How do I make a thread dump with MONO?

Question

How can I show the threads (stacktraces) in a hanging application that run with MONO?

I know that I can do it in .NET with the Managed Stack Explorer (MSE). Because the application hang only with MONO that I need to do it with MONO.

Or there are any other ideas how I can find the place of hanging?

Answer 1

Assuming you're on Linux/Unix, not Windows, send a SIGQUIT signal to your program. This can be done with

kill -QUIT $PID

where $PID is the pid of your program. Mono will then dump stack traces of all threads to stdout. Note that although the process stays running after this, you should not expect it to remain usable/stable.

See http://en.wikipedia.org/wiki/SIGQUIT for some background.

Note: The thread dump will not print out in the terminal window where you ran the kill command. It will appear in the stderr of the mono process.

Answer 2

It is also possible to quickly grab a managed stack trace using GDB. Execute gdb; use sudo if you're not root or debugging a process owned by your user.

Execute this script which I got from the debugging Mono page on mono-project.org:

handle SIGXCPU SIG33 SIG35 SIGPWR nostop noprint

define mono_stack
 set $mono_thread = mono_thread_current ()
 if ($mono_thread == 0x00)
   printf "No mono thread associated with this thread\n"
 else
   set $ucp = malloc (sizeof (ucontext_t))
   call (void) getcontext ($ucp)
   call (void) mono_print_thread_dump ($ucp)
   call (void) free ($ucp)
 end
end

If you like you can drop these commands in your ~/.gdbinit so you don't have to copy and paste all the time.

Now attach to your PID:

attach 12345

Note that the whole process is now paused so if you're doing this in production it is advisable to script this so it's as fast as possible.

To get your stack trace, execute mono_stack as defined above. Note that you won't see the output in gdb but in stdout. If you run your process with upstart you can just edit the upstart job to use console log to log it to /var/log/upstart.

You may be interested in another thread than your main thread however. To do so, execute info threads to get your thread list and thread 2 to switch to thread #2. For more information on thread debugging, see debugging programs with multiple threads in the GDB docs.

Once you're done, execute quit, and your program will continue working.