/* Simple implementation of strstr for systems without it.   This function is in the public domain.  */ /* @deftypefn Supplemental char* strstr (const char *@var{string}, const char *@var{sub}) This function searches for the substring @var{sub} in the string@var{string}, not including the terminating null characters.  A pointerto the first occurrence of @var{sub} is returned, or @code{NULL} if thesubstring is absent.  If @var{sub} points to a string with zerolength, the function returns @var{string}. @end deftypefn  */ #include <stddef.h> extern char *strchr (const char *, int);extern int strncmp (const void *, const void *, size_t);extern size_t strlen (const char *); char *strstr (const char *s1, const char *s2){  const char *p = s1;  const size_t len = strlen (s2);   if (!len)    return s1;   for (; (p = strchr (p, *s2)) != 0; p++)    {      if (strncmp (p, s2, len) == 0)	return (char *)p;    }  return (0);}