memmove实现

转自： http://blog.youkuaiyun.com/gxy837/article/details/6144600

 

memmove(

void * memmove ( void * destination, const void * source, size_t num );)是<string.h>的标准函数，其作用是把从source开始的num个字符拷贝到destination。

最简单的方法是直接复制，但是由于它们可能存在内存的重叠区，因此可能覆盖了原有数据。



比如当source+count>=dest&&source<dest时，dest可能覆盖了原有source的数据。

解决办法是从后往前拷贝。

对于其它情况，则从前往后拷贝。



源代码如下：

;***



;memcpy.asm - contains memcpy and memmove routines



;



;        Copyright (c) 1986-1997, Microsoft Corporation. All right reserved.



;



;Purpose:



;        memcpy() copies a source memory buffer to a destination buffer.



;        Overlapping buffers are not treated specially, so propogation may occur.



;        memmove() copies a source memory buffer to a destination buffer.



;        Overlapping buffers are treated specially, to avoid propogation.



;



;*******************************************************************************



;***



;memcpy - Copy source buffer to destination buffer



;



;Purpose:



;        memcpy() copies a source memory buffer to a destination memory buffer.



;        This routine does NOT recognize overlapping buffers, and thus can lead



;        to propogation.



;        For cases where propogation must be avoided, memmove() must be used.



;



;        Algorithm:

       void* memcpy(void* dest, void* source, size_t count)

      {

           void* ret = dest;

          //copy from lower address to higher address

          while (count--)

                  *dest++ = *source;

           return ret;

      }

memmove

memmove - Copy source buffer to destination buffer
;
;Purpose:
;       memmove() copies a source memory buffer to a destination memory buffer.
;       This routine recognize overlapping buffers to avoid propogation.
;       For cases where propogation is not a problem, memcpy() can be used.
;
;   Algorithm:

    void* memmove(void* dest, void* source, size_t count)

   {

       void* ret = dest;

      if (dest <= source || dest >= (source + count))

       {

          //Non-Overlapping Buffers
         //copy from lower addresses to higher addresses

         while (count --)

               *dest++ = *source++;

     }

     else

     {

        //Overlapping Buffers
       //copy from higher addresses to lower addresses

       dest += count - 1;

       source += count - 1;

       while (count--)

                *dest-- = *source--;l

     }

      return ret;

   }