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http://www-theorie.physik.unizh.ch/~dpotter/howto/daemonize
How to Daemonize in Linux
One of the things I keep running across is Linux daemons that don’t properly daemonize themselves. To properly daemonize, the following steps must be followed.
- The fork() call is used to create a separate process.
- The setsid() call is used to detach the process from the parent (normally a shell).
- The file mask should be reset.
- The current directory should be changed to something benign.
- The standard files (stdin,stdout and stderr) need to be reopened.
Failure to do any of these steps will lead to a daemon process that can misbehave. The typical symptoms are as follows.
- Starting the daemon and then logging out will cause the terminal to hang. This is particularly nasty with ssh.
- The directory from which the daemon was launched remains locked.
- Spurious output appears in the shell from which the daemon was started.
Simple Example
The following example program performs the bare minimum steps required to launch a daemon process.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#define EXIT_SUCCESS 0
#define EXIT_FAILURE 1
static void daemonize(void)
{
pid_t pid, sid;
/* already a daemon */
if ( getppid() == 1 ) return;
/* Fork off the parent process */
pid = fork();
if (pid < 0) {
exit(EXIT_FAILURE);
}
/* If we got a good PID, then we can exit the parent process. */
if (pid > 0) {
exit(EXIT_SUCCESS);
}
/* At this point we are executing as the child process */
/* Change the file mode mask */
umask(0);
/* Create a new SID for the child process */
sid = setsid();
if (sid < 0) {
exit(EXIT_FAILURE);
}
/* Change the current working directory. This prevents the current
directory from being locked; hence not being able to remove it. */
if ((chdir("/")) < 0) {
exit(EXIT_FAILURE);
}
/* Redirect standard files to /dev/null */
freopen( "/dev/null", "r", stdin);
freopen( "/dev/null", "w", stdout);
freopen( "/dev/null", "w", stderr);
}
int main( int argc, char *argv[] ) {
daemonize();
/* Now we are a daemon -- do the work for which we were paid */
return 0;
}
It has been brought to my attention that a second call to fork() may be required to fully detach the process from the controller terminal (in other words: fork, setsid, fork). This does not seem to be required in Linux. A second fork would not cause any problems, although it would complicate the child/parent signalling below.
A More Useful Example
The following program extends the basic daemon by adding the following features.
- Logs messages to the system log (via syslog).
- Creates a lock file to prevent the daemon from being run twice.
- Changes the effective user (drops privileges).
- Startup errors are reported to the main process.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <syslog.h>
#include <errno.h>
#include <pwd.h>
#include <signal.h>
/* Change this to whatever your daemon is called */
#define DAEMON_NAME "mydaemon"
/* Change this to the user under which to run */
#define RUN_AS_USER "daemon"
#define EXIT_SUCCESS 0
#define EXIT_FAILURE 1
static void child_handler(int signum)
{
switch(signum) {
case SIGALRM: exit(EXIT_FAILURE); break;
case SIGUSR1: exit(EXIT_SUCCESS); break;
case SIGCHLD: exit(EXIT_FAILURE); break;
}
}
static void daemonize( const char *lockfile )
{
pid_t pid, sid, parent;
int lfp = -1;
/* already a daemon */
if ( getppid() == 1 ) return;
/* Create the lock file as the current user */
if ( lockfile && lockfile[0] ) {
lfp = open(lockfile,O_RDWR|O_CREAT,0640);
if ( lfp < 0 ) {
syslog( LOG_ERR, "unable to create lock file %s, code=%d (%s)",
lockfile, errno, strerror(errno) );
exit(EXIT_FAILURE);
}
}
/* Drop user if there is one, and we were run as root */
if ( getuid() == 0 || geteuid() == 0 ) {
struct passwd *pw = getpwnam(RUN_AS_USER);
if ( pw ) {
syslog( LOG_NOTICE, "setting user to " RUN_AS_USER );
setuid( pw->pw_uid );
}
}
/* Trap signals that we expect to recieve */
signal(SIGCHLD,child_handler);
signal(SIGUSR1,child_handler);
signal(SIGALRM,child_handler);
/* Fork off the parent process */
pid = fork();
if (pid < 0) {
syslog( LOG_ERR, "unable to fork daemon, code=%d (%s)",
errno, strerror(errno) );
exit(EXIT_FAILURE);
}
/* If we got a good PID, then we can exit the parent process. */
if (pid > 0) {
/* Wait for confirmation from the child via SIGTERM or SIGCHLD, or
for two seconds to elapse (SIGALRM). pause() should not return. */
alarm(2);
pause();
exit(EXIT_FAILURE);
}
/* At this point we are executing as the child process */
parent = getppid();
/* Cancel certain signals */
signal(SIGCHLD,SIG_DFL); /* A child process dies */
signal(SIGTSTP,SIG_IGN); /* Various TTY signals */
signal(SIGTTOU,SIG_IGN);
signal(SIGTTIN,SIG_IGN);
signal(SIGHUP, SIG_IGN); /* Ignore hangup signal */
signal(SIGTERM,SIG_DFL); /* Die on SIGTERM */
/* Change the file mode mask */
umask(0);
/* Create a new SID for the child process */
sid = setsid();
if (sid < 0) {
syslog( LOG_ERR, "unable to create a new session, code %d (%s)",
errno, strerror(errno) );
exit(EXIT_FAILURE);
}
/* Change the current working directory. This prevents the current
directory from being locked; hence not being able to remove it. */
if ((chdir("/")) < 0) {
syslog( LOG_ERR, "unable to change directory to %s, code %d (%s)",
"/", errno, strerror(errno) );
exit(EXIT_FAILURE);
}
/* Redirect standard files to /dev/null */
freopen( "/dev/null", "r", stdin);
freopen( "/dev/null", "w", stdout);
freopen( "/dev/null", "w", stderr);
/* Tell the parent process that we are A-okay */
kill( parent, SIGUSR1 );
}
int main( int argc, char *argv[] ) {
/* Initialize the logging interface */
openlog( DAEMON_NAME, LOG_PID, LOG_LOCAL5 );
syslog( LOG_INFO, "starting" );
/* One may wish to process command line arguments here */
/* Daemonize */
daemonize( "/var/lock/subsys/" DAEMON_NAME );
/* Now we are a daemon -- do the work for which we were paid */
/* Finish up */
syslog( LOG_NOTICE, "terminated" );
closelog();
return 0;
}
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