memcpy和memmove的区别
昨天栽了个大跟头，需要记录一下，memcpy()和memmove()的区别，之前有了解过这两个的区别，无奈一直没有用到，一下子就把memmove()这个函数给忘掉了。以至于对它一点印象都没有，真的是印证了那句话，不去用就等于没用。
1.源码路径
网上很多文章对于这些标准库的方法都是大概讲，有些细节给隐藏掉了，我还是习惯找到源码的出处，自己再根据别人的文章去理解。这两个函数在glibc中实现，可以自行去下载glibc的源码。
glibc/string/memmove.c
glibc/string/memcopy.c2.源码
#include
#include
#undef memcpy
void *
memcpy (void *dstpp, const void *srcpp, size_t len)
{
unsigned long int dstp = (long int) dstpp;
unsigned long int srcp = (long int) srcpp;
/* 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);
return dstpp;
}
libc_hidden_builtin_def (memcpy) #include
#include
/* 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
#ifndef MEMMOVE
#define MEMMOVE memmove
#endif
rettype
inhibit_loop_to_libcall
MEMMOVE (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 MEMCPY_OK_FOR_FWD_MEMMOVE
dest = memcpy (dest, src, len);
#else
/* 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);
#endif /* MEMCPY_OK_FOR_FWD_MEMMOVE */
}
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 源码看起来很容易理解，memcpy()没有判断dstpp和srcpp指向的空间大小是否会出现重叠，如果重叠，则会出现后面拷贝的数据把前面的一部分数据覆盖了，造成数据出错。memmove()则做了判断，如果源地址和目的地址不会重叠，则调用memcpy()否则进行后置拷贝，从地址指向的内存空间的最后往前拷贝。
BYTE_COPY_FWD、 WORD_COPY_FWD、WORD_COPY_BWD、BYTE_COPY_BWD，从宏的名字可以看出是字节的前置拷贝，字前置拷贝，字后置拷贝，字节后置拷贝，据说是有做的编译优化。
3.总结
以后用memmove()函数吧，不用memcpy()了。