How do I create a pipe to connect two child processes using fork, pipe, and dup?

Question

Let's say within a program I want to execute two processes, one to execute a ls -al command, then piping the result into the wc command, and displaying the output on the terminal. How can I do this using pipe file descriptors? So far the code I have written:

int main(int argc, char* argv[]) {
    int pipefd[2];
    int pipefd2[2];

    pipe(pipefd2);
    if ((fork()) == 0) {
       dup2(pipefd2[1], STDOUT_FILENO);
       close(pipefd2[0]);
       close(pipefd2[1]);
       execl("ls", "ls", "-al", NULL);
       exit(EXIT_FAILURE);
    } 

    if ((fork()) == 0){
        dup2(pipefd2[0], STDIN_FILENO);
        close(pipefd2[0]);
        close(pipefd2[1]);
        execl("/usr/bin/wc", "wc", NULL);
        exit(EXIT_FAILURE);
    }
    close(pipefd[0]);
    close(pipefd[1]);
    close(pipefd2[0]);
    close(pipefd2[1]);
}

An example would be greatly helpful.

Answer 1

Your example code was syntactically and semantically broken (e.g. pipefd2 not decared, confusion between pipefd and pipefd2, etc.) Since this smells like homework, please make sure you understand my annotations below and ask more if you need to. I've omitted error checks on pipe, fork and dup, but they should be there, ideally.

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

    pipe(pipefd);

    // this child is generating output to the pipe
    //
    if ((ls_pid = fork()) == 0) {
        // attach stdout to the left side of pipe
        // and inherit stdin and stdout from parent
        dup2(pipefd[1],STDOUT_FILENO);
        close(pipefd[0]);              // not using the right side

        execl("/bin/ls", "ls","-al", NULL);
        perror("exec ls failed");
        exit(EXIT_FAILURE);
    } 

    // this child is consuming input from the pipe
    //
    if ((wc_pid = fork()) == 0) {
        // attach stdin to the right side of pipe
        // and inherit stdout and stderr from parent
        dup2(pipefd[0], STDIN_FILENO);

        close(pipefd[1]);              // not using the left side
        execl("/usr/bin/wc", "wc", NULL);
        perror("exec wc failed");
        exit(EXIT_FAILURE);
    }

    // explicitly not waiting for ls_pid here
    // wc_pid isn't even my child, it belongs to ls_pid

    return EXIT_SUCCESS;
}