Question

A co-worker asked me to explain a bit of C code in memcached. I am at the point where I admit I do not understand it either.

It has to do with C function-like macro definitions with parameters that do not also have a replacement list. For example, starting at line 2751 in memcached.c:

            if (return_cas)
            {
              MEMCACHED_COMMAND_GET(c->sfd, ITEM_key(it), it->nkey,
                                    it->nbytes, ITEM_get_cas(it));
              /* Goofy mid-flight realloc. */
              if (i >= c->suffixsize) {
                char **new_suffix_list = realloc(c->suffixlist,
                                       sizeof(char *) * c->suffixsize * 2);
                if (new_suffix_list) {
                    c->suffixsize *= 2;
                    c->suffixlist  = new_suffix_list;

MEMCACHED_COMMAND_GET() is defined on line 23 in trace.h:

#define MEMCACHED_COMMAND_GET(arg0, arg1, arg2, arg3, arg4)

C macro function-like definition with arguments, ok. No replacement list.

The output of cpp on memcached.c shows the function-like macro turns into ‘;’:

# 2751 "memcached.c"
            if (return_cas)
            {
              ;


              if (i >= c->suffixsize) {
                char **new_suffix_list = realloc(c->suffixlist,
                                       sizeof(char *) * c->suffixsize * 2);
                if (new_suffix_list) {
                    c->suffixsize *= 2;
                    c->suffixlist = new_suffix_list;

Does anyone have any insight into why a programmer would include function-like macros without a replacement list like MEMCACHED_COMMAND_GET() that have no effect on the code produced? What purpose is being served by doing doing this? Thank you kindly in advance.

Answer 1

On Solaris and Mac OSX it’s replaced to triggering a DTrace probe so it expands to something else.