Character string search (start position of specified character)
char *strstr(const char *s1, const char *s2);
Returns the pointer indicating the character string that has been found. If character string s2 is not found, the null pointer is returned. If s2 indicates a character string with a length of 0, s1 is returned.
This function obtains the position of the portion (except the null character (\0)) that first coincides with the character string indicated by s2, in the character string indicated by s1.