main(){  int i, j, in;  char buf[512];  do {    printf("Input device: ");    gets(buf);    in = open(buf, 0);  } while (in <= 0);  do {    printf("Output device: ");    gets(buf);    j = open(buf, 1, 0777);  } while (j <= 0);  while (1) {    if ((i = read(in, buf, 512)) <= 0) {      exit(1);    }    write(j, buf, 512);  }}