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

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.)

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.

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