How do I run the GCC preprocessor to get the code after macros like #define are expanded?

Yes. Use the -E option:

gcc -E foo.c

While the -E option will perform all pre-processing, it also produces some very 'raw' output that might not be what you want (depending on what you want).

If you need to debug a macro expansion that's not doing what you expect, E is a good way to go. If you simply want to filter out the 'inactive code', but leave the remaining code in more-or-less original form, you might want to look at the answers to the following Stack Overflow question:

• Is there a C pre-processor which eliminates #ifdef blocks based on values defined/undefined?

It sounds like you want unifdef, not the GCC preprocessor.

For completeness sake: If you are interested in running just the preprocessor, you can also use cpp:

cpp foo.c

See https://gcc.gnu.org/onlinedocs/cpp/Invocation.html for reference:

Most often when you use the C preprocessor you do not have to invoke it explicitly: the C compiler does so automatically. However, the preprocessor is sometimes useful on its own. You can invoke the preprocessor either with the cpp command, or via gcc -E. In GCC, the preprocessor is actually integrated with the compiler rather than a separate program, and both of these commands invoke GCC and tell it to stop after the preprocessing phase.

The cpp command expects two file names as arguments, infile and outfile. The preprocessor reads infile together with any other files it specifies with ‘#include’. All the output generated by the combined input files is written in outfile.