char *strnstr(const char *s1, const char *s2, size_t n);
strnstr searches at most n characters to locate the first occurrence in the string pointed to by s1 of the sequence of characters (excluding the terminating null character) in the string pointed to by s2.
strnstr returns a pointer to the located string, or a null pointer if the string is not found. If s2 points to a string with zero length, strnstr returns s1.
strnstr is an extension commonly found in Linux and BSD C libraries.