strcpy()、memcpy()、memmove()、memset()的实现
 

strcpy(), 字符串拷贝.
char *strcpy(char *strDest, const char *strSrc)
{
    assert((strDest!=NULL) && (strSrc !=NULL));
    char *address = strDest;     
    while( (*strDest++ = * strSrc++) != '\0') 
       NULL ; 
    return address ;       
}

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 = (byte*)pvTo;
void* pbFrom = (byte*)pvFrom;
/* 内存块重叠吗？如果重叠，就使用memmove */
assert(pbTo>=pbFrom+size || pbFrom>=pbTo+size);
while(size-->0)
    *pbTo++ == *pbFrom++;
return pvTo;
}

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;
}

void *Memmove(void *Dst, const void*Src,size_t count)
{
assert(Dst && Src);
void* pDst = Dst;
if (Dst<Src && (char*)Dst > (char*)Src + count)
{
while(count--)
{
   *(char*)Dst = *(char*)Src;
   Dst = (char*)Dst + 1;
   Src = (char*)Src + 1;
}
}
else
{
   Dst = (char*)Dst + count - 1;
   Src = (char*)Src + count - 1;
   while(count--)
   {
      *(char*)Dst = *(char*)Src;
      Dst = (char*)Dst -1 ;
      Src = (char*)Src -1 ;
   }
}
return pDst;
}


void* memmove(void *dest, const void *src,size_t n) 
{ 
    if (n == 0) return 0; 
    if (dest == NULL) return 0; 
    if (src == NULL)    return 0; 
    char *psrc = (char*)src; 
    char *pdest = (char*)dest; 
    if((dest <= psrc) || (pdest >= psrc + n)) /*检查是否有重叠问题 */ 
        { 
         for(int i=0; i < n; i++) /*正向拷贝*/ 
          { 
           *pdest = *psrc; 
           psrc++; 
           pdest++; 
          } 
        } 
        else /*反向拷贝*/ 
        { 
          psrc += n; 
          pdest += n; 
          for(int i=0;i<n;i++) 
           { 
            psrc--; 
            pdest--; 
            *pdest = *psrc; 
           } 
        } 
   return dest;
}

memset:把buffer所指内存区域的前count个字节设置成字符c

void * Memset(void* buffer, int c, int count)
{
char* pvTo=(char*)buffer;
assert(buffer != NULL);
while(count-->0)
*pvTo++=(char)c;
return buffer;
}