GC BUG: Referenced object destroyed before released

[Graham St Jack]

On Mon, 17 Mar 2008 10:16:57 +0100, Frits van Bommel wrote:

> Graham St Jack wrote:
>> Another question along these lines - I am having trouble with a multi-
>> threaded program (linux, gdc, phobos). It works fine until main()
>> returns, and then I get a SIGUSR1 that causes the program to crash. Any
>> ideas what might be happening?
>> 
>> My investigations so far show that SIGUSR1 and SIGUSR2 are used in
>> thread.d as part of the implementation of pause() and resume(). I was
>> under the impression that the thread that is interrupted to handle a
>> signal is chosen at random, and therefore that it is a bad idea to try
>> to use signals in a threaded program, so I am confused.
>> 
>> Also, the garbage collector seems to kill off all my threads without
>> waiting for their run methods to return. I assume this means that the
>> threads aren't being treated as root objects by the garbage collector.
> 
> When main() exits, the GC starts a collection. In order to do so safely
> it pauses all other threads with Thread.pauseAll, which (in the Linux
> version) sends SIGUSR1 to all other threads to tell them to pause. (Note
> that this doesn't kill them, even though the function that does this is
> called pthread_kill)
> It's usually a good idea to just tell your debugger to ignore SIGUSR1
> and SIGUSR2 (the latter is used to resume afterwards). To do this in
> GDB, use "handle SIGUSR1 noprint nostop SIGUSR2 noprint nostop".
> 
> If you're just wondering why all your threads get killed when main()
> ends I can tell you right now: this is how it works :P. You should see
> the same in a C or C++ program.
> 
>> Does all this mean that I have to explicitly take action so that the
>> run methods will return, then wait() on them before returning from
>> main()?
> 
> As you said, call Thread.wait() on all other threads before exiting
> main() to wait for them to end. Or just switch to Tango which does this
> automatically (add Tangobos if you need Phobos compatibility).

Thanks for the information - it has saved me a lot of wasted time.