Re: Assembly, GCC and the linker

[heard@vvnet.com (C. M. Heard/VVNET, Inc.)]

> I have a few #define in a .h file that are needed by C, asm
> and linker. Is there a way to do this without declaring them in
>  3 files.

It is definitely possible to share .h files between C and assembler;
almost _all_ of the header files in the system I'm now working on
are used that way.  What I do is to hide the C-specific stuff under
an #if defined(_LANGUAGE_C) clause, and make the assembler-specific
stuff under the #else, as in the following file:

/*
 * gdb_stub.h - interface to gdb stub (debug agent)
 *
 *                   THIS SOFTWARE IS NOT COPYRIGHTED
 *
 *  The following software is offered for use in the public domain.
 *  There is no warranty with regard to this software or its performance
 *  and the user must accept the software "AS IS" with all faults.
 *
 *  THE CONTRIBUTORS DISCLAIM ANY WARRANTIES, EXPRESS OR IMPLIED, WITH
 *  REGARD TO THIS SOFTWARE INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 *  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

#ifndef _GDB_STUB_H
#define _GDB_STUB_H

#if defined(_LANGUAGE_C)

void handle_debug_exception (long long *saved_registers);

#else /* if !defined(_LANGUAGE_C) */

        .globl  handle_debug_exception

#endif /* defined(_LANGUAGE_C) */

#endif /* _GDB_STUB_H */


In the general case (though not in this example) there would be #defines
outside the #if defined(_LANGUAGE_C) clause, and these would be visible
to both the C and assembler modules.

> I tried to #include the .h file in the assembley file and feed it to
> the c preprocessor. The output had the #include file data in it, so 'as'
> choked.
> 
> What parameters should I pass to the preprocessor to get it to output
> something that can be fed to AS?
>  or How can I do the above step in 1 shot.

gcc recognizes the suffix .S (upper case S) as meaning an assembler file
which should go through the preprocessor, and will process that correctly.

> I am compiling in a dos box under NT and make (or NT) doesn't see  .S and
> .s files as different, it treats both as assembler. Is this an issue?

YES IT MOST DEFINITELY IS.  Although I use x86 Solaris, my partner refuses
to use Unix of any flavour, so I set up a compiler for him which runs in
a dos box under Windows 3.11.  On the latter system we change all of the
suffixes from .S (which can't be represented under dos -- go32 lowers the
case of all 8+3 filenames) to .asm (which _can_ be represented) and add
the switch

-x assembler-with-cpp

to the gcc invocation line.  Not elegant, but it does the trick.

> I need to allocate a block of memory in the linker file.
> The size of the block is a function of values declared in the .h 
> file but the location must be specified by the linker
> 
> Can I pass a value from a 'c' style #include file to the linker?

If you find out how to do this please tell me how you did it.

Mike
--
C. M. Heard
VVNET, Inc.                           phone:  +1 408 247 9376
4040 Moorpark Ave. Suite 206          fax:    +1 408 244 3651
San Jose, CA 95117 USA                e-mail: heard@vvnet.com