我的memcmp：
int memcmp(void *buf1, void *buf2, unsigned int count){
int reval;
while(count && !(reval = (*(unsigned char *)buf1) - (*(unsigned char *)buf2)))
{
buf1 = (unsigned char *)buf1 + ;
buf2 = (unsigned char *)buf2 + ;
--count;
}
return reval;
}
MS VC：
int __cdecl memcmp (
const void * buf1,
const void * buf2,
size_t count
)
{
if (!count)
return(); while ( --count && *(char *)buf1 == *(char *)buf2 ) {
buf1 = (char *)buf1 + ;
buf2 = (char *)buf2 + ;
} return( *((unsigned char *)buf1) - *((unsigned char *)buf2) );
}
应该使用const void *buf为宜，不改变该块内存的内容，最终使用unsigned char *进行运算，保证运算结果的符号正确。
我的memcpy：
void *memcpy(void *dest, const void *src, unsigned int count){
void *reval = dest;
while(count--){
(*(unsigned char *)dest++) = (*(unsigned char *)src++);
}
return reval;
}
MSVC：
void * __cdecl memcpy (
void * dst,
const void * src,
size_t count
)
{
void * ret = dst; #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
{
extern void RtlMoveMemory( void *, const void *, size_t count ); RtlMoveMemory( dst, src, count );
}
#else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
/*
* copy from lower addresses to higher addresses
*/
while (count--) {
*(char *)dst = *(char *)src;
dst = (char *)dst + ;
src = (char *)src + ;
}
#endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */ return(ret);
}
我的memmove:
void *memmove(void *dest, const void *src, unsigned int count){
void *reval = dest;
int overlap = ((unsigned char *)src < (unsigned char *)dest && ((unsigned char *)src + count) > dest);
while(count--){
if(overlap)//src is in front of dest and overlap. copy direction is from endIndex to beginIndex
(*((unsigned char *)dest + count)) = (*((unsigned char *)src + count));
else
(*(unsigned char *)dest++) = (*(unsigned char *)src++);
}
return reval;
}
MSVC:
void * __cdecl memmove (
void * dst,
const void * src,
size_t count
)
{
void * ret = dst; #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
{
extern void RtlMoveMemory( void *, const void *, size_t count ); RtlMoveMemory( dst, src, count );
}
#else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
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 + ;
src = (char *)src + ;
}
}
else {
/*
* Overlapping Buffers
* copy from higher addresses to lower addresses
*/
dst = (char *)dst + count - ;
src = (char *)src + count - ; while (count--) {
*(char *)dst = *(char *)src;
dst = (char *)dst - ;
src = (char *)src - ;
}
}
#endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */ return(ret);
}
关于memcpy和memmove的区别，memcpy不考虑内存区域重叠的情况而memmove保证内存区域重叠也能正常复制成功。
有时候我们的memcpy也可能在内存重叠的情况下正常使用，这取决于它的实现，不具有普遍性，C语言标准中未对其有这种要求。
参考资料：
《关于memcpy和memmove两函数的区别》
http://blog.csdn.net/caowei840701/article/details/8491836
《memcpy() vs memmove()》
http://*.com/questions/4415910/memcpy-vs-memmove
我的memset:
void *memset(void *buffer, int c, int count){
void *reval = buffer;
while(count--){
(*(unsigned char *)buffer++) = (unsigned char)c;
}
return reval;
}
MSVC:
void * __cdecl memset (
void *dst,
int val,
size_t count
)
{
void *start = dst; #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
{
extern void RtlFillMemory( void *, size_t count, char ); RtlFillMemory( dst, count, (char)val );
}
#else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
while (count--) {
*(char *)dst = (char)val;
dst = (char *)dst + ;
}
#endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */ return(start);
}
完