memcpy和memmove的区别与实现

memcpy和memmove函数的实现

区别:

从DESCRIPTION看来，两者的功能基本相同，唯一不同的是，当 dest 和 src 有重叠的时候选用不同的函数可能会造成不同的结果。不妨写个小程序来测一下：

0 ＃i nclude <string.h>
1 ＃i nclude <stdio.h>
2
3 int main()
4 {
5 int i = 0;
6 int a[10];
7
8 for(i; i < 10; i++)
9 {
10 a[i] = i;
11 }
12
13 memcpy(&a[4], a, sizeof(int)*6);
14
15 for(i = 0; i < 10; i++)
16 {
17 printf("%d ",a[i]);
18 }
20
21 printf("/n");
22 return 0;
23 }

很简单的小程序！不过已经足以达到我的目的了：）将上面代码gcc之后再运行，结果为：0 1 2 3 0 1 2 3 0 1 。
再把第13行改成：memmove(&a[4], a, sizeof(int)*6)，重新gcc再运行，结果为：0 1 2 3 0 1 2 3 4 5 ！
呵呵，两者的区别出现了。不过其实这样还不够，继续修改13行： memmove(a, &a[4], sizeof(int)*6) //也就是将源、目的置换一下而已
重新gcc编译再运行，结果为：4 5 6 7 8 9 6 7 8 9 。
还不够，继续修改13行为： memcpy(a, &a[4], sizeof(int)*6); gcc并运行，结果仍为： 4 5 6 7 8 9 6 7 8 9 ！
至此真相已经大白了。对比上面四个结果，不难得出以下结论：
1. 当 src 和 dest 所指内存区有重叠时，memmove 相对 memcpy 能提供保证：保证能将 src 所指内存区的前 n 个字节正确的拷贝到 dest 所指内存中；
2. 当 src 地址比 dest 地址低时，两者结果一样。换句话说，memmove 与 memcpy 的区别仅仅体现在 dest 的头部和 src 的尾部有重叠的情况下；

memcpy


代码:
;***
;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;

}

另一种实现：

void* mymemcpy( void* dest, const void* src, size_t count )
{
char* d = (char*)dest;
const char* s = (const char*)src;
// int n = (count + 7) / 8; // count > 0 assumed
int n = count >> 3;
switch( count & 7 )
{
do { *d++ = *s++;
case 7: *d++ = *s++;
case 6: *d++ = *s++;
case 5: *d++ = *s++;
case 4: *d++ = *s++;
case 3: *d++ = *s++;
case 2: *d++ = *s++;
case 1: *d++ = *s++;
case 0 } //while (--n > 0);
while (n-- > 0)
}

return dest;
}