void * memmem(const void *, size_t, const void *, size_t) __ATTR_PURE__
The memmem() function finds the start of the first occurrence of
the substring s2 of length
len2 in the memory area
s1 of length
len1.
The memmem() function returns a pointer to the beginning of the
substring, or NULL if the substring is not found.
If len2 is zero, the function returns
s1.