gcc - C preprocessor: expand macro in a #warning

Question

I would like to print a macro value (expand the macro) in the #warning directive.

For example, for the code:

#define AAA 17
#warning AAA = ???

The desired compile-time output would be

warning: AAA = 17

What do I use for ???, or, how do I augment the code?

Answer 1

You can use the preprocessor directive #pragma message.

Example:

#define STR_HELPER(x) #x
#define STR(x) STR_HELPER(x)

#define AAA 123
#pragma message "content of AAA: " STR(AAA)

int main() { return 0; }

The output may look like this:

$ gcc test.c
test.c:5:9: note: #pragma message: content of AAA: 123
 #pragma message("content of AAA: " STR(AAA))
         ^

For reference:

• https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html
• https://msdn.microsoft.com/en-us/library/x7dkzch2.aspx
• https://clang.llvm.org/docs/UsersManual.html#controlling-diagnostics-via-pragmas