asmlinkage int sys_copy(char *s,char *d){   char buff[512];   int source,dest;   int num;   mm_segment_t old_fs;   old_fs=get_fs();   set_fs(get_ds());      if((sourcefd=sys_open(s,O_RDONLY,0))<0)       return -1;    if((destfd=sys_open(d,O_CREAT | O_WRONLY | O_TRUNC,0))<0)          return -1;    while((num=sys_read(source,buff,512))>0)       {         if((sys_write(dest,buff,num))!=num)            return -1;           }       sys_close(sourcefd);     sys_close(destfd);      set_fs(old_fs);           return 0; }