Sample for Mutithreading

[wysota]

So I need at least 2 different threads for serial reading.

No, you don't. You only need one thread. If you use two, you are entering the "shoot yourself in the foot" world. Unless of course you need real concurrency when reading the ports but I'm almost sure you don't.

[HugoHiasl]

No, you don't. You only need one thread. If you use two, you are entering the "shoot yourself in the foot" world. Unless of course you need real concurrency when reading the ports but I'm almost sure you don't.

First of all thanks for the tip with the QVector. This worked for me :-)

What you are telling is an interesting point. I thought using multiple threads would be easier and necessary because the handling of the different manufacturers on the different ports is really completely different. The first one has a RS485 protokoll that must be parsed by myself. I also need to lock the communiaction on this serial port until I got response for my query. The other manufacturer delivers a shared object with an API to query the data with. Here I do not have any direct influence how long the communication on this serial port is blocked and if probably the thread is blocked too.

Others setups might have RS485 protocol with polling on serial port 1 and RS232 protocol with getting every 10 seconds data automatically pushed by the inverter on serial port 2.

To be open for those other setups I thought it would be less complicated to handle it in single threads per port. But probably your right and I should use only one and write an handling object for every port and call them sequentially in a loop to query the data from the ports.

I need to think about it.

[wysota]

If all communication with the port is done asynchronously then you only need one thread for everything. You don't need to loop over ports all the time to query them for data, both serial port implementations for Qt should provide the readyRead() signal for the serial device so you will get notified when there is anything to read.

[HugoHiasl]

Hmm.... thanks for this really interesting approach. I will do some tests with a single thread environment.

Actually I stuck in another problem. As mentioned earlier I'm really familiar with C# and Java. But I'm completely new to QT. I plan to create a BaseClass for the handlers for different manufacturers.

I planned to make a base class that defines the set of methods that will be called by the serial thread. First I tried to make an Interface but I didn't manage it The implementation classes shall emit Signals to be able to connect them to the database thread.

Now I'm trying it with a BaseClass which I want to subclass. But even with my 10 line trial I stuck in linker warnings.

This is the implmentation now:

serialhandlerbase.h

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H
 
#include <QObject>
 
class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling();
    virtual void stopHandling();
};
 
#endif // SERIALHANDLERBASE_H

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H

#include <QObject>

class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling();
    virtual void stopHandling();
};

#endif // SERIALHANDLERBASE_H

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serialbasehandler.cpp

#include "serialhandlerbase.h"
 
SerialHandlerBase::SerialHandlerBase()
{
}

#include "serialhandlerbase.h"

SerialHandlerBase::SerialHandlerBase()
{
}

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The compiler says: undefined reference to 'vtable for SerialHandlerBase'
and the linker: collect2: Id returned 1 exit status

I can get rid of the compiler messages if I declare a destructor and implement it in the cpp file. But it does not fix the linker message.

Do you have an idea for me?

[wysota]

If you don't want to implement the virtual methods in the class (making it an interface or a pure abstract class as we call it in C++), you need to tell that to the compiler:

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H
 
#include <QObject>
 
class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling() = 0;
    virtual void stopHandling() = 0;
};

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H

#include <QObject>

class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling() = 0;
    virtual void stopHandling() = 0;
};

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[HugoHiasl]

I tried adding these = 0 but it did not change anything

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H
 
#include <QObject>
 
class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling() = 0;
    virtual void stopHandling() = 0;
};
 
#endif // SERIALHANDLERBASE_H

#ifndef SERIALHANDLERBASE_H
#define SERIALHANDLERBASE_H

#include <QObject>

class SerialHandlerBase : public QObject
{
    Q_OBJECT
public:
    SerialHandlerBase();
    virtual void startHandling() = 0;
    virtual void stopHandling() = 0;
};

#endif // SERIALHANDLERBASE_H

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[wysota]

Did you run qmake after adding the Q_OBJECT macro to your class?

[HugoHiasl]

Did you run qmake after adding the Q_OBJECT macro to your class?

This was the reason. Perfect thanks :-)

I'll try to implement the handler class now. Am I able to set the owner thread to sleep ? I think my next question will be concerning connecting the signals of the objects of the serial thread to the slots of the object in the database thread.

But berfore asking this I'll do some trials on my own.

Thank you very much for your patience.

[wysota]

Again, you only need one thread. Not one for serial, one for database, one for something else. One in general, it's not java.

[HugoHiasl]

Again, you only need one thread. Not one for serial, one for database, one for something else. One in general, it's not java.

Hmm.. I seem to be too focused to my standard development environment in c# and java. I'll try to sort it out in my mind. My fear is that the task that occur sometime will interrupt the data retrieval process. From time to time the saved data needs to be sent as xml to a web page. The data needs to be displayed in the gui and so on.

I think I'll need the one or other day to get into the "best practices" for QT.

[wysota]

Nothing will interrupt anything and you won't have problems with synchronizing all the threads. Just start thinking in asynchronous manner.

[jimc1200]

Nothing will interrupt anything and you won't have problems with synchronizing all the threads. Just start thinking in asynchronous manner.

@wysota:

this is interesting as i'm also looking into implementing threaded architecture. from what you are suggesting single thread with asynchronous handling will do the job. but isn't it much more efficient if multiple threads are involved? let's take for an example a method(slot) handling the signal in a single thread, wouldn't that stop or delay other processing as compared to concurrent processing in multithreads?
i'm still quite undecided whether to implement asynchronous handling in one thread or implement multiple threads. would really appreciate your expert advice as to which one is more efficient. thanks.