• This is an answer to a question on stackoverflow: Creating a daemon in Linux
• Fork the skeleton code: Basic skeleton of a linux daemon written in C
• Read the article here: How to create a c-style daemon

Basic skeleton of a linux daemon written in C

Daemons work in the background and (usually...) don't belong to a TTY that's why you can't use stdout/stderr in the way you probably want. Usually a syslog daemon (syslogd) is used for logging messages to files (debug, error,...).

Besides that, there are a few required steps to daemonize a process.

Required Steps

• fork off the parent process & let it terminate if forking was successful. -> Because the parent process has terminated, the child process now runs in the background.
• setsid - Create a new session. The calling process becomes the leader of the new session and the process group leader of the new process group. The process is now detached from its controlling terminal (CTTY).
• Catch signals - Ignore and/or handle signals.
• fork again & let the parent process terminate to ensure that you get rid of the session leading process. (Only session leaders may get a TTY again.)
• chdir - Change the working directory of the daemon.
• umask - Change the file mode mask according to the needs of the daemon.
• close - Close all open file descriptors that may be inherited from the parent process.

Look at this skeleton code that shows the basic steps:

/*
* 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
* Read about it: https://nullraum.net/how-to-create-a-daemon-in-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 appropriated 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;
}

Compile and run

• Compile the code: gcc -o firstdaemon daemonize.c
• Start the daemon: ./firstdaemon
• Check if everything is working properly: ps -xj | grep firstdaemon

Test the output

• The output should be similar to this one:

+------+------+------+------+-----+-------+------+------+------+-----+
| PPID | PID  | PGID | SID  | TTY | TPGID | STAT | UID  | TIME | CMD |
+------+------+------+------+-----+-------+------+------+------+-----+
|    1 | 3387 | 3386 | 3386 | ?   |    -1 | S    | 1000 | 0:00 | ./  |
+------+------+------+------+-----+-------+------+------+------+-----+

What you should see here is:

• The daemon has no controlling terminal (TTY = ?)
• The parent process ID (PPID) is 1 (The init process)
• The PID != SID which means that our process is NOT the session leader
  (because of the second fork())
• Because PID != SID our process can't take control of a TTY again

Reading the syslog:

• Locate your syslog file. Mine is here: /var/log/syslog

• Do a: grep firstdaemon /var/log/syslog

• The output should be similar to this one:

  firstdaemon[3387]: First daemon started.
  firstdaemon[3387]: First daemon terminated.

A note: In reality you would also want to implement a signal handler and set up the logging properly (Files, log levels...).

Further reading:

• Linux-UNIX-Programmierung - German