strstr.c

一个C源代码分析器

/* Copyright (C) 1991, 1992 Free Software Foundation, Inc.
This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include <ansidecl.h>
#include <stddef.h>
#include <string.h>

/* Return the first ocurrence of NEEDLE in HAYSTACK.  */
char *
DEFUN(strstr, (haystack, needle),
      CONST char *CONST haystack AND
      CONST char *CONST needle)
{
  register CONST char *CONST needle_end = strchr(needle, '\0');
  register CONST char *CONST haystack_end = strchr(haystack, '\0');
  register CONST size_t needle_len = needle_end - needle;
  register CONST size_t needle_last = needle_len - 1;
  register CONST char *begin;

  if (needle_len == 0)
    return (char *) haystack;	/* ANSI 4.11.5.7, line 25.  */
  if ((size_t) (haystack_end - haystack) < needle_len)
    return NULL;

  for (begin = &haystack[needle_last]; begin < haystack_end; ++begin)
    {
      register CONST char *n = &needle[needle_last];
      register CONST char *h = begin;

      do
	if (*h != *n)
	  goto loop;		/* continue for loop */
      while (--n >= needle && --h >= haystack);

      return (char *) h;

    loop:;
    }

  return NULL;
}