char *strstr(const char *s1, const char *s2);
IEEE Std 1003.1-2017
strstr() function shall locate the first occurrence in the string pointed to by s1 of the sequence of bytes
(excluding the terminating NUL character) in the string pointed to by s2.
Upon successful completion,
strstr() shall return a pointer to the located string or a
null pointer if the string is not
If s2 points to a string with zero length, the function shall return s1.
No errors are defined.