duff_device.c

#include <stdio.h>
#include <string.h>

void copy(char * to, char * from, int count)
{
    int n = (count + 7 ) / 8 ; 
    switch (count % 8 ) { 
        case 0 :    do { *to++ = *from++;
        case 7 :         *to++ = *from++;
        case 6 :         *to++ = *from++;
        case 5 :         *to++ = *from++;
        case 4 :         *to++ = *from++;
        case 3 :         *to++ = *from++;
        case 2 :         *to++ = *from++;
        case 1 :         *to++ = *from++;
                    } while (--n > 0); 
    }   
}   

int main(int argc, const char *argv[])
{
    char from[100] = "Hello, my name is liyang. I come from city of Daqing.";
    char to[100];
    copy(to, from, strlen(from)+1);
    printf("dest content: %s\n", to);
    return 0;
}