HW2
released: 01/26,15:00
due: 02/01, 23:59

Answer the following questions.
Submit your answers to Canvas assignments.
There is an entry for this homework.


1. fork, short circuit, and adoption (6 points)

Read the C code below and answer the questions.

  #include<stdio.h>
  #include<stdlib.h>
  #include<unistd.h>

  int main(){
      printf("[%d]: main process\n", getpid());
      fork() || fork();
      printf("[%d]: my parent is %d\n", getpid(), getppid());
  }

Questions:

  1.a (1 point)
    Run the C code above and what outputs do you get (copy them below)?


  1.b (2 points)
    Why does the program have such outputs?
    Explain why in 2--3 sentences.
    [hints:
      - pay attention to the return values of "fork()"
      - study "||" operator in C
      - what you saw is called "short circuit"
    ]


  [update 01/30: the 1.c and 1.c below (one is a typo; should have been "1.d")
  are ambiguous in multiple ways. So, we rephrase the questions to be a new 1.c.
  You can submit answers to either the new "1.c" or the old two "1.c"s.
  Both are valid answers.]

  //  1.c (1 point)
  //    You should see multiple lines of "[ABC]: my parent is [XYZ]",
  //    where "[ABC]" and "[XYZ]" are integers (in fact, process ids).
  //    There is one "[XYZ]" (namely parent pid) that hasn't shown up in any line of
  //    "[ABC]" (the process who prints this line).
  //    Who is this "[XYZ]"?
  //      [hint: use "echo $$" to show the pid of the shell]
  //
  //
  //  1.c (1 point)
  //    Are there processes whose parent is pid=1? (if no, say "no")
  //    If yes, why its parent is pid=1? can you give a guess in 1 sentence?
  //    [hint keyword: orphan process]

  1.c (2 points)  [update 01/30: this is a new 1.c]
    You should see multiple lines of "[ABC]: my parent is [XYZ]",
    where "[ABC]" and "[XYZ]" are integers (in fact, process ids).
    There are two "[XYZ]" (namely parent pid) that haven't shown up in any line of
    "[ABC]" (the process who prints this line).
    Who are the two "[XYZ]"?
    [hints:
      -- use "echo $$" to show the pid of the shell
      -- hint keyword: orphan process
    ]



  1.d (1 point)
    [update 01/30: if you haven't seen pid=1, then say "not apply"]
    To avoid getting pid=1, Ryan modified the program to be:

      int main(){
          printf("[%d]: main process\n", getpid());
          fork()|| fork();
          printf("[%d]: my parent is %d\n", getpid(), getppid());

          wait(NULL);
      }

    Will this work?
    Explain why in 1 sentence.




2. Pipe, a potential pitfall in Lab2 (4 points)

Read the following code about syscall pipe and answer questions:

    #include <sys/types.h>
    #include <sys/wait.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <string.h>

    int
    main(int argc, char *argv[])
    {
        int pipefd[2];
        pid_t pid;
        char buf;

        if (argc != 2) {
            fprintf(stderr, "Usage: %s <string>\n", argv[0]);
            exit(EXIT_FAILURE);
        }

        if (pipe(pipefd) == -1) {
            perror("pipe");
            exit(EXIT_FAILURE);
        }

        pid = fork();
        if (pid == -1) {
            perror("fork");
            exit(EXIT_FAILURE);
        }

        if (pid == 0) {    // child
            close(pipefd[1]);      // <---HERE

            while (read(pipefd[0], &buf, 1) > 0)
                write(STDOUT_FILENO, &buf, 1);

            write(STDOUT_FILENO, "\n", 1);
            close(pipefd[0]);
            exit(EXIT_SUCCESS);

        } else {            // parent
            close(pipefd[0]);          // Close unused read end
            write(pipefd[1], argv[1], strlen(argv[1]));
            close(pipefd[1]);          // Reader will see EOF
            wait(NULL);                // Wait for child
            exit(EXIT_SUCCESS);
        }
    }


Questions:

  2.a (1 point)
    What does this program do?
    Explain in 1--2 sentences.


  2.b (1 point)
    If you comment out the line with "<---HERE", what has changed when you
    compile and rerun the program?
    [hint: notice if the program finishes.]


  2.c (2 points)
    Why does commenting the line with "<---HERE" have this consequence?
    Explain why in 1--2 sentences.
    [hint: this is THE most frequent bug in Lab2.
     You do want to understand why and avoid it.]