实现memcpy()函数及过程总结

 1 void *my_memcpy_byte(void *dst, const void *src, int n)
 2 {
 3     if (dst == NULL || src == NULL || n <= 0)
 4         return NULL;
 5 
 6     char * pdst = (char *)dst;
 7     char * psrc = (char *)src;
 8 
 9     if (pdst > psrc && pdst < psrc + n)
10     {
11         pdst = pdst + n - 1;
12         psrc = psrc + n - 1;
13         while (n--)
14             *pdst-- = *psrc--;
15     }
16     else
17     {
18         while (n--)
19             *pdst++ = *psrc++;
20     }
21     return dst;
22 }

 1 void *my_memcpy(void *dst, const void *src, int n)
 2 {
 3     if (dst == NULL || src == NULL || n <= 0)
 4         return NULL;
 5 
 6     int * pdst = (int *)dst;
 7     int * psrc = (int *)src;
 8     char *tmp1 = NULL;
 9     char *tmp2 = NULL;
10     int c1 = n / 4;
11     int c2 = n % 4;
12 
13     if (pdst > psrc && pdst < psrc + n)
14     {
15         tmp1 = (char *)pdst + n - 1;
16         tmp2 = (char *)psrc + n - 1;
17         while(c2--)
18             *tmp1-- = *tmp2--;
19         pdst = (int *)tmp1;
20         psrc = (int *)tmp2;
21         while (c1--)
22             *pdst-- = *psrc--;
23     }
24     else
25     {
26         while (c1--)
27             *pdst++ = *psrc++;
28         tmp1 = (char *)pdst;
29         tmp2 = (char *)psrc;
30         while (c2--)
31             *tmp1++ = *tmp2++;
32     }
33 
34     return dst;
35 }