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) //byte是java里的变量类型

{

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++;

returnpvTo;

}

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 ;

}

}

returnpDst;

}

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(inti=0; i < n; i++) /*正向拷贝*/

{

*pdest= *psrc;

psrc++;

pdest++;

}

}

else/*反向拷贝*/

{

psrc+= n;

pdest+= n;

for(inti=0;i<n;i++)

{

psrc--;

pdest--;

*pdest = *psrc;

}

}

returndest;

}

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;

returnbuffer;

}