memcpy与memmove的实现

/*   memcpy是不管有否区域重叠的，重叠不重叠照样复制，   memmove才会避免区域重叠，检测这个区域重叠很简单，如果目标地址在源区域内，那就会发生重叠.   处理重叠也很简单，从源地址的末尾开始反方向复制就OK了。 未重叠的情况:  内存布局(little endian):  higher address                           lower address     |--------------------------------------|                                 src                   |------------|     |-----------|                |------------|                 dest                            dest 目标区域的首地址在源区域内，就发生重叠了。                        higher address              src(lower address)                              |---------------------|           |---------------------|                                   dest                */ //下面是memmove源码的一段： #include <stdio.h> #include <string.h> #include <stdlib.h> /* ============== my_memmove() ==============*/ #if 1 void *my_memmove(void *dest, const void *src, size_t n) {     void *pdest = dest;     if(NULL == dest && NULL == src && (n == 0))     {         return;     }     if((dest <= src) || (char *)dest >= ((char *)src + n))     {         /*          * Non Overlapping Buffers          * copy from lower address to higher address          * 正向拷贝          */         while(n--)         {             *(char *)dest = *(char *)src;             dest = (char *)dest + 1;             src = (char *)src + 1;         }     }     else     {         /*          * Overlapping Buffers          * copy from higher address to lower address          * 反向拷贝          */         dest = (char *)dest + n - 1;         src = (char *)src + n - 1;                  while(n--)         {             *(char *)dest = *(char *)src;             dest = (char *)dest - 1;             src = (char *)src - 1;         }     }     return pdest; } #else void* my_memmove(void *dest, const void *src,size_t n) {     if(NULL == dest && NULL == src && (n == 0))     {         return;     }     int i;     char *pdest = (char *)dest;     char *psrc = (char *)src;          if((pdest <= psrc) || (pdest >= psrc + n))     {         /* 正向拷贝 */         for(i = 0; i < n; ++i)         {             *pdest++ = *psrc++;         }     }     else     {         /* 反向拷贝 */         pdest = pdest + n -1;         psrc = psrc + n - 1;                  for(i = 0; i < n; ++i)         {             *pdest-- = *psrc--;         }     }     return dest; } #endif /* =============== my_memcpy() ==============*/ //pDest < pSrc 顺序拷贝，否则逆序 #if 1 void *my_memcpy(void *dest, const void *src, size_t n) {     char *pdest = (char*)dest;     const char *psrc = (char *)src;     size_t i;     if(NULL == dest && NULL == src && (n == 0))     {         return;     }     if((pdest > psrc) && (pdest < psrc + n))     {         /* 反向拷贝 */         for(i = n - 1; i <= 0; ++i)         {            pdest[i] = psrc[i];         }     }     else     {         /* 正向拷贝 */         for(i = 0; i < n; ++i)         {             pdest[i] = psrc[i];         }     }     pdest = '\0';     return pdest; } #else /* ================ libc ==================*/ void *memcpy(void *dst, const void *src, size_t n) {     void *ret = dst;     if(NULL == src && NULL == dst)     {         return;     }     while(n--)     {         *(char *)dst = *(char *)src;         dst = (char *)dst + 1;         src = (char *)src + 1;     }     return ret; } #endif /* c++ 版: pDest < pSrc 顺序拷贝，否则逆序 void *memCopy(void *dest,const void *src,size_t count) {     char *pDest=static_cast<char *>(dest);     const char *pSrc=static_cast<const char *>(src);     if( pDest>pSrc && pDest<pSrc+count )     {         for(size_t i=count-1; i<=0; ++i)         {             pDest[i]=pSrc[i];         }     }     else     {         for(size_t i=0; i<count; ++i)         {              pDest[i]=pSrc[i];         }     }     return pDest; } */ /* 另一个单不考虑不重叠的情况: memcpy, 拷贝不重叠的内存块 void *memcpy(void* pvTo, void* pvFrom, size_t size) {     assert(pvTo != NULL && pvFrom != NULL);     void* pbTo = (char *)pvTo;     void* pbFrom = (char *)pvFrom;       /* 内存块重叠吗？如果重叠，就使用memmove */     assert(pbTo>=pbFrom+size || pbFrom>=pbTo+size);     while(size--)     {          *pbTo++ == *pbFrom++;     }     return pvTo; } */ int main(int argc, char *argv[]) {     char *str = "I'm study c";     char *dst = NULL;          dst = (char *)malloc(sizeof(char) + 1 * sizeof(char));     printf("after call my_memmove():\n");     my_memmove(dst, str, strlen(str));     printf("dst = %s\n", dst);     printf("after call my_memcpy():\n");     my_memcpy(dst, str, strlen(str));     printf("dst = %s\n", dst);     free(dst);     return 0; }