Debug what #DEFINE resolves to before compilation

Question

I need to see what is happeing under the hood before compilation. How? The following is an example out of many...

I am compiling the Intel Math Lib and that works fine. There are different options to get it running that influence how symbols are defined during precompilation.

I can deduct from the "build code" (make/cmake), what should happen, but my expectations don't match the results. Therefore I would now like to "look into it" and see what actually happens.

Example (simplyfied from bid_conf.h in the mentioned Intel library): The client code using the math library calls this:

num_to_string(s, num);

The code that defines num_to_string is defined further up during precompilation:

BID_EXTERN_C void bid128_to_string (char *str, BID_UINT128 x _EXC_FLAGS_PARAM);

Now _EXC_FLAGS_PARAM is evaluated depending on build arguments, defined here:

#if !DECIMAL_GLOBAL_EXCEPTION_FLAGS
#define _EXC_FLAGS_PARAM , _IDEC_flags *pfpsf
#else
#define _EXC_FLAGS_ARG
#endif

That reads: If the exception flags are not set, then _EXC_FLAGS_PARAM becomes ", _IDEC_flags *pfpsf" or else "" (nada).

The flag is? set but I still get:

error: too few arguments to function call, expected 3, have 2
  num_to_string(s, num);

So _EXC_FLAGS_PARAM becomes ", _IDEC_flags *pfpsf". But DECIMAL_GLOBAL_EXCEPTION_FLAGS is set!...

...or is it not?

How can I print out what is actually set while it is happening, like:

#please_print_this(DECIMAL_GLOBAL_EXCEPTION_FLAGS)

Or even better: see the intermediate output, basically what the compiler ultimately gets fed with...

(No C compiler expert but could not find an answer yet).

Answer 1

If you're using gcc, compile with the -E option. That will run just the preprocessor and output the result to stdout. Then you can see what the macros expand to.