hdaemonize is a simple library that hides some of the complexities
of writing UNIX daemons in Haskell.
The latest version is available (BSD license) at GitHub.
The synopsis is:
import System.Posix.Daemonize main = daemonize $ program
This code will make
program do what good daemons should do, that is,
detach from the terminal, close file descriptors, create a new process
group, and so on.
If you want more functionality than that, it is available as a
Here is an example:
import Control.Concurrent import System.Posix.Daemonize loop i log = do threadDelay $ 10^6 log (show i) writeFile "/tmp/counter" $ show i if i == 5 then undefined else loop (i + 1) log main = serviced (loop 0)
Let us say this program is compiled as
# mydaemon start
starts the service. A second call to start will complain that the program is already running.
During its execution, mydaemon will simply write a new number to
/tmp/counter every second, until it reaches 5. Then, an exception
will be thrown. This exception will be caught by
/var/log/daemon.log or similar (this is depends on how
syslog works on your platorm).
log (show i) will leave messages
in the same file.
When the exception is thrown, the program will be restared in 5 seconds, and will start counting from 0 again.
The following commands are also made available:
# mydaemon stop # mydaemon restart
Finally, if configured to do so,
mydaemon drops privileges by
changing its effective user/group ID. (If no user/group ID are
specified, it will continue execution as the original user and group.)
Note that if you wish to parse your own commandline arguments, you can
replace invocation of the
serviced function with
requires specification of an
Operation which indicates whether the
daemon should be started, stopped, restarted, or whether its status
should be queried.
Stop, etc.) to allow invocation separated from commandline
- Only attempt to change effective user/group ID when explicitly specified.
- Do not attempt to set user or group to the daemon’s name in the absence of a specified user or group.
- Fix a bug where hdaemonize fails when user or group “daemon” is absent
- Update to use hsyslog == 5.
- Fix pre-AMP builds.
- Updated to use hsyslog >=4
- added support for a privileged action before dropping privileges
- merged with updates by madhadron
- provided documentation
- backported to older GHC versions, tested on 6.8.1
- initial public release
Jeremy Bornstein firstname.lastname@example.org
Lana Black email@example.com
Anton Tayanovskyy firstname.lastname@example.org.
The code is originally based on a public posting by Andre Nathan, used by permission.