memmem -- locate a substring


#define _GNU_SOURCE
#include <string.h>

void *memmem(const void *haystack, size_t haystacklen,
             const void *needle, size_t needlelen);


The memmem() function finds the start of the first occurrence of the substring needle of length needlelen in the memory area haystack of length haystacklen.

Return Value

The memmem() function returns a pointer to the beginning of the substring, or NULL if the substring is not found.


This function was broken in Linux libraries up to and including libc 5.0.9; there the needle and haystack arguments were interchanged, and a pointer to the end of the first occurrence of needle was returned. Since libc 5.0.9 is still widely used, this is a dangerous function to use.

Both old and new libc's have the bug that if needle is empty haystack-1 (instead of haystack) is returned. And glibc 2.0 makes it worse, and returns a pointer to the last byte of haystack. This is fixed in glibc 2.1.