memcpy和memmove区别
memcpy
函数声明:
void* memcpy(void* dest, const void* src, size_t count);
头文件:
#include<string.h>或者#include<memory.h>
参数:
dest:
New buffer
src:
Buffer to copy from
count:
Number of characters to copy
摘要:
memcpy函数将src的count字节复制到dest,如果源和目的字节重叠,这个函数不能保证重叠区域的源字节在被覆盖之前被复制。使用memmove处理重叠区域。
模拟实现:
void* memcpy(void* dest, const void* src, size_t count)
{
assert(dest);
assert(src);
void* ret = dest;
while (count--)
{
*(char*)dest = *(char*)src;
(char*)dest = (char*)dest + 1;
(char*)src = (char*)src + 1;
}
return ret;
}
memmove
函数声明:
void *memmove( void *dest, const void *src, size_t count );
头文件:
#include<string.h>
参数:
dest:
Destination object
src:
Source object
count:
Number of bytes of characters to copy
摘要:
memmove函数将计数字节的字符从src复制到dest,如果源区域和目标区域的某些区域重叠,memmove确保重叠区域的原始源字节在被覆盖之前被复制。
模拟实现:
void *memmove(void *dest, const void *src, size_t count)
{
assert(dest);
assert(src);
void* ret = dest;
if (dest <= src || (char*)dest >= ((char*)+count))
{
while (count--)
{
*(char*)dest = *(char*)src;
(char*)dest = (char*)dest + 1;
(char*)src = (char*)src + 1;
}
}
else
{
(char*)dest = (char*)dest + count - 1;
(char*)src = (char*)src + count - 1;
while (count--)
{
*(char*)dest = *(char*)src;
(char*)dest = (char*)dest - 1;
(char*)src = (char*)src - 1;
}
}
return ret;
}
memmove实现原理
总结
memmove()和memcpy()函数的区别和联系:
相同点: 两个都是内存拷贝,对所有类型都适用;
不同点:
(1)memcpy()函数是从前往后拷贝;假入出现内存重叠的现象;拷贝结果可能出错;
(2)memmove()函数在memcpy()函数的基础上加入了对内存重叠拷贝的处理;引入了倒序拷贝的方式处理内存重叠的某些情况;保证拷贝的正确性;