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

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://stackoverflow.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