Question

I have a source file foo.c and a header file bar.h. How can I just expand the macros in bar.h without expanding macros in other header files?

$ cat foo.c

#include <stdio.h>
#include "bar.h"

int main()
{
#ifdef BAR_FUNC
    printf("bar func\n");
#else
    printf("foo func\n");
#endif
    return 0;
}

$ cat bar.h

#define BAR_FUNC 1

What I want is:

$ EXPAND_MAGIC foo.c

#include <stdio.h>

int main()
{
    printf("bar func\n");
    return 0;
}

If I use `gcc -E, it expands <stdio.h> as well. But I just want to expand macros in bar.h. Is there an option in gcc doing that? If not, are there any other tools that can do such preprocessing?

Update: Above foo.c/bar.h is just an example. In reality, I have a few hundreds of macros defined in bar.h (pls consider config.h generated by autoconf in a fairly large project). And what I want is to expand all (and ONLY) these macros in more than 10K source files. Any suggestions?

Answer 1

You might look into the program sunifdef which is mentioned in the answer to this question. It allows you specify macro definitions on the command line, and it eliminates #ifdef lines appropriately. Note that it doesn’t do macro expansion.

That said, you shouldn’t use a tool like this for general development, but it’s quite useful for decoding or cleaning up a code that has gotten messy with many unused #ifdefs‘s over the years.