Difference between revisions of "Daemon linux"
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| − | Example of creating a daemon in Linux: | + | |
| + | =Creating your own daemon= | ||
| + | <onlyinclude> | ||
| + | Example of creating a daemon in Linux: (See [[systemd debian]] for systemd daemons) | ||
*'''setsid()''' - damon will be the session leader. Sets Session ID (SID) and Group ID (GID) to Process ID (PID) | *'''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) | *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) | ||
| − | |||
<source lang=c line> | <source lang=c line> | ||
| + | #include <stdio.h> | ||
| + | #include <unistd.h> | ||
| + | #include <stdlib.h> | ||
| + | #include <syslog.h> | ||
| + | void daemon_create(void); | ||
| + | |||
| + | int main(void) { | ||
| + | printf("Starting my_daemon\n"); | ||
| + | openlog("my_daemon", LOG_INFO, LOG_DAEMON); | ||
| + | daemon_create(); | ||
| + | for (int i=0; i < 100; i++) { | ||
| + | syslog(LOG_INFO,"Countet to %d", i); | ||
| + | sleep(2); | ||
| + | } | ||
| + | return(0); | ||
| + | } | ||
| + | |||
void daemon_create(void) { | void daemon_create(void) { | ||
pid_t pid; | pid_t pid; | ||
| Line 10: | Line 29: | ||
pid = fork(); | pid = fork(); | ||
if(pid < 0){ | if(pid < 0){ | ||
| − | + | syslog(LOG_ERR, "Error in fork: %m"); | |
exit(1); | exit(1); | ||
} | } | ||
| − | if(pid > 0){ | + | if(pid > 0) { |
| + | syslog(LOG_INFO, "Parent dying"); | ||
exit(0); // Parent die | exit(0); // Parent die | ||
} | } | ||
if(setsid() < 0){ // Make child session leader | if(setsid() < 0){ // Make child session leader | ||
| − | + | syslog(LOG_ERR, "Error in setsid: %m"); | |
| − | exit( | + | exit(2); |
} | } | ||
| − | if ( | + | if (freopen("/dev/null", "r", stdin) == NULL |
|| freopen("/dev/null", "w", stdout) == NULL | || freopen("/dev/null", "w", stdout) == NULL | ||
|| freopen("/dev/null", "w", stderr) == NULL ) { | || freopen("/dev/null", "w", stderr) == NULL ) { | ||
| − | + | syslog(LOG_ERR, "When creating daemon freopen from STDIN/OUT/ERR failed: %m"); | |
| − | + | exit(3); | |
| − | + | } | |
} | } | ||
</source> | </source> | ||
| + | ===Checking the daemon=== | ||
| + | *Daemon detached from TTY - PPID is 1 | ||
| + | * Session and session group leader - SID and PGID same as PID | ||
| + | <source lang=bash> | ||
| + | $ 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 | ||
| + | $ tail -f /var/log/sysg | ||
| + | Dec 20 12:37:59 emb3 my_daemon: Countet to 2 | ||
| + | Dec 20 12:38:01 emb3 my_daemon: Countet to 3 | ||
| + | Dec 20 12:38:03 emb3 my_daemon: Countet to 4 | ||
| + | Dec 20 12:38:05 emb3 my_daemon: Countet to 5 | ||
| + | ... | ||
| + | </source> | ||
| + | </onlyinclude> | ||
| + | [[Category:Linux]] | ||
Latest revision as of 05:07, 22 December 2022
Creating your own daemon
Example of creating a daemon in Linux: (See systemd debian for systemd daemons)
- 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 void daemon_create(void);
6
7 int main(void) {
8 printf("Starting my_daemon\n");
9 openlog("my_daemon", LOG_INFO, LOG_DAEMON);
10 daemon_create();
11 for (int i=0; i < 100; i++) {
12 syslog(LOG_INFO,"Countet to %d", i);
13 sleep(2);
14 }
15 return(0);
16 }
17
18 void daemon_create(void) {
19 pid_t pid;
20
21 pid = fork();
22 if(pid < 0){
23 syslog(LOG_ERR, "Error in fork: %m");
24 exit(1);
25 }
26 if(pid > 0) {
27 syslog(LOG_INFO, "Parent dying");
28 exit(0); // Parent die
29 }
30 if(setsid() < 0){ // Make child session leader
31 syslog(LOG_ERR, "Error in setsid: %m");
32 exit(2);
33 }
34 if (freopen("/dev/null", "r", stdin) == NULL
35 || freopen("/dev/null", "w", stdout) == NULL
36 || freopen("/dev/null", "w", stderr) == NULL ) {
37 syslog(LOG_ERR, "When creating daemon freopen from STDIN/OUT/ERR failed: %m");
38 exit(3);
39 }
40 }
Checking the daemon
- Daemon detached from TTY - PPID is 1
- Session and session group leader - SID and PGID same as PID
$ 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
$ tail -f /var/log/sysg
Dec 20 12:37:59 emb3 my_daemon: Countet to 2
Dec 20 12:38:01 emb3 my_daemon: Countet to 3
Dec 20 12:38:03 emb3 my_daemon: Countet to 4
Dec 20 12:38:05 emb3 my_daemon: Countet to 5
...