What does libc_hidden_builtin_def (strspn) mean?

Question

libc_hidden_builtin_def (strspn)

I found the code above in glibc-2.18/string/strspn.c. Can someone explain what this mean. Is this important to rest of the code? Here is the content of the file strspn.c:

#include <string.h>

#undef strspn

/* Return the length of the maximum initial segment
   of S which contains only characters in ACCEPT.  */
size_t strspn (s, accept) const char *s; const char *accept; {
  const char *p;
  const char *a;
  size_t count = 0;

  for (p = s; *p != '\0'; ++p) {
      for (a = accept; *a != '\0'; ++a) {
        if (*p == *a)
           break;
        if (*a == '\0')
           return count;
        else
          ++count;
     }
  }

  return count;
}
libc_hidden_builtin_def (strspn)

Answer 1

Can someone explain what this mean.

It's a #defined macro, which expands to nothing when building (non-shared) libc.a, and to:

extern __typeof (strcspn) __EI_strcspn __asm__("" "strcspn");
extern __typeof (strcspn) __EI_strcspn __attribute__((alias ("" "__GI_strcspn")));

when compiling libc.so.6.

What this does is define a symbol alias __GI_strcspn that has the same value as strcspn, but is not exported out of libc.so.6 (i.e. it's an internal symbol).

Is this important to rest of the code?

Yes: other code inside libc.so.6 may call this symbol without possibility to interpose it in e.g. LD_PRELOADED library.