freebsd_hang.c

#include <assert.h>
#include <fcntl.h>
#include <limits.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>

static int pipes[2];

static void assert_noerr(int res)
{
    if (res < 0)
    {
        fprintf(stderr, "error: %s", strerror(res));
        exit(EXIT_FAILURE);
    }
}

static void sigchld_handler(int sig)
{
    // Write a byte to the write end of pipes.
    (void)sig;
    char c = 0;
    ssize_t amt = write(pipes[1], &c, 1);
    if (amt < 0)
    {
        perror("write");
        exit(EXIT_FAILURE);
    }
}

static void make_fd_nonblocking(int fd)
{
    int flags = fcntl(fd, F_GETFL, 0);
    assert_noerr(flags);
    assert_noerr(fcntl(fd, F_SETFL, flags | O_NONBLOCK));
}

static pid_t spawn_child()
{
    pid_t pid = fork();
    assert_noerr(pid);
    if (pid == 0)
    {
        _exit(0);
    }
    return pid;
}

int main(void)
{
    // Install SIGCHLD handler with SA_RESTART.
    struct sigaction chld_act = {};
    sigemptyset(&chld_act.sa_mask);
    chld_act.sa_handler = &sigchld_handler;
    chld_act.sa_flags = SA_RESTART;
    assert_noerr(sigaction(SIGCHLD, &chld_act, NULL));

    // Make our self-pipes.
    assert_noerr(pipe(pipes));

    // Spawn and reap children in a loop.
    unsigned count = 0;
    for (;;)
    {
        // Spawn a child.
        pid_t child = spawn_child();
        assert(child >= 0 && "Invalid pid");

        // Wait for the SIGCHLD handler to write to us.
        for (;;)
        {
            char buf[PIPE_BUF];
            ssize_t amt = read(pipes[0], buf, sizeof buf);
            if (amt >= 0)
                break;
        }

        // Reap it.
        int status = 0;
        int pid_res = waitpid(child, &status, WUNTRACED | WCONTINUED);
        assert(pid_res == child && "Unexpected pid");

        if ((++count % 256) == 0)
            fprintf(stdout, "%u\n", count >> 8);
    }

    return 0;
}