C Preprocessor Stringify difference in commandline and in-code definitions

Question

A project has the ability to compile code in or not and there is a need to know which code is included and what is not. The functions are all isolated in single files and the code therein is embraced in #ifdef UPPERCASE_FILENAME_C ...code... #endif and to make handling it simpler we check with a bit of preprocessor "magic"

#define STRINGIZE(x)  __STRINGIZE(x)
#define __STRINGIZE(x) ""#x""
#define MP_HAS(x)        (sizeof(STRINGIZE(x##_C)) == 1u )

If the macro in question is defined, it expands to an empty string which has a size of one (only the \0 gets counted) and if not the whole string given gets expanded which resolves to a value >1.

Something to play around with:

#include <stdlib.h>
#include <stdio.h>

#ifdef DEFINE_IT
#define FOOT_C
#endif

#define STRINGIZE(x)  __STRINGIZE(x)
#define __STRINGIZE(x) ""#x""
#define HAVE_WE(x)        (sizeof(STRINGIZE(x##_C)) )

int main(void) {

   printf("sizeof(stringized) = %zu\n", HAVE_WE(FOOT));

   exit(EXIT_SUCCESS);
}

So far, so good. A bit complicated, but it works.

But: if we give the definition at the commandline (to test all branches)gcc -DFOOT_C sizeoftest.c -o sizeoftest the macro expands to 1, so the string is not empty anymore and has a sizeof of 2 (two), the character 1 and the character \0.

One workaround is to set the definition given at commandline to an empty string explicitely: gcc -DFOOT_C="" sizeoftest.c -o sizeoftest.

Is there a way to adapt the stringify macros to ignore that discrepancy and handle both ways to define a macro in the same way?

EDIT: The proposed answer at Check at runtime if macro was defined does not fit exactly, sadly, but it came close. Might be the right direction.

Does this answer your question? Check at runtime if macro was defined — nielsen
– nielsen, Commented Apr 9, 2023 at 22:11
This all seems way too complicated. What's wrong with #ifdef FOOT_C ...? — catnip
– catnip, Commented Apr 9, 2023 at 22:27
@nielsen That's an interessting approach. Thank's for the find! — deamentiaemundi
– deamentiaemundi, Commented Apr 9, 2023 at 23:34
@PaulSanders Yeah, a bit complicated but easier to use than a bunch of ifdef's. Not much of a problem, but I thought someone here could show a more elegant way to do it. The answer linked by @nielsen for example seems promising. — deamentiaemundi
– deamentiaemundi, Commented Apr 9, 2023 at 23:41
I do not understand. The functions are all isolated in single files and the code therein is embraced in #ifdef UPPERCASE_FILENAME_C Great, so what is the point of the question? __STRI identifiers starting with double _ are reserved, you can't use them. — KamilCuk
– KamilCuk, Commented Apr 15, 2023 at 10:07

KamilCuk · Accepted Answer · 2023-04-15 10:14:14Z

0

Is there a way to adapt the stringify macros to ignore that discrepancy and handle both ways to define a macro in the same way?

No. This is pre -processor, you can just type it out statically.

#ifdef FOOT_C
#define HAVE_FOOT_C  1
#else
#define HAVE_FOOT_C  0
#endif

Then you typically just have #define HAVE(x) HAVE_##x.

answered Apr 15, 2023 at 10:14

KamilCuk

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