Daemon Sceleton

Steps typically required to daemonize a process:

1. Fork the parent process (fork) and let the parent exit; the child continues to run in the background.

2. Create a new session and become session leader (setsid). The calling process becomes the leader of a new session and process group and is disassociated from the controlling terminal (CTTY).

3. Catch and handle (or ignore) signals as needed.

4. Fork again (fork) and let the parent exit to ensure the daemon cannot reacquire a controlling terminal (only session leaders can acquire a controlling TTY).

5. Change the working directory (chdir) to a safe location (commonly /).

6. Set an appropriate file mode creation mask (umask).

7. Close all open file descriptors inherited from the parent (close).

daemonize.c

/*
 * daemonize.c
 * This example daemonizes a process, writes a few log messages,
 * sleeps 20 seconds and terminates afterwards.
 * This is an answer to the stackoverflow question:
 * https://stackoverflow.com/questions/17954432/creating-a-daemon-in-linux/17955149#17955149
 * Fork this code: https://github.com/pasce/daemon-skeleton-linux-c
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <syslog.h>

static void skeleton_daemon()
{
    pid_t pid;

    /* Fork off the parent process */
    pid = fork();

    /* An error occurred */
    if (pid < 0)
        exit(EXIT_FAILURE);

    /* Success: Let the parent terminate */
    if (pid > 0)
        exit(EXIT_SUCCESS);

    /* On success: The child process becomes session leader */
    if (setsid() < 0)
        exit(EXIT_FAILURE);

    /* Catch, ignore and handle signals */
    //TODO: Implement a working signal handler */
    signal(SIGCHLD, SIG_IGN);
    signal(SIGHUP, SIG_IGN);

    /* Fork off for the second time*/
    pid = fork();

    /* An error occurred */
    if (pid < 0)
        exit(EXIT_FAILURE);

    /* Success: Let the parent terminate */
    if (pid > 0)
        exit(EXIT_SUCCESS);

    /* Set new file permissions */
    umask(0);

    /* Change the working directory to the root directory */
    /* or another appropriate directory */
    chdir("/");

    /* Close all open file descriptors */
    int x;
    for (x = sysconf(_SC_OPEN_MAX); x>=0; x--)
    {
        close (x);
    }

    /* Open the log file */
    openlog ("firstdaemon", LOG_PID, LOG_DAEMON);
}

int main()
{
    skeleton_daemon();

    while (1)
    {
        //TODO: Insert daemon code here.
        syslog (LOG_NOTICE, "First daemon started.");
        sleep (20);
        break;
    }

    syslog (LOG_NOTICE, "First daemon terminated.");
    closelog();

    return EXIT_SUCCESS;
}

Sources

1. How to Create a Daemon in C?

2. Basic skeleton of a linux daemon written in C