How do I show the value of a #define at compile time in gcc

Question

So far I've got as far as:

#define ADEFINE "23" #pragma message ("ADEFINE" ADEFINE) 

Which works, but what if ADEFINE isn't a string?

#define ADEFINE 23 #pragma message ("ADEFINE" ADEFINE) 

causes:

warning: malformed ‘#pragma message’, ignored

Ideally I'd like to be able to deal with any value, including undefined.

Answer 1

To display macros which aren't strings, stringify the macro:

#define STRINGIFY(s) XSTRINGIFY(s) #define XSTRINGIFY(s) #s  #define ADEFINE 23 #pragma message ("ADEFINE=" STRINGIFY(ADEFINE)) 

If you have/want boost, you can use boost stringize to do it for you:

#include <boost/preprocessor/stringize.hpp> #define ADEFINE 23 #pragma message ("ADEFINE=" BOOST_PP_STRINGIZE(ADEFINE)) 

Answer 2

I'm not sure if this will do what you want, but if you're only interested in this to debug the occasional macro problem (so it's not something you need displayed in a message for each compile), the following might work for you. Use gcc's -E -dD option to dump #define directives along with preprocessing output. Then pipe that through grep to see only the lines you want:

// test.c #include <stdlib.h> #include <stdio.h> #define ADEFINE "23" #include <string.h>  int main(int argc, char *argv[]) { #undef ADEFINE #define ADEFINE 42     return 0; } 

The command gcc -E -dD -c test.c | grep ADEFINE shows:

#define ADEFINE "23" #undef ADEFINE #define ADEFINE 42