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Connecting several readyRead() signals to one slot (QSerialPort)



  • Hello everyone,
    I'm writing a program that uses QSerialPort to exchange data. I had connected readyRead() signals from 3 QSerialPort objects to one slot called readComData() for convenience. However, now I'm wondering whether my code works correctly or not.
    My code runs in a single thread. If any data comes from serial ports simultaneously and readyRead() emits from these serial ports, will these signals be queued somehow? If they're queued somehow (I suppose so), will my slot be allowed to finish executing its code before this slot will be invoked by next signal in a queue or my slot will just be reentered each time the readyRead() signal emits from next QSerialPort object?
    I've done extensive Google search before posting, but failed to find any answers.


  • Qt Champions 2018

    @smnsmn said in Connecting several readyRead() signals to one slot (QSerialPort):

    will it be queued somehow

    yes, QueuedConnection is default when connecting signals/slots between different threads.

    "will my slot be allowed to finish executing its code before it will be invoked by next signal" - yes.



  • @jsulm

    My code runs in a single thread

    Sorry if I fail to use appropriate markup, I'm writing from my cell phone.


  • Qt Champions 2018

    @smnsmn Within same thread slots are simply called immediately when signal is emitted. That means: as long as a slot is executed no other slots will be called as the event loop is blocked. When the currently executed slot is finished event loop will pick next slot from the queue (if there is any).


  • Moderators

    @smnsmn

    your code will be fine, as long as you do not do any of the following inside your readComData function

    • call the processEvent() function
    • create and execute a QEventLoop
    • Open a modal QDialog/QMessagebox


  • @jsulm
    Please correct me if I'm wrong, but it seems to me that your answer conflicts with Qt Documentation (taken from article "Syncronizing Threads):

    To place an invocation in an event loop, make a queued signal-slot connection. Whenever the signal is emitted, its arguments will be recorded by the event system. The thread that the signal receiver lives in will then run the slot. Alternatively, call QMetaObject::invokeMethod() to achieve the same effect without signals. In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.

    Specifically this statement:

    In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.


  • Qt Champions 2018

    @smnsmn said in Connecting several readyRead() signals to one slot (QSerialPort):

    but it seems to me that your answer conflicts with Qt Documenation

    In what way? That text from documentation explains signals/slots between DIFFERENT threads (queued connection).
    If thread A emits a signal, the slot in thread B will be executed inside thread B.
    But you said you're only using one thread, in this case you don't have to worry at all and what I said before applies.



  • @jsulm
    Ok, that's how I understand it now (again, please correct me if I'm wrong):

    1. In a single-threaded program the slots connected to emitted signals are executed immediately, whatever the Qt::ConnectionType is.
    2. If it happens that signals are emitted simultaneously, they are written in a thread event queue and corresponding slots are processed one after another (the slot is invoked right after another slot (or the same slot) has finished its execution caused by previous signal from the thread event queue).

    Am I right?


  • Qt Champions 2018

    @smnsmn said in Connecting several readyRead() signals to one slot (QSerialPort):

    Am I right?

    Not really. In a single thread app slots cannot be emitted simultaneously, because there is only one thread. So, when a signal is emitted all connected slots are executed one after the other (in the same order they were connected to the signal).

    void MyClass::myMethod()
    {
    ...
    emit signal1(); // Here all slots connected to signal1() will be executed
    ...
    emit signal2(); // Here all slots connected to signal2() will be executed
    }
    

    It is actually the same as if you would call the slots directly instead of emitting the signal:

    
    void MyClass::myMethod()
    {
    ...
    slot11();
    slot12()
    slot1n();
    ...
    
    slot21();
    slot22()
    slot2n();
    }
    


  • @jsulm

    Okay, here's that part of the code to make things clear:

    connect(&(comPorts.com[0]), SIGNAL( readyRead() ), this, SLOT( readComData() ));
    connect(&(comPorts.com[1]), SIGNAL( readyRead() ), this, SLOT( readComData() ));
    connect(&(comPorts.com[2]), SIGNAL( readyRead() ), this, SLOT( readComData() ));
    
    1. If e.g. two of these serial ports receive some data at one time, in which order do they emit readyRead()? Will the signals be put in a thread event queue?
    2. Do I understand correctly that readComData() slot will be invoked exactly two times, each time after the slot has finished its execution caused by previous signal (from the thread event queue?)?


  • @smnsmn said in Connecting several readyRead() signals to one slot (QSerialPort):

    If e.g. two of these serial ports receive some data at one time

    There is not really a "simultaneously receive at one time", computers don't do that. You can be quite sure that the order would be undefined, in the sense that you cannot know/rely on which one would be seen first and emit its signal. If it makes any difference to you, readyRead()'s this will tell you which comport has received.

    I don't understand about "readComData() slot will be invoked exactly two times". It should be invoked once per comport per data received (provided you extract the data arrived, you don't get another readyread on a device till you've done that).



  • @JonB

    I don't understand about "readComData() slot will be invoked exactly two times".

    Actually, I wanted to be sure that slot readComData() will be invoked by com ports in some succession and there would be no reentrance in the slot when there are readyRead() signals from each of the port at one time.



  • @smnsmn
    Earlier:
    @J.Hilk said in Connecting several readyRead() signals to one slot (QSerialPort):

    @smnsmn

    your code will be fine, as long as you do not do any of the following inside your readComData function

    • call the processEvent() function
    • create and execute a QEventLoop
    • Open a modal QDialog/QMessagebox

    So long as your readComData() does not do any of these it will not get re-entered. Qt will pick up some "random" comport among those with data, raise the signal on that comport, and call your readComData(). Until your readComPort() returns, Qt will not be raising any more signals or calling your readComData() again. Only once that has returned will you get a new call, on the same or a different comport. But there is no knowing what order the signals will be raised from your comports if multiple ones have data.


  • Moderators

    @smnsmn
    I want to raise an additional point, because I think you're not aware of this

    Just because the readyRead signal is emitted, does not mean that all data has already arrived at your serialPort.
    It just means that since the last event loop cycle some data arrived at your port. You have to manage/check if enough/all data arrived, that will be almost impossible if multiple classes access the serial port on their own. You would have to make a manager class to handle that.



  • Thanks everyone for your time and patience.
    Just one last question, I'm not sure whether it applies to Qt:
    In which moment of time does the data from input buffer of serial port get erased?



  • @J.Hilk
    Thank you for the answer.
    I am aware of that, but since only one class (and its instance) has access to these serial ports and I need to receive just 1 byte of data, it seems to me that what you're talking about isn't the case here.



  • I would also like to clarify one question.
    @jsulm said

    Within same thread slots are simply called immediately when signal is emitted.

    Qt Docs state the following:

    Qt::QueuedConnection
    The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.

    Can signal thread == receiver thread?



  • Sure. It is also very useful for faking recursion.


  • Qt Champions 2018

    @smnsmn said in Connecting several readyRead() signals to one slot (QSerialPort):

    Qt::QueuedConnection
    The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.

    This is for queued connections which are not used by default for signals/slots in same thread. You really need to differentiate between connections in same thread and such between different threads.

    "Can signal thread == receiver thread?" - sure, it is like this most of the time. Only if you use more than one thread you can have signals and slots in different threads. For signals and slots in same thread queued connection is NOT used, unless you tell Qt to do so (last parameter in connect() call).