boolean; pragma Convention (C, C_Switch); |
then the GNAT generated code will treat any nonzero value as true. For truth values generated by GNAT, the conventional value 1 will be used for True, but when one of these values is read, any nonzero value is treated as True.
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Although the descriptions in this section are intended to be complete, it is often easier to simply experiment to see what GNAT accepts and what the effect is on the layout of types and objects.
As required by the Ada RM, if a representation clause is not accepted, then
it must be rejected as illegal by the compiler. However, when a representation
clause or pragma is accepted, there can still be questions of what the
compiler actually does. For example, if a partial record representation
clause specifies the location of some components and not others, then where
are the non-specified components placed? Or if pragma Pack is used on a
record, then exactly where are the resulting fields placed? The section
on pragma Pack in this chapter can be used to answer the second question,
but it is often easier to just see what the compiler does.
For this purpose, GNAT provides the option -gnatR. If you compile
with this option, then the compiler will output information on the actual
representations chosen, in a format similar to source representation
clauses. For example, if we compile the package:
package q is
type r (x : boolean) is tagged record
case x is
when True => S : String (1 .. 100);
when False => null;
end case;
end record;
type r2 is new r (false) with record
y2 : integer;
end record;
for r2 use record
y2 at 16 range 0 .. 31;
end record;
type x is record
y : character;
end record;
type x1 is array (1 .. 10) of x;
for x1'component_size use 11;
type ia is access integer;
type Rb1 is array (1 .. 13) of Boolean;
pragma Pack (rb1);
type Rb2 is array (1 .. 65) of Boolean;
pragma Pack (rb2);
type x2 is record
l1 : Boolean;
l2 : Duration;
l3 : Float;
l4 : Boolean;
l5 : Rb1;
l6 : Rb2;
end record;
pragma Pack (x2);
end q;
|
using the switch -gnatR we obtain the following output:
Representation information for unit q ------------------------------------- for r'Size use ??; for r'Alignment use 4; for r use record x at 4 range 0 .. 7; _tag at 0 range 0 .. 31; s at 5 range 0 .. 799; end record; for r2'Size use 160; for r2'Alignment use 4; for r2 use record x at 4 range 0 .. 7; _tag at 0 range 0 .. 31; _parent at 0 range 0 .. 63; y2 at 16 range 0 .. 31; end record; for x'Size use 8; for x'Alignment use 1; for x use record y at 0 range 0 .. 7; end record; for x1'Size use 112; for x1'Alignment use 1; for x1'Component_Size use 11; for rb1'Size use 13; for rb1'Alignment use 2; for rb1'Component_Size use 1; for rb2'Size use 72; for rb2'Alignment use 1; for rb2'Component_Size use 1; for x2'Size use 224; for x2'Alignment use 4; for x2 use record l1 at 0 range 0 .. 0; l2 at 0 range 1 .. 64; l3 at 12 range 0 .. 31; l4 at 16 range 0 .. 0; l5 at 16 range 1 .. 13; l6 at 18 range 0 .. 71; end record; |
The Size values are actually the Object_Size, i.e. the default size that will be allocated for objects of the type. The ?? size for type r indicates that we have a variant record, and the actual size of objects will depend on the discriminant value.
The Alignment values show the actual alignment chosen by the compiler for each record or array type.
The record representation clause for type r shows where all fields are placed, including the compiler generated tag field (whose location cannot be controlled by the programmer).
The record representation clause for the type extension r2 shows all the fields present, including the parent field, which is a copy of the fields of the parent type of r2, i.e. r1.
The component size and size clauses for types rb1 and rb2 show
the exact effect of pragma Pack on these arrays, and the record
representation clause for type x2 shows how pragma Pack affects
this record type.
In some cases, it may be useful to cut and paste the representation clauses generated by the compiler into the original source to fix and guarantee the actual representation to be used.
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The Ada 95 Reference Manual contains in Annex A a full description of an extensive set of standard library routines that can be used in any Ada program, and which must be provided by all Ada compilers. They are analogous to the standard C library used by C programs.
GNAT implements all of the facilities described in annex A, and for most purposes the description in the Ada 95 reference manual, or appropriate Ada text book, will be sufficient for making use of these facilities.
In the case of the input-output facilities, See section 8. The Implementation of Standard I/O, gives details on exactly how GNAT interfaces to the file system. For the remaining packages, the Ada 95 reference manual should be sufficient. The following is a list of the packages included, together with a brief description of the functionality that is provided.
For completeness, references are included to other predefined library routines defined in other sections of the Ada 95 reference manual (these are cross-indexed from annex A).
Ada (A.2)
Ada.Calendar (9.6)
Calendar provides time of day access, and routines for
manipulating times and durations.
Ada.Characters (A.3.1)
Ada.Characters.Handling (A.3.2)
Ada.Characters.Latin_1 (A.3.3)
UC_E_Acute in this package. Then your program
will print in an understandable manner even if your environment does not
support these extended characters.
Ada.Command_Line (A.15)
argc and argv in C), and
also allows the exit status for the program to be set in a
system-independent manner.
Ada.Decimal (F.2)
Ada.Direct_IO (A.8.4)
Ada.Dynamic_Priorities (D.5)
Ada.Exceptions (11.4.1)
Ada.Finalization (7.6)
Ada.Interrupts (C.3.2)
Ada.Interrupts.Names (C.3.2)
Ada.IO_Exceptions (A.13)
Ada.Numerics
Ada.Numerics.Complex_Elementary_Functions
Float and the Complex and Imaginary types
created by the package Numerics.Complex_Types.
Ada.Numerics.Complex_Types
Numerics.Generic_Complex_Types using Standard.Float to
build the type Complex and Imaginary.
Ada.Numerics.Discrete_Random
Ada.Numerics.Float_Random
Ada.Numerics.Generic_Complex_Elementary_Functions
The following predefined instantiations of this package are provided:
Short_Float
Ada.Numerics.Short_Complex_Elementary_Functions
Float
Ada.Numerics.Complex_Elementary_Functions
Long_Float
Ada.Numerics.
Long_Complex_Elementary_Functions
Ada.Numerics.Generic_Complex_Types
The following predefined instantiations of this package exist
Short_Float
Ada.Numerics.Short_Complex_Complex_Types
Float
Ada.Numerics.Complex_Complex_Types
Long_Float
Ada.Numerics.Long_Complex_Complex_Types
Ada.Numerics.Generic_Elementary_Functions
The following predefined instantiations of this package exist
Short_Float
Ada.Numerics.Short_Elementary_Functions
Float
Ada.Numerics.Elementary_Functions
Long_Float
Ada.Numerics.Long_Elementary_Functions
Ada.Real_Time (D.8)
Calendar, but
operating with a finer clock suitable for real time control.
Ada.Sequential_IO (A.8.1)
Ada.Storage_IO (A.9)
Ada.Streams (13.13.1)
Input,
Output, Read and Write).
Ada.Streams.Stream_IO (A.12.1)
Streams defined in
package Streams together with a set of operations providing
Stream_IO capability. The Stream_IO model permits both random and
sequential access to a file which can contain an arbitrary set of values
of one or more Ada types.
Ada.Strings (A.4.1)
Ada.Strings.Bounded (A.4.4)
Ada.Strings.Fixed (A.4.3)
Ada.Strings.Maps (A.4.2)
Ada.Strings.Maps.Constants (A.4.6)
Ada.Strings.Unbounded (A.4.5)
Ada.Strings.Wide_Bounded (A.4.7)
Ada.Strings.Wide_Fixed (A.4.7)
Ada.Strings.Wide_Maps (A.4.7)
Ada.Strings.Wide_Maps.Constants (A.4.7)
Ada.Strings.Wide_Unbounded (A.4.7)
Wide_String and Wide_Character instead of String
and Character.
Ada.Synchronous_Task_Control (D.10)
Ada.Tags
Ada.Task_Attributes
Ada.Text_IO
Ada.Text_IO.Decimal_IO
Ada.Text_IO.Enumeration_IO
Ada.Text_IO.Fixed_IO
Ada.Text_IO.Float_IO
Short_Float
Short_Float_Text_IO
Float
Float_Text_IO
Long_Float
Long_Float_Text_IO
Ada.Text_IO.Integer_IO
Short_Short_Integer
Ada.Short_Short_Integer_Text_IO
Short_Integer
Ada.Short_Integer_Text_IO
Integer
Ada.Integer_Text_IO
Long_Integer
Ada.Long_Integer_Text_IO
Long_Long_Integer
Ada.Long_Long_Integer_Text_IO
Ada.Text_IO.Modular_IO
Ada.Text_IO.Complex_IO (G.1.3)
Ada.Text_IO.Editing (F.3.3)
Ada.Text_IO.Text_Streams (A.12.2)
Ada.Unchecked_Conversion (13.9)
If the types have the same Size (more accurately the same Value_Size), then the effect is simply to transfer the bits from the source to the target type without any modification. This usage is well defined, and for simple types whose representation is typically the same across all implementations, gives a portable method of performing such conversions.
If the types do not have the same size, then the result is implementation defined, and thus may be non-portable. The following describes how GNAT handles such unchecked conversion cases.
If the types are of different sizes, and are both discrete types, then the effect is of a normal type conversion without any constraint checking. In particular if the result type has a larger size, the result will be zero or sign extended. If the result type has a smaller size, the result will be truncated by ignoring high order bits.
If the types are of different sizes, and are not both discrete types, then the conversion works as though pointers were created to the source and target, and the pointer value is converted. The effect is that bits are copied from successive low order storage units and bits of the source up to the length of the target type.
A warning is issued if the lengths differ, since the effect in this case is implementation dependent, and the above behavior may not match that of some other compiler.
A pointer to one type may be converted to a pointer to another type using unchecked conversion. The only case in which the effect is undefined is when one or both pointers are pointers to unconstrained array types. In this case, the bounds information may get incorrectly transferred, and in particular, GNAT uses double size pointers for such types, and it is meaningless to convert between such pointer types. GNAT will issue a warning if the alignment of the target designated type is more strict than the alignment of the source designated type (since the result may be unaligned in this case).
A pointer other than a pointer to an unconstrained array type may be converted to and from System.Address. Such usage is common in Ada 83 programs, but note that Ada.Address_To_Access_Conversions is the preferred method of performing such conversions in Ada 95. Neither unchecked conversion nor Ada.Address_To_Access_Conversions should be used in conjunction with pointers to unconstrained objects, since the bounds information cannot be handled correctly in this case.
Ada.Unchecked_Deallocation (13.11.2)
Ada.Wide_Text_IO (A.11)
Ada.Text_IO, except that the external
file supports wide character representations, and the internal types are
Wide_Character and Wide_String instead of Character
and String. It contains generic subpackages listed next.
Ada.Wide_Text_IO.Decimal_IO
Ada.Wide_Text_IO.Enumeration_IO
Ada.Wide_Text_IO.Fixed_IO
Ada.Wide_Text_IO.Float_IO
Short_Float
Short_Float_Wide_Text_IO
Float
Float_Wide_Text_IO
Long_Float
Long_Float_Wide_Text_IO
Ada.Wide_Text_IO.Integer_IO
Short_Short_Integer
Ada.Short_Short_Integer_Wide_Text_IO
Short_Integer
Ada.Short_Integer_Wide_Text_IO
Integer
Ada.Integer_Wide_Text_IO
Long_Integer
Ada.Long_Integer_Wide_Text_IO
Long_Long_Integer
Ada.Long_Long_Integer_Wide_Text_IO
Ada.Wide_Text_IO.Modular_IO
Ada.Wide_Text_IO.Complex_IO (G.1.3)
Ada.Text_IO.Complex_IO, except that the
external file supports wide character representations.
Ada.Wide_Text_IO.Editing (F.3.4)
Ada.Text_IO.Editing, except that the
types are Wide_Character and Wide_String instead of
Character and String.
Ada.Wide_Text_IO.Streams (A.12.3)
Ada.Text_IO.Streams, except that the
types are Wide_Character and Wide_String instead of
Character and String.
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GNAT implements all the required input-output facilities described in A.6 through A.14. These sections of the Ada 95 reference manual describe the required behavior of these packages from the Ada point of view, and if you are writing a portable Ada program that does not need to know the exact manner in which Ada maps to the outside world when it comes to reading or writing external files, then you do not need to read this chapter. As long as your files are all regular files (not pipes or devices), and as long as you write and read the files only from Ada, the description in the Ada 95 reference manual is sufficient.
However, if you want to do input-output to pipes or other devices, such as the keyboard or screen, or if the files you are dealing with are either generated by some other language, or to be read by some other language, then you need to know more about the details of how the GNAT implementation of these input-output facilities behaves.
In this chapter we give a detailed description of exactly how GNAT interfaces to the file system. As always, the sources of the system are available to you for answering questions at an even more detailed level, but for most purposes the information in this chapter will suffice.
Another reason that you may need to know more about how input-output is implemented arises when you have a program written in mixed languages where, for example, files are shared between the C and Ada sections of the same program. GNAT provides some additional facilities, in the form of additional child library packages, that facilitate this sharing, and these additional facilities are also described in this chapter.
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The Standard I/O packages described in Annex A for
are implemented using the C library streams facility; where
fopen.
fread/fwrite.
There is no internal buffering of any kind at the Ada library level. The only buffering is that provided at the system level in the implementation of the C library routines that support streams. This facilitates shared use of these streams by mixed language programs.
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The format of a FORM string in GNAT is:
"keyword=value,keyword=value,...,keyword=value" |
where letters may be in upper or lower case, and there are no spaces between values. The order of the entries is not important. Currently there are two keywords defined.
SHARED=[YES|NO] WCEM=[n|h|u|s\e] |
The use of these parameters is described later in this section.
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Direct_IO can only be instantiated for definite types. This is a
restriction of the Ada language, which means that the records are fixed
length (the length being determined by type'Size, rounded
up to the next storage unit boundary if necessary).
The records of a Direct_IO file are simply written to the file in index sequence, with the first record starting at offset zero, and subsequent records following. There is no control information of any kind. For example, if 32-bit integers are being written, each record takes 4-bytes, so the record at index K starts at offset (K-1)*4.
There is no limit on the size of Direct_IO files, they are expanded as necessary to accommodate whatever records are written to the file.
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Sequential_IO may be instantiated with either a definite (constrained) or indefinite (unconstrained) type.
For the definite type case, the elements written to the file are simply the memory images of the data values with no control information of any kind. The resulting file should be read using the same type, no validity checking is performed on input.
For the indefinite type case, the elements written consist of two
parts. First is the size of the data item, written as the memory image
of a Interfaces.C.size_t value, followed by the memory image of
the data value. The resulting file can only be read using the same
(unconstrained) type. Normal assignment checks are performed on these
read operations, and if these checks fail, Data_Error is
raised. In particular, in the array case, the lengths must match, and in
the variant record case, if the variable for a particular read operation
is constrained, the discriminants must match.
Note that it is not possible to use Sequential_IO to write variable
length array items, and then read the data back into different length
arrays. For example, the following will raise Data_Error:
package IO is new Sequential_IO (String); F : IO.File_Type; S : String (1..4); ... IO.Create (F) IO.Write (F, "hello!") IO.Reset (F, Mode=>In_File); IO.Read (F, S); Put_Line (S); |
On some Ada implementations, this will print `hell', but the program is clearly incorrect, since there is only one element in the file, and that element is the string `hello!'.
In Ada 95, this kind of behavior can be legitimately achieved using Stream_IO, and this is the preferred mechanism. In particular, the above program fragment rewritten to use Stream_IO will work correctly.
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Text_IO files consist of a stream of characters containing the following special control characters:
LF (line feed, 16#0A#) Line Mark FF (form feed, 16#0C#) Page Mark |
A canonical Text_IO file is defined as one in which the following conditions are met:
LF is used only as a line mark, i.e. to mark the end
of the line.
FF is used only as a page mark, i.e. to mark the
end of a page and consequently can appear only immediately following a
LF (line mark) character.
LF (line mark) or LF-FF
(line mark, page mark). In the former case, the page mark is implicitly
assumed to be present.
A file written using Text_IO will be in canonical form provided that no
explicit LF or FF characters are written using Put
or Put_Line. There will be no FF character at the end of
the file unless an explicit New_Page operation was performed
before closing the file.
A canonical Text_IO file that is a regular file, i.e. not a device or a pipe, can be read using any of the routines in Text_IO. The semantics in this case will be exactly as defined in the Ada 95 reference manual and all the routines in Text_IO are fully implemented.
A text file that does not meet the requirements for a canonical Text_IO file has one of the following:
FF characters not immediately following a
LF character.
LF or FF characters written by
Put or Put_Line, which are not logically considered to be
line marks or page marks.
LF or FF,
i.e. there is no explicit line mark or page mark at the end of the file.
Text_IO can be used to read such non-standard text files but subprograms
to do with line or page numbers do not have defined meanings. In
particular, a FF character that does not follow a LF
character may or may not be treated as a page mark from the point of
view of page and line numbering. Every LF character is considered
to end a line, and there is an implied LF character at the end of
the file.
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Ada.Text_IO has a definition of current position for a file that
is being read. No internal buffering occurs in Text_IO, and usually the
physical position in the stream used to implement the file corresponds
to this logical position defined by Text_IO. There are two exceptions:
End_Of_Page that returns True, the stream
is positioned past the LF (line mark) that precedes the page
mark. Text_IO maintains an internal flag so that subsequent read
operations properly handle the logical position which is unchanged by
the End_Of_Page call.
End_Of_File that returns True, if the
Text_IO file was positioned before the line mark at the end of file
before the call, then the logical position is unchanged, but the stream
is physically positioned right at the end of file (past the line mark,
and past a possible page mark following the line mark. Again Text_IO
maintains internal flags so that subsequent read operations properly
handle the logical position.
These discrepancies have no effect on the observable behavior of Text_IO, but if a single Ada stream is shared between a C program and Ada program, or shared (using `shared=yes' in the form string) between two Ada files, then the difference may be observable in some situations.
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A non-regular file is a device (such as a keyboard), or a pipe. Text_IO can be used for reading and writing. Writing is not affected and the sequence of characters output is identical to the normal file case, but for reading, the behavior of Text_IO is modified to avoid undesirable look-ahead as follows:
An input file that is not a regular file is considered to have no page
marks. Any Ascii.FF characters (the character normally used for a
page mark) appearing in the file are considered to be data
characters. In particular:
Get_Line and Skip_Line do not test for a page mark
following a line mark. If a page mark appears, it will be treated as a
data character.
End_Of_Page always returns False
End_Of_File will return False if there is a page mark at
the end of the file.
Output to non-regular files is the same as for regular files. Page marks
may be written to non-regular files using New_Page, but as noted
above they will not be treated as page marks on input if the output is
piped to another Ada program.
Another important discrepancy when reading non-regular files is that the end
of file indication is not "sticky". If an end of file is entered, e.g. by
pressing the EOT key,
then end of file
is signalled once (i.e. the test End_Of_File
will yield True, or a read will
raise End_Error), but then reading can resume
to read data past that end of
file indication, until another end of file indication is entered.
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Get_Immediate returns the next character (including control characters) from the input file. In particular, Get_Immediate will return LF or FF characters used as line marks or page marks. Such operations leave the file positioned past the control character, and it is thus not treated as having its normal function. This means that page, line and column counts after this kind of Get_Immediate call are set as though the mark did not occur. In the case where a Get_Immediate leaves the file positioned between the line mark and page mark (which is not normally possible), it is undefined whether the FF character will be treated as a page mark.
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The package Text_IO.Streams allows a Text_IO file to be treated
as a stream. Data written to a Text_IO file in this stream mode is
binary data. If this binary data contains bytes 16#0A# (LF) or
16#0C# (FF), the resulting file may have non-standard
format. Similarly if read operations are used to read from a Text_IO
file treated as a stream, then LF and FF characters may be
skipped and the effect is similar to that described above for
Get_Immediate.
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A package GNAT.IO_Aux in the GNAT library provides some useful extensions
to the standard Text_IO package:
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The package Ada.Strings.Unbounded.Text_IO
in library files a-suteio.ads/adb contains some GNAT-specific
subprograms useful for Text_IO operations on unbounded strings:
Put (To_String (U)) except that an extra copy is avoided.
New_Line.
Similar to the effect of Put_Line (To_String (U)) except
that an extra copy is avoided.
In the above procedures, File is of type Ada.Text_IO.File_Type
and is optional. If the parameter is omitted, then the standard input or
output file is referenced as appropriate.
The package Ada.Strings.Wide_Unbounded.Wide_Text_IO in library
files `a-swuwti.ads' and `a-swuwti.adb' provides similar extended Wide_Text_IO
functionality for unbounded wide strings.
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Wide_Text_IO is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide character values. The encoding scheme for a given file may be
specified using a FORM parameter:
WCEM=x |
as part of the FORM string (WCEM = wide character encoding method), where x is one of the following characters
The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.
The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).
ESC a b c d |
where a, b, c, d are the four hexadecimal
characters (using upper case letters) of the wide character code. For
example, ESC A345 is used to represent the wide character with code
16#A345#. This scheme is compatible with use of the full
Wide_Character set.
16#0000#-16#007f#: 2#0xxxxxxx# 16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx# 16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx# |
where the xxx bits correspond to the left-padded bits of the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will raise a Constraint_Error, as will all invalid UTF-8 sequences.)
[ " a b c d " ] |
Where a, b, c, d are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, ["A345"] is used to represent the wide character with code
16#A345#.
This scheme is compatible with use of the full Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g. ["C1"] for lower case a. However, on output, brackets notation
is only used for wide characters with a code greater than 16#FF#.
For the coding schemes other than Hex and Brackets encoding, not all wide character values can be represented. An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised. An invalid wide character sequence on input also causes Constraint_Error to be raised.
8.6.1 Stream Pointer Positioning 8.6.2 Reading and Writing Non-Regular Files
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Ada.Wide_Text_IO is similar to Ada.Text_IO in its handling
of stream pointer positioning (see section 8.5 Text_IO). There is one additional
case:
If Ada.Wide_Text_IO.Look_Ahead reads a character outside the
normal lower ASCII set (i.e. a character in the range:
Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#) |
then although the logical position of the file pointer is unchanged by
the Look_Ahead call, the stream is physically positioned past the
wide character sequence. Again this is to avoid the need for buffering
or backup, and all Wide_Text_IO routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using Wide_Text_IO. However, this discrepancy
can be observed if the wide text file shares a stream with another file.
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As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and End_Of_Page always returns
False. Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.
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A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada 95 reference manual. The type
Stream_Element is simply a byte. There are two ways to read or
write a stream file.
Read and Write directly read or write a
sequence of stream elements with no control information.
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Section A.14 of the Ada 95 Reference Manual allows implementations to provide a wide variety of behavior if an attempt is made to access the same external file with two or more internal files.
To provide a full range of functionality, while at the same time minimizing the problems of portability caused by this implementation dependence, GNAT handles file sharing as follows:
Use_Error will be
raised. Note that a file that is not explicitly closed by the program
remains open until the program terminates.
When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that
Use_Error is raised.
The documentation of the original compiler and the documentation of the
program should then be examined to determine if file sharing was
expected, and `shared=xxx' parameters added to Open
and Create calls as required.
When a program is ported from GNAT to some other Ada compiler, no special attention is required unless the `shared=xxx' form parameter is used in the program. In this case, you must examine the documentation of the new compiler to see if it supports the required file sharing semantics, and form strings modified appropriately. Of course it may be the case that the program cannot be ported if the target compiler does not support the required functionality. The best approach in writing portable code is to avoid file sharing (and hence the use of the `shared=xxx' parameter in the form string) completely.
One common use of file sharing in Ada 83 is the use of instantiations of Sequential_IO on the same file with different types, to achieve heterogeneous input-output. Although this approach will work in GNAT if `shared=yes' is specified, it is preferable in Ada 95 to use Stream_IO for this purpose (using the stream attributes)
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Open and Create calls result in a call to fopen
using the mode shown in Table 6.1
Open and Create Call Modes
OPEN CREATE Append_File "r+" "w+" In_File "r" "w+" Out_File (Direct_IO) "r+" "w" Out_File (all other cases) "w" "w" Inout_File "r+" "w+" |
If text file translation is required, then either `b' or `t' is added to the mode, depending on the setting of Text. Text file translation refers to the mapping of CR/LF sequences in an external file to LF characters internally. This mapping only occurs in DOS and DOS-like systems, and is not relevant to other systems.
A special case occurs with Stream_IO. As shown in the above table, the
file is initially opened in `r' or `w' mode for the
In_File and Out_File cases. If a Set_Mode operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in `r+' mode to permit the required operation.
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Interfaces.C_Streams provides an Ada program with direct
access to the C library functions for operations on C streams:
package Interfaces.C_Streams is
-- Note: the reason we do not use the types that are in
-- Interfaces.C is that we want to avoid dragging in the
-- code in this unit if possible.
subtype chars is System.Address;
-- Pointer to null-terminated array of characters
subtype FILEs is System.Address;
-- Corresponds to the C type FILE*
subtype voids is System.Address;
-- Corresponds to the C type void*
subtype int is Integer;
subtype long is Long_Integer;
-- Note: the above types are subtypes deliberately, and it
-- is part of this spec that the above correspondences are
-- guaranteed. This means that it is legitimate to, for
-- example, use Integer instead of int. We provide these
-- synonyms for clarity, but in some cases it may be
-- convenient to use the underlying types (for example to
-- avoid an unnecessary dependency of a spec on the spec
-- of this unit).
type size_t is mod 2 ** Standard'Address_Size;
NULL_Stream : constant FILEs;
-- Value returned (NULL in C) to indicate an
-- fdopen/fopen/tmpfile error
----------------------------------
-- Constants Defined in stdio.h --
----------------------------------
EOF : constant int;
-- Used by a number of routines to indicate error or
-- end of file
IOFBF : constant int;
IOLBF : constant int;
IONBF : constant int;
-- Used to indicate buffering mode for setvbuf call
SEEK_CUR : constant int;
SEEK_END : constant int;
SEEK_SET : constant int;
-- Used to indicate origin for fseek call
function stdin return FILEs;
function stdout return FILEs;
function stderr return FILEs;
-- Streams associated with standard files
--------------------------
-- Standard C functions --
--------------------------
-- The functions selected below are ones that are
-- available in DOS, OS/2, UNIX and Xenix (but not
-- necessarily in ANSI C). These are very thin interfaces
-- which copy exactly the C headers. For more
-- documentation on these functions, see the Microsoft C
-- "Run-Time Library Reference" (Microsoft Press, 1990,
-- ISBN 1-55615-225-6), which includes useful information
-- on system compatibility.
procedure clearerr (stream : FILEs);
function fclose (stream : FILEs) return int;
function fdopen (handle : int; mode : chars) return FILEs;
function feof (stream : FILEs) return int;
function ferror (stream : FILEs) return int;
function fflush (stream : FILEs) return int;
function fgetc (stream : FILEs) return int;
function fgets (strng : chars; n : int; stream : FILEs)
return chars;
function fileno (stream : FILEs) return int;
function fopen (filename : chars; Mode : chars)
return FILEs;
-- Note: to maintain target independence, use
-- text_translation_required, a boolean variable defined in
-- a-sysdep.c to deal with the target dependent text
-- translation requirement. If this variable is set,
-- then b/t should be appended to the standard mode
-- argument to set the text translation mode off or on
-- as required.
function fputc (C : int; stream : FILEs) return int;
function fputs (Strng : chars; Stream : FILEs) return int;
function fread
(buffer : voids;
size : size_t;
count : size_t;
stream : FILEs)
return size_t;
function freopen
(filename : chars;
mode : chars;
stream : FILEs)
return FILEs;
function fseek
(stream : FILEs;
offset : long;
origin : int)
return int;
function ftell (stream : FILEs) return long;
function fwrite
(buffer : voids;
size : size_t;
count : size_t;
stream : FILEs)
return size_t;
function isatty (handle : int) return int;
procedure mktemp (template : chars);
-- The return value (which is just a pointer to template)
-- is discarded
procedure rewind (stream : FILEs);
function rmtmp return int;
function setvbuf
(stream : FILEs;
buffer : chars;
mode : int;
size : size_t)
return int;
function tmpfile return FILEs;
function ungetc (c : int; stream : FILEs) return int;
function unlink (filename : chars) return int;
---------------------
-- Extra functions --
---------------------
-- These functions supply slightly thicker bindings than
-- those above. They are derived from functions in the
-- C Run-Time Library, but may do a bit more work than
-- just directly calling one of the Library functions.
function is_regular_file (handle : int) return int;
-- Tests if given handle is for a regular file (result 1)
-- or for a non-regular file (pipe or device, result 0).
---------------------------------
-- Control of Text/Binary Mode --
---------------------------------
-- If text_translation_required is true, then the following
-- functions may be used to dynamically switch a file from
-- binary to text mode or vice versa. These functions have
-- no effect if text_translation_required is false (i.e. in
-- normal UNIX mode). Use fileno to get a stream handle.
procedure set_binary_mode (handle : int);
procedure set_text_mode (handle : int);
----------------------------
-- Full Path Name support --
----------------------------
procedure full_name (nam : chars; buffer : chars);
-- Given a NUL terminated string representing a file
-- name, returns in buffer a NUL terminated string
-- representing the full path name for the file name.
-- On systems where it is relevant the drive is also
-- part of the full path name. It is the responsibility
-- of the caller to pass an actual parameter for buffer
-- that is big enough for any full path name. Use
-- max_path_len given below as the size of buffer.
max_path_len : integer;
-- Maximum length of an allowable full path name on the
-- system, including a terminating NUL character.
end Interfaces.C_Streams;
|
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The packages in this section permit interfacing Ada files to C Stream operations.
with Interfaces.C_Streams;
package Ada.Sequential_IO.C_Streams is
function C_Stream (F : File_Type)
return Interfaces.C_Streams.FILEs;
procedure Open
(File : in out File_Type;
Mode : in File_Mode;
C_Stream : in Interfaces.C_Streams.FILEs;
Form : in String := "");
end Ada.Sequential_IO.C_Streams;
with Interfaces.C_Streams;
package Ada.Direct_IO.C_Streams is
function C_Stream (F : File_Type)
return Interfaces.C_Streams.FILEs;
procedure Open
(File : in out File_Type;
Mode : in File_Mode;
C_Stream : in Interfaces.C_Streams.FILEs;
Form : in String := "");
end Ada.Direct_IO.C_Streams;
with Interfaces.C_Streams;
package Ada.Text_IO.C_Streams is
function C_Stream (F : File_Type)
return Interfaces.C_Streams.FILEs;
procedure Open
(File : in out File_Type;
Mode : in File_Mode;
C_Stream : in Interfaces.C_Streams.FILEs;
Form : in String := "");
end Ada.Text_IO.C_Streams;
with Interfaces.C_Streams;
package Ada.Wide_Text_IO.C_Streams is
function C_Stream (F : File_Type)
return Interfaces.C_Streams.FILEs;
procedure Open
(File : in out File_Type;
Mode : in File_Mode;
C_Stream : in Interfaces.C_Streams.FILEs;
Form : in String := "");
end Ada.Wide_Text_IO.C_Streams;
with Interfaces.C_Streams;
package Ada.Stream_IO.C_Streams is
function C_Stream (F : File_Type)
return Interfaces.C_Streams.FILEs;
procedure Open
(File : in out File_Type;
Mode : in File_Mode;
C_Stream : in Interfaces.C_Streams.FILEs;
Form : in String := "");
end Ada.Stream_IO.C_Streams;
|
In each of these five packages, the C_Stream function obtains the
FILE pointer from a currently opened Ada file. It is then
possible to use the Interfaces.C_Streams package to operate on
this stream, or the stream can be passed to a C program which can
operate on it directly. Of course the program is responsible for
ensuring that only appropriate sequences of operations are executed.
One particular use of relevance to an Ada program is that the
setvbuf function can be used to control the buffering of the
stream used by an Ada file. In the absence of such a call the standard
default buffering is used.
The Open procedures in these packages open a file giving an
existing C Stream instead of a file name. Typically this stream is
imported from a C program, allowing an Ada file to operate on an
existing C file.
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The GNAT library contains a number of general and special purpose packages. It represents functionality that the GNAT developers have found useful, and which is made available to GNAT users. The packages described here are fully supported, and upwards compatibility will be maintained in future releases, so you can use these facilities with the confidence that the same functionality will be available in future releases.
The chapter here simply gives a brief summary of the facilities available. The full documentation is found in the spec file for the package. The full sources of these library packages, including both spec and body, are provided with all GNAT releases. For example, to find out the full specifications of the SPITBOL pattern matching capability, including a full tutorial and extensive examples, look in the `g-spipat.ads' file in the library.
For each entry here, the package name (as it would appear in a with
clause) is given, followed by the name of the corresponding spec file in
parentheses. The packages are children in four hierarchies, Ada,
Interfaces, System, and GNAT, the latter being a
GNAT-specific hierarchy.
Note that an application program should only use packages in one of these
four hierarchies if the package is defined in the Ada Reference Manual,
or is listed in this section of the GNAT Programmers Reference Manual.
All other units should be considered internal implementation units and
should not be directly with'ed by application code. The use of
a with statement that references one of these internal implementation
units makes an application potentially dependent on changes in versions
of GNAT, and will generate a warning message.
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Ada.Characters.Latin_9 (`a-chlat9.ads')
This child of Ada.Characters
provides a set of definitions corresponding to those in the
RM-defined package Ada.Characters.Latin_1 but with the
few modifications required for Latin-9
The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3(27)).
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Ada.Characters.Wide_Latin_1 (`a-cwila1.ads')
This child of Ada.Characters
provides a set of definitions corresponding to those in the
RM-defined package Ada.Characters.Latin_1 but with the
types of the constants being Wide_Character
instead of Character. The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3(27)).
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Ada.Characters.Wide_Latin_9 (`a-cwila1.ads')
This child of Ada.Characters
provides a set of definitions corresponding to those in the
GNAT defined package Ada.Characters.Latin_9 but with the
types of the constants being Wide_Character
instead of Character. The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3(27)).
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Ada.Command_Line.Remove (`a-colire.ads')
This child of Ada.Command_Line
provides a mechanism for logically removing
arguments from the argument list. Once removed, an argument is not visible
to further calls on the subprograms in Ada.Command_Line will not
see the removed argument.
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Ada.Direct_IO.C_Streams (`a-diocst.ads')
This package provides subprograms that allow interfacing between
C streams and Direct_IO. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Exceptions.Is_Null_Occurrence (`a-einuoc.ads')
This child subprogram provides a way of testing for the null
exception occurrence (Null_Occurrence) without raising
an exception.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Sequential_IO.C_Streams (`a-siocst.ads')
This package provides subprograms that allow interfacing between
C streams and Sequential_IO. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Streams.Stream_IO.C_Streams (`a-ssicst.ads')
This package provides subprograms that allow interfacing between
C streams and Stream_IO. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Strings.Unbounded.Text_IO (`a-suteio.ads') This package provides subprograms for Text_IO for unbounded strings, avoiding the necessity for an intermediate operation with ordinary strings.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Strings.Wide_Unbounded.Wide_Text_IO (`a-swuwti.ads') This package provides subprograms for Text_IO for unbounded wide strings, avoiding the necessity for an intermediate operation with ordinary wide strings.
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Ada.Text_IO.C_Streams (`a-tiocst.ads')
This package provides subprograms that allow interfacing between
C streams and Text_IO. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ada.Wide_Text_IO.C_Streams (`a-wtcstr.ads')
This package provides subprograms that allow interfacing between
C streams and Wide_Text_IO. The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.
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GNAT.AWK (`g-awk.ads') Provides AWK-like parsing functions, with an easy interface for parsing one or more files containing formatted data. The file is viewed as a database where each record is a line and a field is a data element in this line.
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GNAT.Bubble_Sort_A (`g-busora.ads') Provides a general implementation of bubble sort usable for sorting arbitrary data items. Move and comparison procedures are provided by passing access-to-procedure values.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.Bubble_Sort_G (`g-busorg.ads')
Similar to Bubble_Sort_A except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.Calendar (`g-calend.ads')
Extends the facilities provided by Ada.Calendar to include handling
of days of the week, an extended Split and Time_Of capability.
Also provides conversion of Ada.Calendar.Time values to and from the
C timeval format.
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GNAT.Calendar.Time_IO (`g-catiio.ads') | [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.CRC32 (`g-crc32.ads') This package implements the CRC-32 algorithm. For a full description of this algorithm you should have a look at: "Computation of Cyclic Redundancy Checks via Table Look-Up", Communications of the ACM, Vol. 31 No. 8, pp. 1008-1013, Aug. 1988. Sarwate, D.V.
Provides an extended capability for formatted output of time values with full user control over the format. Modeled on the GNU Date specification.
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GNAT.Case_Util (`g-casuti.ads')
A set of simple routines for handling upper and lower casing of strings
without the overhead of the full casing tables
in Ada.Characters.Handling.
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GNAT.CGI (`g-cgi.ads') This is a package for interfacing a GNAT program with a Web server via the Common Gateway Interface (CGI). Basically this package parses the CGI parameters, which are a set of key/value pairs sent by the Web server. It builds a table whose index is the key and provides some services to deal with this table.
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GNAT.CGI.Cookie (`g-cgicoo.ads') This is a package to interface a GNAT program with a Web server via the Common Gateway Interface (CGI). It exports services to deal with Web cookies (piece of information kept in the Web client software).
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GNAT.CGI.Debug (`g-cgideb.ads') This is a package to help debugging CGI (Common Gateway Interface) programs written in Ada.
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GNAT.Command_Line (`g-comlin.ads')
Provides a high level interface to Ada.Command_Line facilities,
including the ability to scan for named switches with optional parameters
and expand file names using wild card notations.
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GNAT.Current_Exception (`g-curexc.ads') Provides access to information on the current exception that has been raised without the need for using the Ada-95 exception choice parameter specification syntax. This is particularly useful in simulating typical facilities for obtaining information about exceptions provided by Ada 83 compilers.
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GNAT.Debug_Pools (`g-debpoo.ads') Provide a debugging storage pools that helps tracking memory corruption problems. See section "Finding memory problems with GNAT Debug Pool" in the GNAT User's Guide.
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GNAT.Debug_Utilities (`g-debuti.ads') Provides a few useful utilities for debugging purposes, including conversion to and from string images of address values.
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GNAT.Directory_Operations (g-dirope.ads) Provides a set of routines for manipulating directories, including changing the current directory, making new directories, and scanning the files in a directory.
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GNAT.Dynamic_Tables (`g-dyntab.ads') A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.
This package provides a facility similar to that of GNAT.Table, except that this package declares a type that can be used to define dynamic instances of the table, while an instantiation of GNAT.Table creates a single instance of the table type.
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GNAT.Exception_Traces (`g-exctra.ads') Provides an interface allowing to control automatic output upon exception occurrences.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.Expect (`g-expect.ads') Provides a set of subprograms similar to what is available with the standard Tcl Expect tool. It allows you to easily spawn and communicate with an external process. You can send commands or inputs to the process, and compare the output with some expected regular expression. Currently GNAT.Expect is implemented on all native GNAT ports except for OpenVMS. It is not implemented for cross ports, and in particular is not implemented for VxWorks or LynxOS.
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GNAT.Float_Control (`g-flocon.ads') Provides an interface for resetting the floating-point processor into the mode required for correct semantic operation in Ada. Some third party library calls may cause this mode to be modified, and the Reset procedure in this package can be used to reestablish the required mode.
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GNAT.Heap_Sort_A (`g-hesora.ads') Provides a general implementation of heap sort usable for sorting arbitrary data items. Move and comparison procedures are provided by passing access-to-procedure values. The algorithm used is a modified heap sort that performs approximately N*log(N) comparisons in the worst case.
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GNAT.Heap_Sort_G (`g-hesorg.ads')
Similar to Heap_Sort_A except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.HTable (`g-htable.ads') A generic implementation of hash tables that can be used to hash arbitrary data. Provides two approaches, one a simple static approach, and the other allowing arbitrary dynamic hash tables.
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GNAT.IO (`g-io.ads') A simple preealborable input-output package that provides a subset of simple Text_IO functions for reading characters and strings from Standard_Input, and writing characters, strings and integers to either Standard_Output or Standard_Error.
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GNAT.IO_Aux (`g-io_aux.ads') Provides some auxiliary functions for use with Text_IO, including a test for whether a file exists, and functions for reading a line of text.
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GNAT.Lock_Files (`g-locfil.ads') Provides a general interface for using files as locks. Can be used for providing program level synchronization.
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GNAT.Most_Recent_Exception (`g-moreex.ads') Provides access to the most recently raised exception. Can be used for various logging purposes, including duplicating functionality of some Ada 83 implementation dependent extensions.
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GNAT.OS_Lib (`g-os_lib.ads') Provides a range of target independent operating system interface functions, including time/date management, file operations, subprocess management, including a portable spawn procedure, and access to environment variables and error return codes.
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GNAT.Regexp (`g-regexp.ads') A simple implementation of regular expressions, using a subset of regular expression syntax copied from familiar Unix style utilities. This is the simples of the three pattern matching packages provided, and is particularly suitable for "file globbing" applications.
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GNAT.Registry (`g-regist.ads') This is a high level binding to the Windows registry. It is possible to do simple things like reading a key value, creating a new key. For full registry API, but at a lower level of abstraction, refer to the Win32.Winreg package provided with the Win32Ada binding
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GNAT.Regpat (`g-regpat.ads') A complete implementation of Unix-style regular expression matching, copied from the original V7 style regular expression library written in C by Henry Spencer (and binary compatible with this C library).
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GNAT.Sockets (`g-socket.ads') A high level and portable interface to develop sockets based applications. This package is based on the sockets thin binding found in GNAT.Sockets.Thin. Currently GNAT.Sockets is implemented on all native GNAT ports except for OpenVMS. It is not implemented for the LynxOS cross port.
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GNAT.Source_Info (`g-souinf.ads') Provides subprograms that give access to source code information known at compile time, such as the current file name and line number.
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GNAT.Spell_Checker (`g-speche.ads') Provides a function for determining whether one string is a plausible near misspelling of another string.
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GNAT.Spitbol.Patterns (`g-spipat.ads') A complete implementation of SNOBOL4 style pattern matching. This is the most elaborate of the pattern matching packages provided. It fully duplicates the SNOBOL4 dynamic pattern construction and matching capabilities, using the efficient algorithm developed by Robert Dewar for the SPITBOL system.
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GNAT.Spitbol (`g-spitbo.ads') The top level package of the collection of SPITBOL-style functionality, this package provides basic SNOBOL4 string manipulation functions, such as Pad, Reverse, Trim, Substr capability, as well as a generic table function useful for constructing arbitrary mappings from strings in the style of the SNOBOL4 TABLE function.
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GNAT.Spitbol.Table_Boolean (`g-sptabo.ads')
A library level of instantiation of GNAT.Spitbol.Patterns.Table
for type Standard.Boolean, giving an implementation of sets of
string values.
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GNAT.Spitbol.Table_Integer (`g-sptain.ads')
A library level of instantiation of GNAT.Spitbol.Patterns.Table
for type Standard.Integer, giving an implementation of maps
from string to integer values.
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GNAT.Spitbol.Table_VString (`g-sptavs.ads') A library level of instantiation of GNAT.Spitbol.Patterns.Table for a variable length string type, giving an implementation of general maps from strings to strings.
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GNAT.Table (`g-table.ads') A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.
This package provides a facility similar to that of GNAT.Dynamic_Tables, except that this package declares a single instance of the table type, while an instantiation of GNAT.Dynamic_Tables creates a type that can be used to define dynamic instances of the table.
| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GNAT.Task_Lock (`g-tasloc.ads') A very simple facility for locking and unlocking sections of code using a single global task lock. Appropriate for use in situations where contention between tasks is very rarely expected.
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GNAT.Threads (`g-thread.ads') Provides facilities for creating and destroying threads with explicit calls. These threads are known to the GNAT run-time system. These subprograms are exported C-convention procedures intended to be called from foreign code. By using these primitives rather than directly calling operating systems routines, compatibility with the Ada tasking runt-time is provided.
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GNAT.Traceback (`g-traceb.ads') Provides a facility for obtaining non-symbolic traceback information, useful in various debugging situations.
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GNAT.Traceback.Symbolic (`g-trasym.ads') Provides symbolic traceback information that includes the subprogram name and line number information.
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Interfaces.C.Extensions (`i-cexten.ads') This package contains additional C-related definitions, intended for use with either manually or automatically generated bindings to C libraries.
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Interfaces.C.Streams (`i-cstrea.ads') This package is a binding for the most commonly used operations on C streams.
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Interfaces.CPP (`i-cpp.ads') This package provides facilities for use in interfacing to C++. It is primarily intended to be used in connection with automated tools for the generation of C++ interfaces.
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Interfaces.Os2lib (`i-os2lib.ads') This package provides interface definitions to the OS/2 library. It is a thin binding which is a direct translation of the various `<bse.h>' files.
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Interfaces.Os2lib.Errors (`i-os2err.ads') This package provides definitions of the OS/2 error codes.
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Interfaces.Os2lib.Synchronization (`i-os2syn.ads')
This is a child package that provides definitions for interfacing
to the OS/2 synchronization primitives.
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Interfaces.Os2lib.Threads (`i-os2thr.ads')
This is a child package that provides definitions for interfacing
to the OS/2 thread primitives.
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Interfaces.Packed_Decimal (`i-pacdec.ads') This package provides a set of routines for conversions to and from a packed decimal format compatible with that used on IBM mainframes.
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Interfaces.VxWorks (`i-vxwork.ads') This package provides a limited binding to the VxWorks API. In particular, it interfaces with the VxWorks hardware interrupt facilities.
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Interfaces.VxWorks.IO (`i-vxwoio.ads') This package provides a limited binding to the VxWorks' I/O API. In particular, it provides procedures that enable the use of Get_Immediate under VxWorks.
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System.Address_Image (`s-addima.ads') This function provides a useful debugging function that gives an (implementation dependent) string which identifies an address.
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System.Assertions (`s-assert.ads') This package provides the declaration of the exception raised by an run-time assertion failure, as well as the routine that is used internally to raise this assertion.
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System.Partition_Interface (`s-parint.ads')
This package provides facilities for partition interfacing. It
is used primarily in a distribution context when using Annex E
with GLADE.
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System.Task_Info (`s-tasinf.ads')
This package provides target dependent functionality that is used
to support the Task_Info pragma
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System.Wch_Cnv (`s-wchcnv.ads')
This package provides routines for converting between
wide characters and a representation as a value of type
Standard.String, using a specified wide character
encoding method. It uses definitions in
package System.Wch_Con.
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System.Wch_Con (`s-wchcon.ads')
This package provides definitions and descriptions of
the various methods used for encoding wide characters
in ordinary strings. These definitions are used by
the package System.Wch_Cnv.
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10.1 Interfacing to C 10.2 Interfacing to C++ 10.3 Interfacing to COBOL 10.4 Interfacing to Fortran 10.5 Interfacing to non-GNAT Ada code
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Interfacing to C with GNAT can use one of two approaches:
Interfaces.C may be used.
Pragma Convention C maybe applied to Ada types, but mostly has no
effect, since this is the default. The following table shows the
correspondence between Ada scalar types and the corresponding C types.
Integer
int
Short_Integer
short
Short_Short_Integer
signed char
Long_Integer
long
Long_Long_Integer
long long
Short_Float
float
Float
float
Long_Float
double
Long_Long_Float
Convention C is specified, which causes them to have int
length. Without pragma Convention C, Ada enumeration types map to
8, 16, or 32 bits (i.e. C types signed char, short, int, respectively)
depending on the number of values passed. This is the only case in which
pragma Convention C affects the representation of an Ada type.
type'Size value in Ada.
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The interface to C++ makes use of the following pragmas, which are primarily intended to be constructed automatically using a binding generator tool, although it is possible to construct them by hand. Ada Core Technologies does not currently supply a suitable binding generator tool.
Using these pragmas it is possible to achieve complete inter-operability between Ada tagged types and C class definitions. See 1. Implementation Defined Pragmas for more details.
pragma CPP_Class ([Entity =>] local_name)
pragma CPP_Constructor ([Entity =>] local_name)
Import) as corresponding to a C++ constructor.
pragma CPP_Vtable ...
CPP_Vtable pragma can be present for each component of type
CPP.Interfaces.Vtable_Ptr in a record to which pragma CPP_Class
applies.
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Interfacing to COBOL is achieved as described in section B.4 of the Ada 95 reference manual.
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Interfacing to Fortran is achieved as described in section B.5 of the
reference manual. The pragma Convention Fortran, applied to a
multi-dimensional array causes the array to be stored in column-major
order as required for convenient interface to Fortran.
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It is possible to specify the convention Ada in a pragma Import or
pragma Export. However this refers to the calling conventions used
by GNAT, which may or may not be similar enough to those used by
some other Ada 83 or Ada 95 compiler to allow interoperation.
If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).
In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.
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Package Machine_Code provides machine code support as described
in the Ada 95 Reference Manual in two separate forms:
The two features are similar, and both closely related to the mechanism
provided by the asm instruction in the GNU C compiler. Full understanding
and use of the facilities in this package requires understanding the asm
instruction as described in
Using and Porting the GNU Compiler Collection (GCC) by Richard
Stallman. Calls to the function Asm and the procedure Asm
have identical semantic restrictions and effects as described below.
Both are provided so that the procedure call can be used as a statement,
and the function call can be used to form a code_statement.
The first example given in the GCC documentation is the C asm
instruction:
asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));
|
The equivalent can be written for GNAT as:
Asm ("fsinx %1 %0",
My_Float'Asm_Output ("=f", result),
My_Float'Asm_Input ("f", angle));
|
The first argument to Asm is the assembler template, and is
identical to what is used in GNU C. This string must be a static
expression. The second argument is the output operand list. It is
either a single Asm_Output attribute reference, or a list of such
references enclosed in parentheses (technically an array aggregate of
such references).
The Asm_Output attribute denotes a function that takes two
parameters. The first is a string, the second is the name of a variable
of the type designated by the attribute prefix. The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g. what kind of register is
required). The second argument is the variable to be updated with the
result. The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end. If there are no output operands, then this argument may
either be omitted, or explicitly given as No_Output_Operands.
The second argument of my_float'Asm_Output functions as
though it were an out parameter, which is a little curious, but
all names have the form of expressions, so there is no syntactic
irregularity, even though normally functions would not be permitted
out parameters. The third argument is the list of input
operands. It is either a single Asm_Input attribute reference, or
a list of such references enclosed in parentheses (technically an array
aggregate of such references).
The Asm_Input attribute denotes a function that takes two
parameters. The first is a string, the second is an expression of the
type designated by the prefix. The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g. what kind of register is required). The second argument is the
value to be used as the input argument. The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
If there are no input operands, this argument may either be omitted, or
explicitly given as No_Input_Operands. The fourth argument, not
present in the above example, is a list of register names, called the
clobber argument. This argument, if given, must be a static string
expression, and is a space or comma separated list of names of registers
that must be considered destroyed as a result of the Asm call. If
this argument is the null string (the default value), then the code
generator assumes that no additional registers are destroyed.
The fifth argument, not present in the above example, called the
volatile argument, is by default False. It can be set to
the literal value True to indicate to the code generator that all
optimizations with respect to the instruction specified should be
suppressed, and that in particular, for an instruction that has outputs,
the instruction will still be generated, even if none of the outputs are
used. See the full description in the GCC manual for further details.
The Asm subprograms may be used in two ways. First the procedure
forms can be used anywhere a procedure call would be valid, and
correspond to what the RM calls "intrinsic" routines. Such calls can
be used to intersperse machine instructions with other Ada statements.
Second, the function forms, which return a dummy value of the limited
private type Asm_Insn, can be used in code statements, and indeed
this is the only context where such calls are allowed. Code statements
appear as aggregates of the form:
Asm_Insn'(Asm (...)); Asm_Insn'(Asm_Volatile (...)); |
In accordance with RM rules, such code statements are allowed only within subprograms whose entire body consists of such statements. It is not permissible to intermix such statements with other Ada statements.
Typically the form using intrinsic procedure calls is more convenient
and more flexible. The code statement form is provided to meet the RM
suggestion that such a facility should be made available. The following
is the exact syntax of the call to Asm (of course if named notation is
used, the arguments may be given in arbitrary order, following the
normal rules for use of positional and named arguments)
ASM_CALL ::= Asm (
[Template =>] static_string_EXPRESSION
[,[Outputs =>] OUTPUT_OPERAND_LIST ]
[,[Inputs =>] INPUT_OPERAND_LIST ]
[,[Clobber =>] static_string_EXPRESSION ]
[,[Volatile =>] static_boolean_EXPRESSION] )
OUTPUT_OPERAND_LIST ::=
No_Output_Operands
| OUTPUT_OPERAND_ATTRIBUTE
| (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})
OUTPUT_OPERAND_ATTRIBUTE ::=
SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)
INPUT_OPERAND_LIST ::=
No_Input_Operands
| INPUT_OPERAND_ATTRIBUTE
| (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})
INPUT_OPERAND_ATTRIBUTE ::=
SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)
|
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12.1 Mapping Ada Tasks onto the Underlying Kernel Threads 12.2 Ensuring Compliance with the Real-Time Annex
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GNAT run-time system comprises two layers:
In GNAT, Ada's tasking services rely on a platform and OS independent layer known as GNARL. This code is responsible for implementing the correct semantics of Ada's task creation, rendezvous, protected operations etc.
GNARL decomposes Ada's tasking semantics into simpler lower level operations such as create a thread, set the priority of a thread, yield, create a lock, lock/unlock, etc. The spec for these low-level operations constitutes GNULLI, the GNULL Interface. This interface is directly inspired from the POSIX real-time API.
If the underlying executive or OS implements the POSIX standard faithfully, the GNULL Interface maps as is to the services offered by the underlying kernel. Otherwise, some target dependent glue code maps the services offered by the underlying kernel to the semantics expected by GNARL.
Whatever the underlying OS (VxWorks, UNIX, OS/2, Windows NT, etc.) the key point is that each Ada task is mapped on a thread in the underlying kernel. For example, in the case of VxWorks, one Ada task = one VxWorks task.
In addition Ada task priorities map onto the underlying thread priorities. Mapping Ada tasks onto the underlying kernel threads has several advantages:
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The reader will be quick to notice that while mapping Ada tasks onto the underlying threads has significant advantages, it does create some complications when it comes to respecting the scheduling semantics specified in the real-time annex (Annex D).
For instance Annex D requires that for the FIFO_Within_Priorities scheduling policy we have:
When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority, except in the case where the active priority is lowered due to the loss of inherited priority, in which case the task is added at the head of the ready queue for its new active priority. |
While most kernels do put tasks at the end of the priority queue when a task changes its priority, (which respects the main FIFO_Within_Priorities requirement), almost none keep a thread at the beginning of its priority queue when its priority drops from the loss of inherited priority.
As a result most vendors have provided incomplete Annex D implementations.
The GNAT run-time, has a nice cooperative solution to this problem which ensures that accurate FIFO_Within_Priorities semantics are respected.
The principle is as follows. When an Ada task T is about to start running, it checks whether some other Ada task R with the same priority as T has been suspended due to the loss of priority inheritance. If this is the case, T yields and is placed at the end of its priority queue. When R arrives at the front of the queue it executes.
Note that this simple scheme preserves the relative order of the tasks that were ready to execute in the priority queue where R has been placed at the end.
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Aggregate have a rich syntax and allow the user to specify the values of complex data structures by means of a single construct. As a result, the code generated for aggregates can be quite complex and involve loops, case statements and multiple assignments. In the simplest cases, however, the compiler will recognize aggregates whose components and constraints are fully static, and in those cases the compiler will generate little or no executable code. The following is an outline of the code that GNAT generates for various aggregate constructs. For further details, the user will find it useful to examine the output produced by the -gnatG flag to see the expanded source that is input to the code generator. The user will also want to examine the assembly code generated at various levels of optimization.
The code generated for aggregates depends on the context, the component values, and the type. In the context of an object declaration the code generated is generally simpler than in the case of an assignment. As a general rule, static component values and static subtypes also lead to simpler code.
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type One_Dim is array (1..10) of integer;
ar0 : constant One_Dim := ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
|
GNAT generates no executable code: the constant ar0 is placed in static memory. The same is true for constant aggregates with named associations:
Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1=> 1);
Cr3 : constant One_Dim := (others => 7777);
|
type two_dim is array (1..3, 1..3) of integer;
Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));
|
type ar1b is array (1..3) of boolean; type ar_ar is array (1..3) of ar1b; None : constant ar1b := (others => false); -- fully static None2 : constant ar_ar := (1..3 => None); -- fully static |
Zero1: constant two_dim := (1..3 => (1..3 => 0)); Zero2: constant two_dim := (others => (others => 0)); |
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others cannot be used. GNAT determines the bounds for the actual
subtype of the aggregate, and allocates the aggregate statically as well. No
code is generated for the following:
type One_Unc is array (natural range <>) of integer;
Cr_Unc : constant One_Unc := (12,24,36);
|
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Cr_Var1 : One_Dim := (2, 5, 7, 11);
Cr_Var2 : One_Dim := (others > -1);
|
Cr_Var1 (1) := 2;
Cr_Var1 (2) := 3;
Cr_Var1 (3) := 5;
Cr_Var1 (4) := 11;
for I in Cr_Var2'range loop
Cr_Var2 (I) := =-1;
end loop;
|
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A(1) := A(2);
A(2) := A(1);
|
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Ada 95 defines a number of specialized needs annexes, which are not required in all implementations. However, as described in this chapter, GNAT implements all of these special needs annexes:
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This chapter contains sections that describe compatibility issues between GNAT and other Ada 83 and Ada 95 compilation systems, to aid in porting applications developed in other Ada environments.
15.1 Compatibility with Ada 83 15.4 Compatibility with DEC Ada 83 15.2 Compatibility with Other Ada 95 Systems 15.3 Representation Clauses
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Ada 95 is designed to be highly upwards compatible with Ada 83. In particular, the design intention is that the difficulties associated with moving from Ada 83 to Ada 95 should be no greater than those that occur when moving from one Ada 83 system to another.
However, there are a number of points at which there are minor incompatibilities. The Ada 95 Annotated Reference Manual contains full details of these issues, and should be consulted for a complete treatment. In practice the following are the most likely issues to be encountered.
Standard.Character is now the full 256 characters of Latin-1,
whereas in most Ada 83 implementations it was restricted to 128 characters.
This may show up as compile time or runtime errors. The desirable fix is to
adapt the program to accommodate the full character set, but in some cases
it may be convenient to define a subtype or derived type of Character that
covers only the restricted range.
abstract, aliased, protected,
requeue, tagged, and until are reserved in Ada 95.
Existing Ada 83 code using any of these identifiers must be edited to
use some alternative name.
A particular case is that representation pragmas (including the
extended DEC Ada 83 compatibility pragmas such as Export_Procedure), cannot
be applied to a subprogram body. If necessary, a separate subprogram
declaration must be introduced to which the pragma can be applied.
Numeric_Error is now the same as Constraint_Error
Numeric_Error is a renaming of Constraint_Error.
This means that it is illegal to have separate exception handlers for
the two exceptions. The fix is simply to remove the handler for the
Numeric_Error case (since even in Ada 83, a compiler was free to raise
Constraint_Error in place of Numeric_Error in all cases).
String) as
the actual for a generic formal private type, but then the instantiation
would be illegal if there were any instances of declarations of variables
of this type in the generic body. In Ada 95, to avoid this clear violation
of the contract model, the generic declaration clearly indicates whether
or not such instantiations are permitted. If a generic formal parameter
has explicit unknown discriminants, indicated by using (<>) after the
type name, then it can be instantiated with indefinite types, but no
variables can be declared of this type. Any attempt to declare a variable
will result in an illegality at the time the generic is declared. If the
(<>) notation is not used, then it is illegal to instantiate the generic
with an indefinite type. This will show up as a compile time error, and
the fix is usually simply to add the (<>) to the generic declaration.
All implementations of GNAT provide a switch that causes GNAT to operate in Ada 83 mode. In this mode, some but not all compatibility problems of the type described above are handled automatically. For example, the new Ada 95 protected keywords are not recognized in this mode. However, in practice, it is usually advisable to make the necessary modifications to the program to remove the need for using this switch.
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Providing that programs avoid the use of implementation dependent and implementation defined features of Ada 95, as documented in the Ada 95 reference manual, there should be a high degree of portability between GNAT and other Ada 95 systems. The following are specific items which have proved troublesome in moving GNAT programs to other Ada 95 compilers, but do not affect porting code to GNAT.
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The Ada 83 reference manual was quite vague in describing both the minimal required implementation of representation clauses, and also their precise effects. The Ada 95 reference manual is much more explicit, but the minimal set of capabilities required in Ada 95 is quite limited.
GNAT implements the full required set of capabilities described in the Ada 95 reference manual, but also goes much beyond this, and in particular an effort has been made to be compatible with existing Ada 83 usage to the greatest extent possible.
A few cases exist in which Ada 83 compiler behavior is incompatible with requirements in the Ada 95 reference manual. These are instances of intentional or accidental dependence on specific implementation dependent characteristics of these Ada 83 compilers. The following is a list of the cases most likely to arise in existing legacy Ada 83 code.
Pack, or for more fine tuned control, provide
a Component_Size clause.
To get around this problem, GNAT also permits the use of "thin pointers" for access types in this case (where the designated type is an unconstrained array type). These thin pointers are indeed the same size as a System.Address value. To specify a thin pointer, use a size clause for the type, for example:
type X is access all String; for X'Size use Standard'Address_Size; |
which will cause the type X to be represented using a single pointer. When using this representation, the bounds are right behind the array. This representation is slightly less efficient, and does not allow quite such flexibility in the use of foreign pointers or in using the Unrestricted_Access attribute to create pointers to non-aliased objects. But for any standard portable use of the access type it will work in a functionally correct manner and allow porting of existing code. Note that another way of forcing a thin pointer representation is to use a component size clause for the element size in an array, or a record representation clause for an access field in a record.
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The VMS version of GNAT fully implements all the pragmas and attributes provided by DEC Ada 83, as well as providing the standard DEC Ada 83 libraries, including Starlet. In addition, data layouts and parameter passing conventions are highly compatible. This means that porting existing DEC Ada 83 code to GNAT in VMS systems should be easier than most other porting efforts. The following are some of the most significant differences between GNAT and DEC Ada 83.
TO_ADDRESS(INTEGER) TO_ADDRESS(UNSIGNED_LONGWORD) TO_ADDRESS(universal_integer) |
The version of TO_ADDRESS taking a universal integer argument is in fact an extension to Ada 83 not strictly compatible with the reference manual. In GNAT, we are constrained to be exactly compatible with the standard, and this means we cannot provide this capability. In DEC Ada 83, the point of this definition is to deal with a call like:
TO_ADDRESS (16#12777#); |
Normally, according to the Ada 83 standard, one would expect this to be ambiguous, since it matches both the INTEGER and UNSIGNED_LONGWORD forms of TO_ADDRESS. However, in DEC Ada 83, there is no ambiguity, since the definition using universal_integer takes precedence.
In GNAT, since the version with universal_integer cannot be supplied, it is not possible to be 100% compatible. Since there are many programs using numeric constants for the argument to TO_ADDRESS, the decision in GNAT was to change the name of the function in the UNSIGNED_LONGWORD case, so the declarations provided in the GNAT version of AUX_Dec are:
function To_Address (X : Integer) return Address; pragma Pure_Function (To_Address); function To_Address_Long (X : Unsigned_Longword) return Address; pragma Pure_Function (To_Address_Long); |
This means that programs using TO_ADDRESS for UNSIGNED_LONGWORD must change the name to TO_ADDRESS_LONG.
For full details on these and other less significant compatibility issues, see appendix E of the Digital publication entitled DEC Ada, Technical Overview and Comparison on DIGITAL Platforms.
For GNAT running on other than VMS systems, all the DEC Ada 83 pragmas and attributes are recognized, although only a subset of them can sensibly be implemented. The description of pragmas in this reference manual indicates whether or not they are applicable to non-VMS systems.
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Version 1.1, March 2000
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A B C D E F G H I L M N O P R S T U V W Z |
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Ada.Characters.Latin_9 (`a-chlat9.ads')
Ada.Characters.Wide_Latin_1 (`a-cwila1.ads')
Ada.Characters.Wide_Latin_9 (`a-cwila1.ads')
Ada.Command_Line.Remove (`a-colire.ads')
Ada.Direct_IO.C_Streams (`a-diocst.ads')
Ada.Exceptions.Is_Null_Occurrence (`a-einuoc.ads')
Ada.Sequential_IO.C_Streams (`a-siocst.ads')
Ada.Streams.Stream_IO.C_Streams (`a-ssicst.ads')
Ada.Strings.Unbounded.Text_IO (`a-suteio.ads')
Ada.Strings.Wide_Unbounded.Wide_Text_IO (`a-swuwti.ads')
Ada.Text_IO.C_Streams (`a-tiocst.ads')
Ada.Wide_Text_IO.C_Streams (`a-wtcstr.ads')
GNAT.AWK (`g-awk.ads')
GNAT.Bubble_Sort_A (`g-busora.ads')
GNAT.Bubble_Sort_G (`g-busorg.ads')
GNAT.Calendar (`g-calend.ads')
GNAT.Calendar.Time_IO (`g-catiio.ads')
GNAT.CRC32 (`g-crc32.ads')
GNAT.Case_Util (`g-casuti.ads')
GNAT.CGI (`g-cgi.ads')
GNAT.CGI.Cookie (`g-cgicoo.ads')
GNAT.CGI.Debug (`g-cgideb.ads')
GNAT.Command_Line (`g-comlin.ads')
GNAT.Current_Exception (`g-curexc.ads')
GNAT.Debug_Pools (`g-debpoo.ads')
GNAT.Debug_Utilities (`g-debuti.ads')
GNAT.Directory_Operations (g-dirope.ads)
GNAT.Dynamic_Tables (`g-dyntab.ads')
GNAT.Exception_Traces (`g-exctra.ads')
GNAT.Expect (`g-expect.ads')
GNAT.Float_Control (`g-flocon.ads')
GNAT.Heap_Sort_A (`g-hesora.ads')
GNAT.Heap_Sort_G (`g-hesorg.ads')
GNAT.HTable (`g-htable.ads')
GNAT.IO (`g-io.ads')
GNAT.IO_Aux (`g-io_aux.ads')
GNAT.Lock_Files (`g-locfil.ads')
GNAT.Most_Recent_Exception (`g-moreex.ads')
GNAT.OS_Lib (`g-os_lib.ads')
GNAT.Regexp (`g-regexp.ads')
GNAT.Registry (`g-regist.ads')
GNAT.Regpat (`g-regpat.ads')
GNAT.Sockets (`g-socket.ads')
GNAT.Source_Info (`g-souinf.ads')
GNAT.Spell_Checker (`g-speche.ads')
GNAT.Spitbol.Patterns (`g-spipat.ads')
GNAT.Spitbol (`g-spitbo.ads')
GNAT.Spitbol.Table_Boolean (`g-sptabo.ads')
GNAT.Spitbol.Table_Integer (`g-sptain.ads')
GNAT.Spitbol.Table_VString (`g-sptavs.ads')
GNAT.Table (`g-table.ads')
GNAT.Task_Lock (`g-tasloc.ads')
GNAT.Threads (`g-thread.ads')
GNAT.Traceback (`g-traceb.ads')
GNAT.Traceback.Symbolic (`g-trasym.ads')
Interfaces.C.Extensions (`i-cexten.ads')
Interfaces.C.Streams (`i-cstrea.ads')
Interfaces.CPP (`i-cpp.ads')
Interfaces.Os2lib (`i-os2lib.ads')
Interfaces.Os2lib.Errors (`i-os2err.ads')
Interfaces.Os2lib.Synchronization (`i-os2syn.ads')
Interfaces.Os2lib.Threads (`i-os2thr.ads')
Interfaces.Packed_Decimal (`i-pacdec.ads')
Interfaces.VxWorks (`i-vxwork.ads')
Interfaces.VxWorks.IO (`i-vxwoio.ads')
System.Address_Image (`s-addima.ads')
System.Assertions (`s-assert.ads')
System.Partition_Interface (`s-parint.ads')
System.Task_Info (`s-tasinf.ads')
System.Wch_Cnv (`s-wchcnv.ads')
System.Wch_Con (`s-wchcon.ads')
| [Top] | [Contents] | [Index] | [ ? ] |
About This Guide
1. Implementation Defined Pragmas
2. Implementation Defined Attributes
3. Implementation Advice
4. Implementation Defined Characteristics
5. Intrinsic Subprograms
6. Representation Clauses and Pragmas
7. Standard Library Routines
8. The Implementation of Standard I/O
9. The GNAT Library
10. Interfacing to Other Languages
11. Machine Code Insertions
12. GNAT Implementation of Tasking
13. Code generation for array aggregates
14. Specialized Needs Annexes
15. Compatibility Guide
A. GNU Free Documentation License
Index
| [Top] | [Contents] | [Index] | [ ? ] |
| Button | Name | Go to | From 1.2.3 go to |
|---|---|---|---|
| [ < ] | Back | previous section in reading order | 1.2.2 |
| [ > ] | Forward | next section in reading order | 1.2.4 |
| [ << ] | FastBack | previous or up-and-previous section | 1.1 |
| [ Up ] | Up | up section | 1.2 |
| [ >> ] | FastForward | next or up-and-next section | 1.3 |
| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
| [Index] | Index | concept index | |
| [ ? ] | About | this page |