Seeing expanded C macros

Question

If I want to expand a C macro, what are some good ways to do that (besides tracing it manually)?

For instance, GTK_WIDGET_SET_FLAGS, it uses a macro that

Answer 1

You can dump the expansion of a macro at run time like this:

#include 

/*
 * generic helper macros
 */
#define CALL(macro, arguments) macro arguments
#define STR(...) STR_(__VA_ARGS__)
#define STR_(...) # __VA_ARGS__

/*
 * dumps a macro and its expansion to stdout
 * the second argument is optional and specifies the number of
 * arguments that macro takes: 0 means macro takes zero arguments
 * no second argument means macro is not function-like
 */
#define DUMP_MACRO(macro, ...) \
    do { \
        puts ( \
            "'" \
            # macro STR(DUMP_MACRO_ARGS_ ## __VA_ARGS__) \
            "' expands to '" \
            STR(CALL(macro, DUMP_MACRO_ARGS_ ## __VA_ARGS__)) \
            "'" \
        ); \
    } while (0)
/* helpers for DUMP_MACRO, add more if required */
#define DUMP_MACRO_ARGS_
#define DUMP_MACRO_ARGS_0 ()
#define DUMP_MACRO_ARGS_1 (<1>)
#define DUMP_MACRO_ARGS_2 (<1>, <2>)
#define DUMP_MACRO_ARGS_3 (<1>, <2>, <3>)

/*
 * macros to be used in examples for DUMP_MACRO
 */
#define EXAMPLE ( EXAMPLE0() << 9 )
#define EXAMPLE0() __GNUC__
#define EXAMPLE1(EXAMPLE1) EXAMPLE1
#define EXAMPLE3(EXAMPLE1, _, __) ( EXAMPLE1 ? _(__) : false )

int main() {
    /* examples */
    DUMP_MACRO(EXAMPLE);
    DUMP_MACRO(EXAMPLE0, 0);
    DUMP_MACRO(EXAMPLE1, 1);
    DUMP_MACRO(EXAMPLE3, 3);
    DUMP_MACRO(EXAMPLE3(EXAMPLE, EXAMPLE1, non_macro_symbol));
    /* does not work for DUMP_MACRO itself, because the
       preprocessor does not allow recursion */
    DUMP_MACRO(DUMP_MACRO, 1);
    DUMP_MACRO(DUMP_MACRO, 2);
    return 0;
}

The program prints:

'EXAMPLE' expands to '( 4 << 9 )'
'EXAMPLE0()' expands to '4'
'EXAMPLE1(<1>)' expands to '<1>'
'EXAMPLE3(<1>, <2>, <3>)' expands to '( <1> ? <2>(<3>) : false )'
'EXAMPLE3(EXAMPLE, EXAMPLE1, non_macro_symbol)' expands to '( ( 4 << 9 ) ? non_macro_symbol : false )'
'DUMP_MACRO(<1>)' expands to 'DUMP_MACRO (<1>)'
'DUMP_MACRO(<1>, <2>)' expands to 'DUMP_MACRO (<1>, <2>)'

However this yields only the full expansion. If you need single steps, Eclipse/CDT can help, but only if you teach it all the headers and compiler flags you use.