Linux daemonize

1. from stackoverflow
http://stackoverflow.com/questions/17954432/creating-a-daemon-in-linux

Sample Source Code

/*
 * daemonize.c
 * This example daemonizes a process, writes a few log messages,
 * sleeps 20 seconds and terminates afterwards.
 */

#include 
#include 
#include 
#include 
#include 
#include 
#include 

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

2. from redis github sample
https://github.com/antirez/redis/blob/unstable/src/redis.c

	void daemonize(void) {
	int fd;
	if (fork() != 0) exit(0); /* parent exits */
	setsid(); /* create a new session */
	/* Every output goes to /dev/null. If Redis is daemonized but
	* the 'logfile' is set to 'stdout' in the configuration file
	* it will not log at all. */
	if ((fd = open("/dev/null", O_RDWR, 0)) != -1) {
	dup2(fd, STDIN_FILENO);
	dup2(fd, STDOUT_FILENO);
	dup2(fd, STDERR_FILENO);
	if (fd > STDERR_FILENO) close(fd);
	}
	}

• The output should be similar to this one:

+------+------+------+------+-----+-------+------+------+------+-----+
| PPID | PID  | PGID | SID  | TTY | TPGID | STAT | UID  | TIME | CMD |
+------+------+------+------+-----+-------+------+------+------+-----+
|    1 | 17508| 17508| 17508| ?   |    -1 | S    | 1000 | 0:00 | ./  |
+------+------+------+------+-----+-------+------+------+------+-----+

the pid = sid