問題一:函數區別
memcpy與memmove都是對內存進行拷貝可以拷貝任何內容,而strcpy僅是對字符串進行操作。
memcpy與memmove拷貝多少是通過其第三個參數進行控制而strcpy是當拷貝至'\0'停止。
memmove函數的功能同memcpy基本一致,但是當src區域和dst內存區域重疊時,memcpy可能會出現錯誤,而memmove能正確進行拷貝。
問題二:memcpy效率
https://aigo.iteye.com/blog/2299702
https://blog.csdn.net/z2007b/article/details/6559457
待分析。
問題三:memcpy實現
實現memcpy:
實現一:
void *memcpy(void *dest, const void *src, size_t count)
{
assert(src != NULL && dest != NULL);
char *tmp = dest;
const char *s = src;
while (count--)
*tmp++ = *s++ ;
return dest;
}
實現二:
void * memcpy(void *dst,const void *src,size_t num)
{
assert(src != NULL && dest != NULL);
int nchunks = num/sizeof(dst); /*按CPU位寬拷貝*/
int slice = num%sizeof(dst); /*剩餘的按字節拷貝*/
unsigned long * s = (unsigned long *)src;
unsigned long * d = (unsigned long *)dst;
while(nchunks--)
*d++ = *s++;
while (slice--)
*((char *)d++) =*((char *)s++);
return dst;
}
實現三(如果內存有重疊):
void *memcpy(void *dest, const void *src, size_t count)
{
assert(src != NULL && dest != NULL);
char *d;
const char *s;
if (dest > (src+size)) || (dest < src))
{
d = dest;
s = src;
while (count--)
*d++ = *s++;
}
else /* overlap */
{
d = (char *)(dest + count - 1); /* offset of pointer is from 0 */
s = (char *)(src + count -1);
while (count --)
*d-- = *s--;
}
return dest;
}