GCC dump preprocessor defines

Question

Is there a way for gcc/g++ to dump its preprocessor defines from the command line? I mean things like __GNUC__, __STDC__, and so on.

Answer 1

I usually do it this way:

$ gcc -dM -E - < /dev/null

Note that some preprocessor defines are dependent on command line options - you can test these by adding the relevant options to the above command line. For example, to see which SSE3/SSE4 options are enabled by default:

$ gcc -dM -E - < /dev/null | grep SSE[34]
#define __SSE3__ 1
#define __SSSE3__ 1

and then compare this when -msse4 is specified:

$ gcc -dM -E -msse4 - < /dev/null | grep SSE[34]
#define __SSE3__ 1
#define __SSE4_1__ 1
#define __SSE4_2__ 1
#define __SSSE3__ 1

Similarly you can see which options differ between two different sets of command line options, e.g. compare preprocessor defines for optimisation levels -O0 (none) and -O3 (full):

$ gcc -dM -E -O0 - < /dev/null > /tmp/O0.txt
$ gcc -dM -E -O3 - < /dev/null > /tmp/O3.txt
$ sdiff -s /tmp/O0.txt /tmp/O3.txt 
#define __NO_INLINE__ 1        <
                               > #define __OPTIMIZE__ 1