Implementing strnstr

How about:

char *strnstr(char *haystack, char *needle, size_t len) {
    if (len == 0) return haystack; /* degenerate edge case */
    while (haystack = strchr(haystack, needle[0])) {
        if (!strncmp(haystack, needle, len)) return haystack;
        haystack++; }
    return 0;
}

If you want haystack to not be null terminated, you'll need two length args:

char *memmem(char *haystack, size_t hlen, char *needle, size_t nlen) {
    if (nlen == 0) return haystack; /* degenerate edge case */
    if (hlen < nlen) return 0; /* another degenerate edge case */
    char *hlimit = haystack + hlen - nlen + 1;
    while (haystack = memchr(haystack, needle[0], hlimit-haystack)) {
        if (!memcmp(haystack, needle, nlen)) return haystack;
        haystack++; }
    return 0;
}

which is available in GNU libc, though older versions are broken.

The implementation provided by Chris Dodd has the following disadvantages:

1. It defeats the purpose of strnstr in that the while condition uses the unbounded string function strchr
2. It depends on haystack being NULL terminated, which is a deviation from the usual implementation of strnstr, for example as provided by GNU-Darwin
3. The call to strchr is an unnecessary function call when strchar is not inlined
4. Returns haystack instead of NULL when len is zero, a deviation from the accepted strstr semantics
5. Returns an empty string instead of haystack when needle has length of zero

The following implementation remedies the above problems without becoming as difficult to read as the GNU-Darwin implementation, and is Creative Commons licensed:

#include <string.h>

char *strnstr(const char *haystack, const char *needle, size_t len)
{
        int i;
        size_t needle_len;

        if (0 == (needle_len = strnlen(needle, len)))
                return (char *)haystack;

        for (i=0; i<=(int)(len-needle_len); i++)
        {
                if ((haystack[0] == needle[0]) &&
                        (0 == strncmp(haystack, needle, needle_len)))
                        return (char *)haystack;

                haystack++;
        }
        return NULL;
}