source: bootcd/isolinux/syslinux-6.03/com32/lib/memmem.c

/*
 * memmem.c
 *
 * Find a byte string inside a longer byte string
 *
 * This uses the "Not So Naive" algorithm, a very simple but
 * usually effective algorithm, see:
 *
 * http://www-igm.univ-mlv.fr/~lecroq/string/
 */

#include <string.h>

void *memmem(const void *haystack, size_t n, const void *needle, size_t m)
{
    const unsigned char *y = (const unsigned char *)haystack;
    const unsigned char *x = (const unsigned char *)needle;

    size_t j, k, l;

    if (m > n || !m || !n)
        return NULL;

    if (1 != m) {
        if (x[0] == x[1]) {
            k = 2;
            l = 1;
        } else {
            k = 1;
            l = 2;
        }

        j = 0;
        while (j <= n - m) {
            if (x[1] != y[j + 1]) {
                j += k;
            } else {
                if (!memcmp(x + 2, y + j + 2, m - 2)
                    && x[0] == y[j])
                    return (void *)&y[j];
                j += l;
            }
        }
    } else {
        do {
            if (*y == *x)
                return (void *)y;
            y++;
        } while (--n);
    }

    return NULL;
}