I don’t usually waste my time on java, but today I was in the right mood and tried to write a very simple network daemon, just to learn how hard it was. After some quick google I found that java can’t daemonize itself (it has no access to relevant native system calls) and therefore the Apache guys developed a nice component called org.apache.commons.daemon. I though Apache guys are smart, after all they developed the Apache web server, Subversion and more interesting stuff, so they have to know very well how a network daemon works. So I happily started with the test app, SimpleDaemon, bundled in the commons tar file, just to prove it works.

Unfortunately at the 1st attempt I got:

pioppo@roentgen ~/sandbox/daemon $ jsvc -verbose SimpleDaemon.class SimpleDaemon
24/12/2006 00:32:53 32525 jsvc error: Cannot execute JSVC executor process

Gosh, what is this? 

Ok ok, some more googling and I found jsvc is using execve(2) on itself but doesn’t canonicalize the filename so execve fails. To workaround the problem you have to invoke jsvc as an absolute path:

pioppo@roentgen ~/sandbox/daemon $ /usr/bin/jsvc -verbose SimpleDaemon.class SimpleDaemon
pioppo@roentgen ~/sandbox/daemon $

On second attempt I get no errors, but it’s not running. Well, not very verbose either. I know my stuff, so I try debugging with strace and I find jsvc is failing opening the pidfile in /var/run/jsvc.pid but it’s not nice enough to tell me. Ok, let’s try again forcing log file creation and a pid file in the current dir:

pioppo@roentgen ~/sandbox/daemon $ /usr/bin/jsvc -pidfile ./sd.pid -errfile ./err.log SimpleDaemon.class SimpleDaemon
pioppo@roentgen ~/sandbox/daemon $

This time the err.log file has been created and I can read this nice report:

24/12/2006 00:42:58 31709 jsvc.exec error: syscall failed in set_caps
24/12/2006 00:42:58 31709 jsvc.exec error: set_caps(CAPS) failed
24/12/2006 00:42:58 31708 jsvc.exec error: Service exit with a return value of 4

Gosh, this is driving me crazy. Wasn’t this supposed to be a piece of cake? Ok, some more googling and I find this happens in case I have no capability kernel module loaded. OK OK, let’s modprobe capability and try again:

24/12/2006 00:51:28 32407 jsvc.exec error: Cannot find daemon loader org/apache/commons/daemon/support/DaemonLoader
24/12/2006 00:51:28 32406 jsvc.exec error: Service exit with a return value of 1

$&$&&^ what the hell is this, a candid camera? After more googling I find this happens because I forgot to add the path to the jar file for commons. Eventually I find a way to run this test daemon that is supposed to do nothing, just an Hello World app. I look it up in top and I see it spawned 10 threads and is taking 368M of virtual mem. Ok, now I know why I don’t usually waste my time on java