Calling Ada routines from C programs.

Calling Ada routines from C programs.

[Mark Culp]

We have a large application program developed in C that we would like to
port to an underlying operating system (CAIS-A) which is written in Ada.

Before we start on this attempt, we realize that our largest problem is
going to be trying to call an Ada routine from our C program.

I have attempted to call an Ada routine from a C sub-routine (of an Ada
main procedure) using the Telesoft Ada compiler, but have had no success
with the linker resolving the Ada's routine name.

Here is my test Ada source program:

	with Text_IO;

	procedure Ada_Routine is
	begin
		Text_IO.Put_Line( "Ada Routine" );
	end Ada_Routine;

	with Text_IO;
	with Ada_Routine;

	procedure Test_C_Interface is
		procedure C_Main_Routine;
		pragma Interface( C, C_Main_Routine );
	begin
		Text_IO.Put_Line( "Begin" );
		C_Main_Routine;
		Text_IO.Put_Line( "End" );
	end Test_C_Interface;

and here is my test C routine:

	#include <stdio.h>

	void c_main_routine()
	{
		printf( "c_main_routine begin\n" );
		ada_routine();
		printf( "c_main_routine end\n" );
	}

I have tried variations on the "ada_routine()" call, but with no success.

If you have attempted to do this, or know how to solve this problem,
I would be grateful to here from you.  Please email, I will summarize.

Thanks.

Mark.

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Re: Calling Ada routines from C programs.

[Jim Showalter]

mark@array.UUCP (Mark Culp) writes:

I believe you are trying to do this backwards. Pragma Interface allows
Ada to call other languages. I do not believe the LRM specifies how
OTHER languages are to call Ada--that would seem to be more the job
of the linker for the other language than of Ada.

Thus, in your example you have written the Ada Test_C_Interface to be able to
call a C function called "C_Main_Routine". This should work (presuming
you have tools sufficient to link the two languages). But then you try
to call ada_routine from your C program. Does your C linker know how
to do this? If not, this won't work. 
--
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Re: Calling Ada routines from C programs.

[Karl Keyte]

The problem with trying to call Ada routines from another language is that
quite frequently the Ada system requires its complete environment in order
to function.  This prevents single routines being called.  A common way
out it to wrap the entire system, say a C program, as an Ada main program
which simply calls the C '_main' in order to start it.  This then allows
the 'C' to call Ada services.

Karl

Re: Calling Ada routines from C programs.

[David Emery]

Karl Keyte noted "the Ada system requires its complete environment in
order to function".  He's almost right.  The real issue is
elaboration.  Before an Ada subprogram is called, it must be
elaborated, along with everything it needs.  Furthermore, library
units must be elaborated before the main program starts.  This is what
must be done before a C program can invoke an Ada operation.  On at
least one compiler, there is a jump table containing the addresses of
the elaboration routines for everything that needs to be elaborated
before the main routine is invoked.  This table is "sorted" in
elaboration order (e.g.  pragma ELABORATE, etc is taken into account.)
It would be sufficient to have the C "_main" execute this code before
(or during) the first call to any Ada routine.  But, of course, how to
do this is highly compiler-dependent.  

In the short run, Karl's technique (having an Ada wrapper invoke the C
"_main" entry) should work.

				dave emery
				emery@aries.mitre.org

Re: Calling Ada routines from C programs.

[Bruce Bauman]

For compilers which support pragma suppress, doing a pragma
supress(elaboration_check) on those routines which you wish to call
from C is often an expedient solution.

Of course, you can run into other problems besides elaboration,
depending upon the implementation...
--
Bruce Bauman	email: boot@osf.org		phone: 617-621-7205
Open Software Foundation, 11 Cambridge Center, Cambridge, MA, 02142

	Not really a programmer, but I play one on TV