#include header with C declarations in an assembly file without errors?

Question

I have an assembly file (asm.S) which needs a constant #define'd in a C header file (c_decls.h). The header file contains C function declarations in addition to the #define I want. Unfortunately, gcc barfs when trying to compile the assembly file. For example,

c_decls.h

#ifndef __c_decls_h__
#define __c_decls_h__

#define I_NEED_THIS 0xBEEF
int foo(int bar);

#endif

asm.S

#include "c_decls.h"

.globl main
main:
    pushl %ebp
    movl %esp, %ebp
    movl $I_NEED_THIS, %eax
    leave
    ret

Output

> gcc -m32 asm.S
c_decls.h: Assembler messages:
c_decls.h:6: Error: junk '(int bar)' after expression
c_decls.h:6: Error: suffix or operands invalid for 'int'

Is there a way to #include a C header file that contains function declarations in an assembly file? (Changing the header or moving/redefining the #define is not an option.)

Solution 1

Use the -dM option for cpp to get just the #defines out of your header files, and include that file instead.

cpp -dM c_decls.h > bare_c_decls.h

Now include bare_c_decls.h in your .S file. And if you can't change the #include in the .S file, generate the bare header files in another directory, and put that include path on your compiler/assembler command line, ahead of everything else.

And finally, you can wrap this all up in a makefile so your "bare" header files are generated automatically.

Solution 2

That's simle: In your .S file use

#define __ASSEMBLY__

In your .C file use

#undef __ASSEMBLY__

Then in .h file place condition

       #ifdef __ASSEMBLY__
                  // here declarations only for assembler
       #else
                  // here only for C
       #endif
                  // and here - defines suitable for both

Comments

[user3124812]

Header should be included with 'C' like include, #include my_defs.h, not 'ASM' like .include my_defs.h. In last case #ifdef, #else, #endif will be ignored by asm parser. Just stumble over this...

[user3124812]

Simple -dM doesn't work for me. It returns nothing. -E -dM dumps all defines, including one which is defined in a header. There only inconvenience that there are more than 700 defines in the list.

[payne]

What version of cpp are you using? (Note the question is related to gcc)

[user3124812]

It's GNU GCC, arm-none-eabi-g++ (GNU Tools for ARM Embedded Processors) 4.9.3 20150529 (release)

[Peter Cordes]

@user3124812: You probably ran gcc -dM rather than cpp -dM. You should use gcc -E -dM with all other compile options you normally use, to make sure all #ifdefs in your headers get the same pre-defined macros. (e.g. maybe you have #ifdef __AVX__ in a header, and define something differently if building on an x86 target with -mavx vs. without.) So I'd recommend not using cpp directly.

[Peter Cordes]

gcc foo.S already pre-defines __ASSEMBLER__ to 1. So you can just protect all your C-only stuff with #ifndef __ASSEMBLER__