lib/memmem.c

   1 /* Copyright (C) 1991-1994, 1996-1998, 2000, 2004, 2007-2020 Free Software
   2    Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License along
  16    with this program; if not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* This particular implementation was written by Eric Blake, 2008.  */
  19
  20 #ifndef _LIBC
  21 # include <config.h>
  22 #endif
  23
  24 /* Specification of memmem.  */
  25 #include <string.h>
  26
  27 #define RETURN_TYPE void *
  28 #define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
  29 #include "str-two-way.h"
  30
  31 /* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
  32    if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
  33    HAYSTACK.  */
  34 void *
  35 memmem (const void *haystack_start, size_t haystack_len,
  36         const void *needle_start, size_t needle_len)
  37 {
  38   /* Abstract memory is considered to be an array of 'unsigned char' values,
  39      not an array of 'char' values.  See ISO C 99 section 6.2.6.1.  */
  40   const unsigned char *haystack = (const unsigned char *) haystack_start;
  41   const unsigned char *needle = (const unsigned char *) needle_start;
  42
  43   if (needle_len == 0)
  44     /* The first occurrence of the empty string is deemed to occur at
  45        the beginning of the string.  */
  46     return (void *) haystack;
  47
  48   /* Sanity check, otherwise the loop might search through the whole
  49      memory.  */
  50   if (__builtin_expect (haystack_len < needle_len, 0))
  51     return NULL;
  52
  53   /* Use optimizations in memchr when possible, to reduce the search
  54      size of haystack using a linear algorithm with a smaller
  55      coefficient.  However, avoid memchr for long needles, since we
  56      can often achieve sublinear performance.  */
  57   if (needle_len < LONG_NEEDLE_THRESHOLD)
  58     {
  59       haystack = memchr (haystack, *needle, haystack_len);
  60       if (!haystack || __builtin_expect (needle_len == 1, 0))
  61         return (void *) haystack;
  62       haystack_len -= haystack - (const unsigned char *) haystack_start;
  63       if (haystack_len < needle_len)
  64         return NULL;
  65       return two_way_short_needle (haystack, haystack_len, needle, needle_len);
  66     }
  67   else
  68     return two_way_long_needle (haystack, haystack_len, needle, needle_len);
  69 }
  70
  71 #undef LONG_NEEDLE_THRESHOLD