【glibc源码分析】--strcpy.c 字符串复制

strcpy是常用的字符串复制函数，经常在面试中考到。该文件位于glibc源码的string目录中。

在线资源路径：

http://www.oschina.net/code/explore/glibc-2.9/string/strcpy.c

快速查看源码如下：

/* Copyright (C) 1991, 1997, 2000, 2003 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 Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <stddef.h>  /* 用到了ptrdiff_t */
#include <string.h>
#include <memcopy.h>
#include <bp-checks.h>  /* 定义了CHECK_BOUNDS_LOW和CHECK_BOUNDS_HIGH */

#undef strcpy

/* Copy SRC to DEST.  */
char *
strcpy (dest, src)
     char *dest;
     const char *src;
{
  char c;
  char *__unbounded s = (char *__unbounded) CHECK_BOUNDS_LOW (src);
  const ptrdiff_t off = CHECK_BOUNDS_LOW (dest) - s - 1;
  size_t n;

  do
    {
      c = *s++;
      s[off] = c;  /* 注意这种写法, s[off]相当于 *(s+off) */
    }
  while (c != '\0');

  n = s - src;
  (void) CHECK_BOUNDS_HIGH (src + n);
  (void) CHECK_BOUNDS_HIGH (dest + n);

  return dest;
}
libc_hidden_builtin_def (strcpy)

 

以上的算法主要通过计算好两个字符串的偏移量，然后循环遍历src字符串，逐个赋值给dest的相应位置；gnu的源码一般都比较复杂，进行了相应的范围检查等处理；

 

再来看个简单的，比较独立的代码：

char * strcpy(char * dest, const char * src)
{
    if ( dest == NULL || src == NULL) // 地址检查
    {
        return NULL;
    }

    if ( dest == src) // 相同地址检查
    {
        return dest;
    }

    char * str = dest;
    while ( ( *str++ = *src++ ) != '\0' )  // 循环复制
    {
        ;
    }

    return dest;
}

注意，以上代码采用了清爽的缩进格式，保证代码的清晰和可读性； 第8行的检查比较容易漏掉，这个是重点；