memmove 的实现

baidu的笔试题目

用C语言实现一个公用库函数void * memmove(void *dest,const void *src,size_t n)。
该函数的功能是拷贝src所指的内存内容前n个字节到dest所指的地址上。
注意，作为公用库函数，请注意安全检查，注意处理内存区重合的情况。

void*  memmove(void *dest, const void *src,size_t n)
{
    if (n == 0)
    {
        return 0;
    }

    /* 验证参数 */
    if (dest == NULL)  return 0;
    if (src == NULL)    return 0;

    char *psrc = (char*)src; 
    char *pdest = (char*)dest;

      /*检查是否有重叠问题 */
    if((dest <= psrc) || (pdest >= psrc + n))
        {
                /*正向拷贝*/
                for(int i=0; i < n; i++)
                {
                        *pdest = *psrc;
                        psrc++;
                        pdest++;
                }
        }
        else
        {
                /*反向拷贝*/
                psrc += n;
                pdest += n;
                for(int i=0;i<n;i++)
                {
                        psrc--;
                        pdest--;
                        *pdest = *psrc;

                }
        } 

    return dest;
} 

 

从glibc里找了memmove的源代码，如下

/* Copy memory to memory until the specified number of bytes
   has been copied.  Overlap is handled correctly.
   Copyright (C) 1991, 1995, 1996, 1997, 2003 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Torbjorn Granlund (tege@sics.se).

   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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <string.h>
#include <memcopy.h>
#include <pagecopy.h>

/* All this is so that bcopy.c can #include
   this file after defining some things.  */
#ifndef a1
#define a1 dest /* First arg is DEST.  */
#define a1const
#define a2 src /* Second arg is SRC.  */
#define a2const const
#undef memmove
#endif
#if !defined(RETURN) || !defined(rettype)
#define RETURN(s) return (s) /* Return DEST.  */
#define rettype  void *
#endif

rettype
memmove (a1, a2, len)
     a1const void *a1;
     a2const void *a2;
     size_t len;
{
  unsigned long int dstp = (long int) dest;
  unsigned long int srcp = (long int) src;

  /* This test makes the forward copying code be used whenever possible.
     Reduces the working set.  */
  if (dstp - srcp >= len) /* *Unsigned* compare!  */
    {
      /* Copy from the beginning to the end.  */

      /* If there not too few bytes to copy, use word copy.  */
      if (len >= OP_T_THRES)
 {
   /* Copy just a few bytes to make DSTP aligned.  */
   len -= (-dstp) % OPSIZ;
   BYTE_COPY_FWD (dstp, srcp, (-dstp) % OPSIZ);

   /* Copy whole pages from SRCP to DSTP by virtual address
      manipulation, as much as possible.  */

   PAGE_COPY_FWD_MAYBE (dstp, srcp, len, len);

   /* Copy from SRCP to DSTP taking advantage of the known
      alignment of DSTP.  Number of bytes remaining is put
      in the third argument, i.e. in LEN.  This number may
      vary from machine to machine.  */

   WORD_COPY_FWD (dstp, srcp, len, len);

   /* Fall out and copy the tail.  */
 }

      /* There are just a few bytes to copy.  Use byte memory operations.  */
      BYTE_COPY_FWD (dstp, srcp, len);
    }
  else
    {
      /* Copy from the end to the beginning.  */
      srcp += len;
      dstp += len;

      /* If there not too few bytes to copy, use word copy.  */
      if (len >= OP_T_THRES)
 {
   /* Copy just a few bytes to make DSTP aligned.  */
   len -= dstp % OPSIZ;
   BYTE_COPY_BWD (dstp, srcp, dstp % OPSIZ);

   /* Copy from SRCP to DSTP taking advantage of the known
      alignment of DSTP.  Number of bytes remaining is put
      in the third argument, i.e. in LEN.  This number may
      vary from machine to machine.  */

   WORD_COPY_BWD (dstp, srcp, len, len);

   /* Fall out and copy the tail.  */
 }

      /* There are just a few bytes to copy.  Use byte memory operations.  */
      BYTE_COPY_BWD (dstp, srcp, len);
    }

  RETURN (dest);
}
#ifndef memmove
libc_hidden_builtin_def (memmove)
#endif