Daemon linux

Example of creating a daemon in Linux:

• setsid() - damon will be the session leader. Sets Session ID (SID) and Group ID (GID) to Process ID (PID)
• Reopening stdin, stdout and stderr to /dev/null detaches the process from the controlling terminal and Parent PID is set to 1 (init or systemd - depending on Linux version/distribution)
• log_err() and log_to_log() are calls to a logging system not shown here

 1 void  daemon_create(void) {
 2     pid_t pid;
 3 
 4     pid = fork();
 5     if(pid < 0){
 6         log_err("Error in fork: %m");  // Write error to terminal
 7         exit(1);
 8     }
 9     if(pid > 0){
10         exit(0); // Parent die
11     }
12     if(setsid() < 0){ // Make child session leader
13         log_err("Error in setsid: %m");
14         exit(1);
15         }
16         if (   freopen("/dev/null", "r", stdin) == NULL
17         || freopen("/dev/null", "w", stdout) == NULL
18         || freopen("/dev/null", "w", stderr) == NULL ) {
19                 log_err("When creating daemon freopen from STDIN/OUT/ERR failed: %m");
20     }
21         log_to_syslog(); // Log to syslog only when in daemon mode
22 }

Checking the daemon

$ ps xo pid,ppid,pgid,sid,comm | grep -P '(my_daemon|PPID)'
    PID    PPID    PGID     SID COMMAND
 775354       1  775353  714976 my_daemon