Pipe: Review Questions

Question numbers subject to change

Q1

Fill in the blanks to make the following program print 123456789. If cat is given no arguments it simply prints its input until EOF. Bonus: Explain why the close call below is necessary.

int main() {
    int i = 0;
    while (++i < 10) {
        pid_t pid = fork();
        if (pid == 0) { /* child */
            char buffer[16];
            sprintf(buffer, ______, i);
            int fds[______];
            pipe(fds);
            write(fds[1], ______, ______); // Write the buffer into the pipe
            close(______);
            dup2(fds[0], ______);
            execlp("cat", "cat",  ______);
            perror("exec"); exit(1);
        }
        waitpid(pid, NULL, 0);
    }
    return 0;
}

Q2

Use POSIX calls fork pipe dup2 and close to implement an autograding program. Capture the standard output of a child process into a pipe. The child process should exec the program ./test with no additional arguments (other than the process name). In the parent process read from the pipe: Exit the parent process as soon as the captured output contains the ! character. Before exiting the parent process send SIGKILL to the child process. Exit 0 if the output contained a !. Otherwise if the child process exits causing the pipe write end to be closed, then exit with a value of 1. Be sure to close the unused ends of the pipe in the parent and child process

Q3 (Advanced)

This advanced challenge uses pipes to get an "AI player" to play itself until the game is complete. The program tictactoe accepts a line of input - the sequence of turns made so far, prints the same sequence followed by another turn, and then exits. A turn is specified using two characters. For example "A1" and "C3" are two opposite corner positions. The string B2A1A3 is a game of 3 turns/plys. A valid response is B2A1A3C1 (the C1 response blocks the diagonal B2 A3 threat). The output line may also include a suffix -I win -You win -invalid or -draw Use pipes to control the input and output of each child process created. When the output contains a -, print the final output line (the entire game sequence and the result) and exit.