我的memcmp：
1 int memcmp(void *buf1, void *buf2, unsigned int count){
2 int reval;
3 while(count && !(reval = (*(unsigned char *)buf1) - (*(unsigned char *)buf2)))
4 {
5 buf1 = (unsigned char *)buf1 + 1;
6 buf2 = (unsigned char *)buf2 + 1;
7 --count;
8 }
9 return reval;
10 }
MS VC：
int __cdecl memcmp (
const void * buf1,
const void * buf2,
size_t count
)
{
if (!count)
return(0);while ( --count && *(char *)buf1 == *(char *)buf2 ) {
buf1 = (char *)buf1 + 1;
buf2 = (char *)buf2 + 1;
}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：
1 void *memcpy(void *dest, const void *src, unsigned int count){
2 void *reval = dest;
3 while(count--){
4 (*(unsigned char *)dest++) = (*(unsigned char *)src++);
5 }
6 return reval;
7 }
MSVC：
1 void * __cdecl memcpy (
2 void * dst,
3 const void * src,
4 size_t count
5 )
6 {
7 void * ret = dst;
8
9 #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
10 {
11 extern void RtlMoveMemory( void *, const void *, size_t count );
12
13 RtlMoveMemory( dst, src, count );
14 }
15 #else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
16 /*
17 * copy from lower addresses to higher addresses
18 */
19 while (count--) {
20 *(char *)dst = *(char *)src;
21 dst = (char *)dst + 1;
22 src = (char *)src + 1;
23 }
24 #endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
25
26 return(ret);
27 }
我的memmove:
1 void *memmove(void *dest, const void *src, unsigned int count){
2 void *reval = dest;
3 int overlap = ((unsigned char *)src < (unsigned char *)dest && ((unsigned char *)src + count) > dest);
4 while(count--){
5 if(overlap)//src is in front of dest and overlap. copy direction is from endIndex to beginIndex
6 (*((unsigned char *)dest + count)) = (*((unsigned char *)src + count));
7 else
8 (*(unsigned char *)dest++) = (*(unsigned char *)src++);
9 }
10 return reval;
11 }
MSVC:
1 void * __cdecl memmove (
2 void * dst,
3 const void * src,
4 size_t count
5 )
6 {
7 void * ret = dst;
8
9 #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
10 {
11 extern void RtlMoveMemory( void *, const void *, size_t count );
12
13 RtlMoveMemory( dst, src, count );
14 }
15 #else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
16 if (dst <= src || (char *)dst >= ((char *)src + count)) {
17 /*
18 * Non-Overlapping Buffers
19 * copy from lower addresses to higher addresses
20 */
21 while (count--) {
22 *(char *)dst = *(char *)src;
23 dst = (char *)dst + 1;
24 src = (char *)src + 1;
25 }
26 }
27 else {
28 /*
29 * Overlapping Buffers
30 * copy from higher addresses to lower addresses
31 */
32 dst = (char *)dst + count - 1;
33 src = (char *)src + count - 1;
34
35 while (count--) {
36 *(char *)dst = *(char *)src;
37 dst = (char *)dst - 1;
38 src = (char *)src - 1;
39 }
40 }
41 #endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
42
43 return(ret);
44 }
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://stackoverflow.com/questions/4415910/memcpy-vs-memmove
我的memset:
1 void *memset(void *buffer, int c, int count){
2 void *reval = buffer;
3 while(count--){
4 (*(unsigned char *)buffer++) = (unsigned char)c;
5 }
6 return reval;
7 }
MSVC:
1 void * __cdecl memset (
2 void *dst,
3 int val,
4 size_t count
5 )
6 {
7 void *start = dst;
8
9 #if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
10 {
11 extern void RtlFillMemory( void *, size_t count, char );
12
13 RtlFillMemory( dst, count, (char)val );
14 }
15 #else /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
16 while (count--) {
17 *(char *)dst = (char)val;
18 dst = (char *)dst + 1;
19 }
20 #endif /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
21
22 return(start);
23 }
完
走进C标准库(7)——"string.h"中函数的实现memcmp,memcpy,memmove,memset,码迷,mamicode.com
时间： 2024-10-11 22:50:38