Function strstr

libs/libc/string/lib_strstr.c:57–286

#undef strstr
no_builtin("strstr")
FAR char *strstr(FAR const char *haystack, FAR const char *needle)
{
#ifdef CONFIG_ALLOW_MIT_COMPONENTS
  FAR const unsigned char *needle_cmp_end;
  FAR const unsigned char *i_haystack;
  const char needle_first = *needle;
  FAR const unsigned char *i_needle;
  unsigned long last_haystack_chars;
  unsigned long last_needle_chars;
  FAR const char *sub_start;
  size_t needle_cmp_len;
  bool identical = true;
  unsigned long mask;
  size_t compare_len;
  size_t needle_len;

  if (!*needle)
    {
      return (FAR char *)haystack;
    }

  /* Runs strchr() on the first section of the haystack as it has a lower
   * algorithmic complexity for discarding the first non-matching characters.
   */

  haystack = strchr(haystack, needle_first);
  if (!haystack) /* First character of needle is not in the haystack. */
    {
      return NULL;
    }

  /* First characters of haystack and needle are the same now. Both are
   * guaranteed to be at least one character long.
   * Now computes the sum of the first needle_len characters of haystack
   * minus the sum of characters values of needle.
   */

  i_haystack = (FAR const unsigned char *)haystack + 1;
  i_needle = (FAR const unsigned char *)needle + 1;

  while (*i_haystack && *i_needle)
    {
      identical &= *i_haystack++ == *i_needle++;
    }

  /* i_haystack now references the (needle_len + 1)-th character. */

  if (*i_needle) /* haystack is smaller than needle. */
    {
      return NULL;
    }
  else if (identical)
    {
      return (FAR char *)haystack;
    }

  needle_len = i_needle - (FAR const unsigned char *)needle;