走进C标准库(7)——"string.h"中函数的实现memcmp,memcpy,memmove,memset

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