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`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
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
`serviced` function.
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`. Then:
# 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 `hdaemonize`, and
logged to `/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, `mydaemon` drops privileges. By default it changes the
effective user and group ids to those of the `daemon` user, but it
prefers to use those of `mydaemon`, if present.
* 0.4
* added support for a privileged action before dropping privileges
* 0.3
* merged with updates by madhadron
* 0.2
* provided documentation
* backported to older GHC versions, tested on 6.8.1
* 0.1
* initial public release
Anton Tayanovskyy <>, bug reports and feature
requests welcome.
The code is originally based on a public posting by
[Andre Nathan](, used by permission.