Implementation of strstr in glibc

What is the implementation of strstr in glibc?

related to an answer for: strcmp and strncmp implementation in glibc
asked Apr 8, 2015 by Eric Z Ma (44,280 points)

1 Answer

Best answer

Implementation of STRSTR in glibc (string/strstr.c):

/* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
   if NEEDLE is empty, otherwise NULL if NEEDLE is not found in
   HAYSTACK.  */
char *
STRSTR (const char *haystack_start, const char *needle_start)
  const char *haystack = haystack_start;
  const char *needle = needle_start;
  size_t needle_len; /* Length of NEEDLE.  */
  size_t haystack_len; /* Known minimum length of HAYSTACK.  */
  bool ok = true; /* True if NEEDLE is prefix of HAYSTACK.  */

  /* Determine length of NEEDLE, and in the process, make sure
     HAYSTACK is at least as long (no point processing all of a long
     NEEDLE if HAYSTACK is too short).  */
  while (*haystack && *needle)
    ok &= *haystack++ == *needle++;
  if (*needle)
    return NULL;
  if (ok)
    return (char *) haystack_start;

  /* Reduce the size of haystack using strchr, since it has a smaller
     linear coefficient than the Two-Way algorithm.  */
  needle_len = needle - needle_start;
  haystack = strchr (haystack_start + 1, *needle_start);
  if (!haystack || __builtin_expect (needle_len == 1, 0))
    return (char *) haystack;
  needle -= needle_len;
  haystack_len = (haystack > haystack_start + needle_len ? 1
		  : needle_len + haystack_start - haystack);

  /* Perform the search.  Abstract memory is considered to be an array
     of 'unsigned char' values, not an array of 'char' values.  See
     ISO C 99 section  */
  if (needle_len < LONG_NEEDLE_THRESHOLD)
    return two_way_short_needle ((const unsigned char *) haystack,
				 (const unsigned char *) needle, needle_len);
  return two_way_long_needle ((const unsigned char *) haystack, haystack_len,
			      (const unsigned char *) needle, needle_len);

The implementation of two_way_short_needle and two_way_long_needle can be found in string/str-two-way.h.

answered Apr 8, 2015 by Eric Z Ma (44,280 points)

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