trying to understand a macro definition in c

Question

This is a part of code that I'm trying to understand but I can't understand how a macro like this works. The first line #define FOREACH_OMPD_STATE(macro) defines a function_like Macro that has a parameter named ' macro ' but what does the rest of the definition mean?

#define FOREACH_OMPD_STATE(macro)                                             \
                                                                              \
    /* first available state */                                               \
    macro (ompt_state_undefined, 0x102)      /* undefined thread state */     \
                                                                              \
    /* work states (0..15) */                                                 \
    macro (ompt_state_work_serial, 0x000)    /* working outside parallel */   \
    macro (ompt_state_work_parallel, 0x001)  /* working within parallel */    \
    macro (ompt_state_work_reduction, 0x002) /* performing a reduction */    

Are those different values for the parameter named macro or what?

Answer 1

FOREACH_OMPD_STATE is an X macro . It provides a list that is used by passing various macros to do things with the list items. For example, we can use a macro that produces only the names to make a list of enum identifiers:

#define NameOnly(name, value)   name,

enum { FOREACH_OMPD_STATE(NameOnly) };

That code generates (line formatting added for readability):

enum {
    ompt_state_undefined,
    ompt_state_work_serial,
    ompt_state_work_parallel,
    ompt_state_work_reduction,
};

Having done that, we could use a different macro that creates array initializers to fill an array indexed by the enum identifiers with the values provided by the FOREACH_OMPD_STATE macro:

#define MakeInitializer(name, value)    [name] = value,

int ArrayOfValues[] = { FOREACH_OMPD_STATE(MakeInitializer) };

That generates (line formatting added for readability):

int ArrayOfValues = {
    [ompt_state_undefined] = 0x102,
    [ompt_state_work_serial] = 0x000,
    [ompt_state_work_parallel] = 0x001,
    [ompt_state_work_reduction] = 0x002,
};

The key idea is we only need to list the names and values once in the source code, in defining the FOREACH_OMPD_STATE macro, and then can use them in diverse ways later without repeating them.