CS360 Final Exam: December 12, 2000. Questions 5: Answer and Grading

CS360 Final Exam: December 12, 2000. Question 5: Answer and Grading

This is a simple matter of getting the pipe/dup2/close/wait calls correct. Here is the answer. You could do other things, like making both pipe() calls initially, or closing everything in the parent just before the wait() calls, but this is pretty much it:

#include < stdio.h >
#include < fcntl.h >

main()
{
  int pa[2];
  int pb[2];
  int fd;
  int dummy;

  pipe(pa);

  if (fork() == 0) {

    fd = open("f1", O_RDONLY);
    if (fd < 0) { perror("f1"); exit(1); }

    dup2(fd, 0);
    dup2(pa[1], 1);

    close(fd);
    close(pa[0]);
    close(pa[1]);

    execlp("sort", "sort", NULL);
    perror("execlp");
    exit(1);

  } 

  close(pa[1]);
  pipe(pb);

  if (fork() == 0) {

    dup2(pa[0], 0);
    dup2(pb[1], 1);

    close(pa[0]);
    close(pb[0]);
    close(pb[1]);

    execlp("head", "head", "-5", NULL);
    perror("execlp");
    exit(1);

  }

  close(pa[0]);
  close(pb[1]);

  if (fork() == 0) {

    dup2(pb[0], 0);

    close(pb[0]);

    execlp("cat", "cat", "-n", NULL);
    perror("execlp");
    exit(1);
  }

  close(pb[0]);
  wait(&dummy);
  wait(&dummy);
  wait(&dummy);

}

Grading

12 points

One point for each of the following:

Calling the first pipe in the right place
Opening f1 in the first child
Dup2'ing f1 and pa[1] in the first child
Making all the correct closes
Calling the first pipe in the right place: 1 point
Dup2'ing pa[0] and pb[1] in the second child
Making all the correct closes in the second child
Dup2'ing pb[1] in the third child
Making all the correct closes in the third child
Making all the correct closes in the parent
Calling exit after the execlp calls
Making the wait calls

I went fractional here -- if you had to make three closes, and you only made 2, you got 2/3 (0.67). If you didn't check for the open of f1 failing, you only got 3/4.

Points were deducted for the following reasons:

Too much extraneous code: 1 to 2 points
Bad form, even though lots of the calls are right: 1 to 11 points