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.

Solution 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))

Solution 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

Comments

[John Lawrence Aspden]

Well, I don't think my client is going to be very happy to have that added as a dependency, but that looks like an existence proof. Perhaps I should go and have a look at stringize.hpp

[rob05c]

@JohnLawrenceAspden I updated the answer with how to do it without boost (or any other library).

[Paul]

Your link is no longer active: "stringify the macro"