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2.10.2 Calling Conventions

GNAT follows standard calling sequence conventions and will thus interface to any other language that also follows these conventions. The following Convention identifiers are recognized by GNAT:

This indicates that the standard Ada calling sequence will be used and all Ada data items may be passed without any limitations in the case where GNAT is used to generate both the caller and callee. It is also possible to mix GNAT generated code and code generated by another Ada compiler. In this case, the data types should be restricted to simple cases, including primitive types. Whether complex data types can be passed depends on the situation. Probably it is safe to pass simple arrays, such as arrays of integers or floats. Records may or may not work, depending on whether both compilers lay them out identically. Complex structures involving variant records, access parameters, tasks, or protected types, are unlikely to be able to be passed.

Note that in the case of GNAT running on a platform that supports HP Ada 83, a higher degree of compatibility can be guaranteed, and in particular records are layed out in an identical manner in the two compilers. Note also that if output from two different compilers is mixed, the program is responsible for dealing with elaboration issues. Probably the safest approach is to write the main program in the version of Ada other than GNAT, so that it takes care of its own elaboration requirements, and then call the GNAT-generated adainit procedure to ensure elaboration of the GNAT components. Consult the documentation of the other Ada compiler for further details on elaboration.

However, it is not possible to mix the tasking run time of GNAT and HP Ada 83, All the tasking operations must either be entirely within GNAT compiled sections of the program, or entirely within HP Ada 83 compiled sections of the program.

Specifies assembler as the convention. In practice this has the same effect as convention Ada (but is not equivalent in the sense of being considered the same convention).

Equivalent to Assembler.

Data will be passed according to the conventions described in section B.4 of the Ada Reference Manual.

Data will be passed according to the conventions described in section B.3 of the Ada Reference Manual.

A note on interfacing to a C “varargs” function:

Equivalent to C.

Equivalent to C.

C_Plus_Plus (or CPP)
This stands for C++. For most purposes this is identical to C. See the separate description of the specialized GNAT pragmas relating to C++ interfacing for further details.

Data will be passed according to the conventions described in section B.5 of the Ada Reference Manual.
This applies to an intrinsic operation, as defined in the Ada Reference Manual. If a pragma Import (Intrinsic) applies to a subprogram, this means that the body of the subprogram is provided by the compiler itself, usually by means of an efficient code sequence, and that the user does not supply an explicit body for it. In an application program, the pragma may be applied to the following sets of names:

This is relevant only to Windows XP/2000/NT implementations of GNAT, and specifies that the Stdcall calling sequence will be used, as defined by the NT API. Nevertheless, to ease building cross-platform bindings this convention will be handled as a C calling convention on non-Windows platforms.

This is equivalent to Stdcall.

This is equivalent to Stdcall.

This is a special convention that indicates that the compiler should provide a stub body that raises Program_Error.

GNAT additionally provides a useful pragma Convention_Identifier that can be used to parametrize conventions and allow additional synonyms to be specified. For example if you have legacy code in which the convention identifier Fortran77 was used for Fortran, you can use the configuration pragma:

     pragma Convention_Identifier (Fortran77, Fortran);

And from now on the identifier Fortran77 may be used as a convention identifier (for example in an Import pragma) with the same meaning as Fortran.