[Libav-user] variable scope into callback av_log_set_callback and multi threading

Algarve Branqueira branqueira at gmail.com
Fri Mar 25 13:02:03 EET 2022

I tested the solutions.
I'll take the second.

For info for those interested,
I followed the first solution with visualVM, we see a lot of thread
creation on java side and as thread creation is expensive...
3000 threads Java in 2 minutes of execution :)

thanks again

Le jeu. 24 mars 2022 à 20:54, Algarve Branqueira <branqueira at gmail.com> a
écrit :

> Just for info for people who don't know JNI.
> When calling java code from c, it is mandatory to attach a java thread to
> the c thread. When we attach a java thread from the c thread, it creates a
> java thread and we specify a name for it.
> So your first solution which consists in attaching the thread at the
> beginning of the "wrapper_logger" method and detaching at the end of this
> method works but I have to monitor if it is not too expensive for the JVM.
> I've never been in this case, in general we attach to the creation of the
> thread and we detach at the end
> Otherwise your second solution is the most interesting if it is not
> possible to execute code at the end.
> the main thread is already attached because the caller is Java, then as
> you say, you have to put the messages of the ffmpeg worker threads in a
> global variable
> Any idea is good to take, a community is made to share ideas, a bad idea
> can lead to other good idea
> Thanks very much
> Le jeu. 24 mars 2022 à 20:32, Simon <staircase at sector-alpha.net> a écrit :
>> Hi,
>> Even if you aren't creating per-thread objects, java is when you register
>> the thread. Two options I can see are:
>> 1) tidy up before you return control - this would mean register the
>> thread with java only while in your logging callback and unregister before
>> returning.
>> 2) Only log to java from a single thread and get the other threads to
>> queue the log messages up in some global object.
>> I expect my second option is actually the most practical. You can have a
>> thread then runs as long as any task is running and just pulls log messages
>> out of the queue and posts them to java.
>> Hope that's helpful.
>> Simon
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