sysdeps/generic/strstr.c

   1 /* Copyright (C) 1991, 1992 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 Library General Public License as
   6 published by the Free Software Foundation; either version 2 of the
   7 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 Library General Public License for more details.
  13
  14 You should have received a copy of the GNU Library General Public
  15 License along with the GNU C Library; see the file COPYING.LIB.  If
  16 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  17 Cambridge, MA 02139, USA.  */
  18
  19 #include <ansidecl.h>
  20 #include <stddef.h>
  21 #include <string.h>
  22
  23 /* Return the first ocurrence of NEEDLE in HAYSTACK.  */
  24 char *
  25 DEFUN(strstr, (haystack, needle),
  26       CONST char *CONST haystack AND
  27       CONST char *CONST needle)
  28 {
  29   register CONST char *CONST needle_end = strchr(needle, '\0');
  30   register CONST char *CONST haystack_end = strchr(haystack, '\0');
  31   register CONST size_t needle_len = needle_end - needle;
  32   register CONST size_t needle_last = needle_len - 1;
  33   register CONST char *begin;
  34
  35   if (needle_len == 0)
  36     return (char *) haystack;   /* ANSI 4.11.5.7, line 25.  */
  37   if ((size_t) (haystack_end - haystack) < needle_len)
  38     return NULL;
  39
  40   for (begin = &haystack[needle_last]; begin < haystack_end; ++begin)
  41     {
  42       register CONST char *n = &needle[needle_last];
  43       register CONST char *h = begin;
  44
  45       do
  46         if (*h != *n)
  47           goto loop;            /* continue for loop */
  48       while (--n >= needle && --h >= haystack);
  49
  50       return (char *) h;
  51
  52     loop:;
  53     }
  54
  55   return NULL;
  56 }