Does checking for getOutputCount before writing on the port make sense?

[traversaro]

Hi YARP developers and users,
I have a doubt on the BufferPort::getOutputCount function. A pattern quite widespread in some YARP-related codebases (see for example icub-main, i.e. https://github.com/robotology/icub-main/search?q=getOutputCount) that I also propagated over the years is to write on a BufferPort only if getOutputCount() is greater then zero:

template <class T> void broadcastData(T& _values, yarp::os::BufferedPort<T>& _port)
{
    if (_port.getOutputCount()>0 )
    {
        _port.prepare()  = _values ;
        _port.write();
    }
}

I never found this discussed, but I can imagine that the goal is to reduce the use of CPU time (or other resources used in _port.prepare()/_port.write() calls) by avoiding those calls if no one is connected to the specific port. The hidden assumption here is that _port.getOutputCount() is rather cheap call, i.e. something that is just accessing a small buffer.

However, if you wear your muddy boots and check the code in PortCore, it seems that getOutputCount also calls the cleanUnits(false) method:

yarp/src/libYARP_os/src/yarp/os/impl/PortCore.cpp

Line 1428 in 9b52a71

cleanUnits(false);

That in turns seems a rather complicated function (and to be honest I am not sure what it is supposed to do):

yarp/src/libYARP_os/src/yarp/os/impl/PortCore.cpp

Line 591 in 9b52a71

// We will remove any connections that have finished operating from

If you check the cleanUnits docs, it says "// called anytime, garbage collects terminated units":

yarp/src/libYARP_os/src/yarp/os/impl/PortCore.h

Line 606 in 9b52a71

// called anytime, garbage collects terminated units

And while I am not sure what that means in this context, "garbage collection" is not something that is typically associated with inexpensive operations.

Having said that, my questions is:

TL;DR: Does it make sense use getOutputCount to avoid writing on a BufferedPort, or perhaps it is better to just write all the time on the BufferPort?

[traversaro]

Related issues (that actually sparkled this discussion):

• Closing the Walking Module may crash the yarprbotinterface walking-controllers#83
• Buffered port getOutputCount may cause a segfault if called when one of the output ports is closing #2531

This is quite relevant to the icub-main codebase, in which pattern is quite widespread. @pattacini @Nicogene

[pattacini]

I think that using getOutputCount() can be still meaningful in some contexts.

Ideally, the function should simply return a number, hence no other instructions should be executed.
In turn, this number should be updated by other methods of the port itself when a connection/disconnection event occurs.

[drdanz]

To my understanding, if you use it to avoid writing on the port, this is just wrong, because the port will do the same internally.
On the other hand, if you do it to avoid doing some other (potentially heavy) processing or large copies, it might make sense.