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)

 1 #include <stdio.h>
 2 #include <unistd.h>
 3 #include <stdlib.h>
 4 #include <syslog.h>
 5 
 6 
 7 void  daemon_create(void);
 8 
 9 int main(void) {
10 
11         printf("Starting my_daemon\n");
12         openlog("my_daemon", LOG_INFO, LOG_DAEMON);
13         daemon_create();
14 
15         for (int i=0; i < 100; i++) {
16                 syslog(LOG_INFO,"Countet to %d", i);
17                 sleep(2);
18         }
19         return(0);
20 }
21 
22 void  daemon_create(void) {
23     pid_t pid;
24 
25     pid = fork();
26     if(pid < 0){
27         syslog(LOG_ERR, "Error in fork: %m");
28         exit(1);
29     }
30     if(pid > 0){
31         syslog(LOG_INFO, "Parent dying");
32         exit(0); // Parent die
33     }
34     if(setsid() < 0){ // Make child session leader
35         syslog(LOG_ERR, "Error in setsid: %m");
36         exit(2);
37         }
38         if (freopen("/dev/null", "r", stdin) == NULL
39         || freopen("/dev/null", "w", stdout) == NULL
40         || freopen("/dev/null", "w", stderr) == NULL ) {
41                 syslog(LOG_ERR, "When creating daemon freopen from STDIN/OUT/ERR failed: %m");
42                 exit(3);
43 
44    }
45 }

Checking the daemon

$ ps xo uname,pid,ppid,pgid,sid,tty,comm | grep -P '(my_daemon|COMMAND)'
USER         PID    PPID    PGID     SID TTY       COMMAND
heth      778254       1  778254  778254 ?        my_daemon