poll child process stdout/stderr

main.c

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

// invoke with something like: `main /bin/ls -al /`
// cc -E -Wp,-v -xc /dev/null

int main(int argc, char *argv[], char *envp[]) {
    int i = 0;
    for (int i = 0; i < argc; i += 1) {
        printf("[%d] %s\n", i, argv[i]);
    }

    if (argc < 2) {
        fprintf(stderr, "usage: %s exe [arg]...\n", argv[0]);
        exit(1);
    }

    int stdout_pipe[2];
    if (pipe(stdout_pipe) == -1) {
        fprintf(stderr, "pipe(stdout_pipe) failed: %s\n", strerror(errno));
        exit(1);
    }

    int stderr_pipe[2];
    if (pipe(stderr_pipe) == -1) {
        fprintf(stderr, "pipe(stderr_pipe) failed: %s\n", strerror(errno));
        exit(1);
    }

    printf("pipes: %d,%d,%d,%d\n", stdout_pipe[0], stdout_pipe[1], stderr_pipe[0], stderr_pipe[1]);

    pid_t child_pid = fork();
    if (child_pid == -1) {
        fprintf(stderr, "fork failed: %s\n", strerror(errno));
        exit(1);
    }

    // child
    if (child_pid == 0) {
        if (dup2(stdout_pipe[1], STDOUT_FILENO) == -1) {
            fprintf(stderr, "dup2(STDOUT_FILENO) failed: %s\n", strerror(errno));
            exit(1);
        }
        if (dup2(stderr_pipe[1], STDERR_FILENO) == -1) {
            fprintf(stderr, "dup2(STDOUT_FILENO) failed: %s\n", strerror(errno));
            exit(1);
        }

        close(stdout_pipe[0]);
        close(stdout_pipe[1]);
        close(stderr_pipe[0]);
        close(stderr_pipe[1]);

        int rv = execve(argv[1], &argv[1], envp);
        if (rv == -1) {
            fprintf(stderr, "execve failed: %s\n", strerror(errno));
            exit(1);
        }
    }

    // parent

    close(stdout_pipe[1]);
    close(stderr_pipe[1]);

    struct pollfd fds[2];
    fds[0].fd = stdout_pipe[0];
    fds[0].events = POLLIN;
    fds[1].fd = stderr_pipe[0];
    fds[1].events = POLLIN;

    int nactive = 2;
    while (nactive > 0) {
        int rv = poll(fds, 2, -1);
        if (rv == -1) {
            fprintf(stderr, "poll failed: %s\n", strerror(errno));
            exit(1);
        }
        printf("poll rv=%d\n", rv);
        if (rv == 0) continue;

        printf("fd[0].revents=0x%04x [1].revents=0x%04x\n", fds[0].revents, fds[1].revents);
        char buf[8192];
        
        if (fds[0].revents & POLLIN) {
            memset(buf, 0, sizeof(buf));
            ssize_t nbytes = read(fds[0].fd, buf, sizeof(buf) - 1);
            printf("fd[0] read=%ld bytes\n", nbytes);
        }

        if (fds[1].revents & POLLIN) {
            memset(buf, 0, sizeof(buf));
            ssize_t nbytes = read(fds[1].fd, buf, sizeof(buf) - 1);
            printf("fd[1] read=%ld bytes\n", nbytes);
        }

        if (fds[0].revents & (POLLERR | POLLHUP | POLLNVAL)) {
            fds[0].fd = -1;
            nactive -= 1;
        }
        if (fds[1].revents & (POLLERR | POLLHUP | POLLNVAL)) {
            fds[1].fd = -1;
            nactive -= 1;
        }
    }

    int status;
    int rv = waitpid(child_pid, &status, 0);
    if (rv == -1) {
        fprintf(stderr, "waitpid failed: %s\n", strerror(errno));
        exit(1);
    }

    printf("child %d exited with status %d\n", child_pid, status);

    return 0;
}

sample output from dragonfly:

[0] ./main
[1] /bin/ls
[2] -al
[3] /
pipes: 3,4,5,6
poll rv=1
fd[0].revents=0x0001 [1].revents=0x0000
fd[0] read=1175 bytes
poll rv=2
fd[0].revents=0x0001 [1].revents=0x0001
fd[0] read=0 bytes
fd[1] read=0 bytes
poll rv=2
fd[0].revents=0x0001 [1].revents=0x0001
fd[0] read=0 bytes
fd[1] read=0 bytes
poll rv=2
fd[0].revents=0x0001 [1].revents=0x0001
fd[0] read=0 bytes
fd[1] read=0 bytes
poll rv=2
...

sample output from freebsd:

[0] ./main
[1] /bin/ls
[2] -al
[3] /
pipes: 3,4,5,6
poll rv=2
fd[0].revents=0x0011 [1].revents=0x0011
fd[0] read=1616 bytes
fd[1] read=0 bytes
child 1855 exited with status 0