string/memmem.c

   1 /* Copyright (C) 1991-2019 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://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 #ifndef _LIBC
  28 # define __builtin_expect(expr, val)   (expr)
  29 # define __memmem       memmem
  30 #endif
  31
  32 #define RETURN_TYPE void *
  33 #define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
  34 #define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
  35 #include "str-two-way.h"
  36
  37 #undef memmem
  38
  39 /* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
  40    if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
  41    HAYSTACK.  */
  42 void *
  43 __memmem (const void *haystack_start, size_t haystack_len,
  44           const void *needle_start, size_t needle_len)
  45 {
  46   /* Abstract memory is considered to be an array of 'unsigned char' values,
  47      not an array of 'char' values.  See ISO C 99 section 6.2.6.1.  */
  48   const unsigned char *haystack = (const unsigned char *) haystack_start;
  49   const unsigned char *needle = (const unsigned char *) needle_start;
  50
  51   if (needle_len == 0)
  52     /* The first occurrence of the empty string is deemed to occur at
  53        the beginning of the string.  */
  54     return (void *) haystack;
  55
  56   /* Sanity check, otherwise the loop might search through the whole
  57      memory.  */
  58   if (__glibc_unlikely (haystack_len < needle_len))
  59     return NULL;
  60
  61   /* Use optimizations in memchr when possible, to reduce the search
  62      size of haystack using a linear algorithm with a smaller
  63      coefficient.  However, avoid memchr for long needles, since we
  64      can often achieve sublinear performance.  */
  65   if (needle_len < LONG_NEEDLE_THRESHOLD)
  66     {
  67       haystack = memchr (haystack, *needle, haystack_len);
  68       if (!haystack || __builtin_expect (needle_len == 1, 0))
  69         return (void *) haystack;
  70       haystack_len -= haystack - (const unsigned char *) haystack_start;
  71       if (haystack_len < needle_len)
  72         return NULL;
  73       /* Check whether we have a match.  This improves performance since we
  74          avoid the initialization overhead of the two-way algorithm.  */
  75       if (memcmp (haystack, needle, needle_len) == 0)
  76         return (void *) haystack;
  77       return two_way_short_needle (haystack, haystack_len, needle, needle_len);
  78     }
  79   else
  80     return two_way_long_needle (haystack, haystack_len, needle, needle_len);
  81 }
  82 libc_hidden_def (__memmem)
  83 weak_alias (__memmem, memmem)
  84 libc_hidden_weak (memmem)
  85
  86 #undef LONG_NEEDLE_THRESHOLD