Re: [Libevent-users] Multithreading and breaking out of the event loop

[azat@xxxxxxxxxxxx]

On Tue, Feb 12, 2019 at 02:44:35PM +0200, George-Theodor Serbana wrote:
> For ease, I will copy and paste the StackOverflow question here as well:

Thanks!

> I have one thread (Thread A) in a C static library that's creating the
> libevent event_base after doing this:
> 
> #ifdef WIN32
>     evthread_use_windows_threads();
> #else
>     evthread_use_pthreads();
> #endif
> 
> This C static library is all single threaded and it's doing all the
> libevent work itself: adding events, dispatching, main loop, etc, all
> these being done from Thread A.
> 
> Now I have a C++ program that I link with the library above, calling
> the entrance function into the C library from a boost::thread called
> Thread B.

You should not call evthread_use_*() functions multiple times (although
this will not break anything).

> At some point into my C++ program, I have yet another boost::thread
> called Thread C which will attempt to event_add using the C library
> into Thread A's event loop and then immediately call event_active.
> 
> What I'd like to see after doing stuff from Thread C is this:
> 
> calling event_add from Thread C
> calling event_active from Thread C
> Processing events from Thread A
> calling the event's handler from Thread A
> 
> Possibly we could have Processing events from Thread A anywhere else
> between those lines, but for sure we must have at least 1 such line
> between event_active and event handler execution, right?

It depends on what "Processing events from Thread A" means, if this is
entering the loop and handling events from it (insice Thread A), the the
answer is "definitelly yes".

> What I see instead is this:
> 
> calling event_add from Thread X
> calling event_active from Thread X
> calling the event's handler from Thread A
> 
> Thread X is a different ID than Thread C but I guess that's just
> because of using boost threads in the C++ calling site and pthreads in
> the C library or something, anyways it doesn't bother me.
> 
> I also like the fact that my main purpose here is OK: firing the event
> from a thread and handling it from another.
> 
> But for my own curiosity what I don't understand is why I don't see a
> line "processing events from Thread A" before the handler callback is
> called?

Where did you put this printf?

P.S. consider using event_enable_debug_logging() to see debug
information (that includes when libevent enters the loop and when it
process event).
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