memcpy和memmove的区别

memcpy（）和memmove（）都是C语言中的库函数，在头文件string.h中，其原型分别如下：

 

void *memcpy(void *dst, const void *src, size_t count);
void *memmove(void *dst, const void *src, size_t count);

 

它们都是从src所指向的内存中复制count个字节到dst所指内存中，并返回dst的值。当源内存区域和目标内存区域无交叉时，两者的结果是一样的，但如果有交叉呢？先看下图：

 

图的上半部分为源内存区域在目标内存区域右边，下半部分为源内存区域在目标区域左边，源内存区域和目标内存区域都有交叉。

memcpy（）是从src的起始部分开始复制，所以虽然第一种情况下没有问题，但如果遇到第二种情况，则会发生错误，如图所示，后两个字节在被复制前已经被覆盖掉了。而memmove（）则由于采用了不同的复制机制，所以可以正确处理第二种情况。

VS.NET2003中所附源码如下（有删）：

void * __cdecl memcpy (void * dst, const void * src, size_t count)
{
        void * ret = dst;

        /*
         * copy from lower addresses to higher addresses
         */
        while (count--) {
                *(char *)dst = *(char *)src;
                dst = (char *)dst + 1;
                src = (char *)src + 1;
        }

        return(ret);
}

void * __cdecl memmove (void * dst, const void * src, size_t count)
{
        void * ret = dst;

        if (dst <= src || (char *)dst >= ((char *)src + count)) {
                /*
                 * Non-Overlapping Buffers
                 * copy from lower addresses to higher addresses
                 */
                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst + 1;
                        src = (char *)src + 1;
                }
        }
        else {
                /*
                 * Overlapping Buffers
                 * copy from higher addresses to lower addresses
                 */
                dst = (char *)dst + count - 1;
                src = (char *)src + count - 1;

                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst - 1;
                        src = (char *)src - 1;
                }
        }

        return(ret);
}