char * strstr(const char *, const char *) __ATTR_PURE__
The strstr() function finds the first occurrence of the substring
s2 in the string s1.
The terminating '\0' characters are not compared.
The strstr() function returns a pointer to the beginning of the
substring, or NULL if the substring is not found.
If s2 points to a string of zero length, the
function returns s1.