[Live-devel] Use of std::atomic_flag

[Ross Finlayson]

> On Nov 30, 2023, at 4:14 AM, Dmitry Bely <d.bely at recognize.ru> wrote:
> 
> Hi,
> Recent versions of live555 use std::atomic_flag array to work with
> event triggers. Consider the following code fragment:
> 
> #ifndef NO_STD_LIB
>      if (fTriggersAwaitingHandling[i].test()) {
>       fTriggersAwaitingHandling[i].clear();
> #else
> 
> It is problematic in two ways: 1) it's not atomic: the value can be
> changed elsewhere between test() and clear()

What you’re missing here is that the event loop (which contains the code that you quote above) is intended to be run only by a single thread.  See
	http://live555.com/liveMedia/faq.html#threads
The *only* LIVE555 code that is meant to ever be run in a separate thread (i.e., other than the thread that runs the event loop) is “triggerEvent()”, which calls
	fTriggersAwaitingHandling[i].test_and_set()
I.e., a non-event-loop thread can only ever set an atomic flag (using the atomic operation “test_and_set()”); it cannot clear it.


> and 2) it requires C++>=20.

No it doesn’t ‘require’ C++>=20.  The code should be supported on any compiler that supports "std::atomic_flag”.  That seems to include all recent versions of clang, BTW.

But if you don’t have "std::atomic_flag”, you can compile the code with -DNO_STD_LIB, and it should still work OK if you are using multiple threads as intended - i.e. running all LIVE555 code - except for “triggerEvent()” - in a single thread.


Ross Finlayson
Live Networks, Inc.
http://www.live555.com/