trick

daemonize.c

/*
 * We said a process is a daemon when:
 *
 * - its parent process terminates,
 * - the daemon process attach to init (1) process.
 *
 * Usually we will close its STDIN / STDOUT / STDERR and chroot to ``/``.
 */

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

void daemonize();
void run_in_background();

int
main()
{
    pid_t child_pid;

    switch ((child_pid = fork())) {
    case -1:
        perror("cannot fork");
        break;
    case 0:                 /* in child process */
        daemonize();
        run_in_background();
        break;
    default:                /* in parent process */
        printf("child process running in: %d\n", child_pid);
        // Detach the child process to ``init``.
        exit(0);
    }

    return 0;
}

void daemonize()
{
    int devnull_fd;

    // Change path to ``/``.
    if (chdir("/") != 0)
        perror("cannot change path to root");

    if ((devnull_fd = open("/dev/null", O_RDWR, 0)) == -1)
        perror("cannot open /dev/null");

    // Close STDIN / STDOUT / STDERR.
    dup2(devnull_fd, STDIN_FILENO);
    dup2(devnull_fd, STDOUT_FILENO);
    dup2(devnull_fd, STDERR_FILENO);
    // Clean up.
    if (devnull_fd > 2)
        close(devnull_fd);
}

// Task that will be running in the background.
void run_in_background()
{
    while (1) {
        // Sleep for 2 secs...
        sleep(2);
    }
}

/*
 * Run result:
 *
 * $ ./d
 * child process running in: 28000
 * $ ps -p 28000 -o ppid=
 * 1
 */