Need Qt SerialPort class example using signals and slots WITHIN A THREAD

[wysota]

As far as I understand data sent by signals is sent to the main thread.

There are two things here. "Data" (as in arguments to signals) are not sent to any thread, in the meaning that thread affinity of them does not change in any way. If you pass a pointer to a QObject that lives in thread A to thread B, then accessing that object there might cause data corruption or worse. If you pass some non-QObject by pointer then it doesn't have any thread affinity and it is its thread-safety that decides whether this is safe or not. if you pass a non-QObject by value then it is copied and in regular cases will have no notion of its other clone being accessed from the original thread. However another topic is if we interpret what you said as "signals are sent to the main thread" in which case this is not true -- signals are sent in the thread they are emitted in (it's just a plain function call) but connecting to them may cause an event to be posted to an object living in a different thread (it doesn't have to be the "main" thread though).

[prof.ebral]

However another topic is if we interpret what you said as "signals are sent to the main thread" in which case this is not true -- signals are sent in the thread they are emitted in (it's just a plain function call) but connecting to them may cause an event to be posted to an object living in a different thread (it doesn't have to be the "main" thread though).

Hmmm, I thought the main thread received the signal as a handler and then executed the method defined by the slot.

[wysota]

Hmmm, I thought the main thread received the signal as a handler and then executed the method defined by the slot.

No, it depends on the thread affinity of the object that the slot belongs to. Here is a simple example:

#include <QtCore>
 
class Object : public QObject {
  Q_OBJECT
public:
  Object(QObject *parent = 0) : QObject(parent) {
 
  }
public slots:
  void reportThread() {
    qDebug() << Q_FUNC_INFO << QThread::currentThread();
  }
};
 
#include "main.moc"
 
int main(int argc, char **argv) {
  QCoreApplication app(argc, argv);
 
  QVector<QThread*> threads;
  QVector<Object*> objects;
  for(int i=0;i<4;++i) {
    QThread *thr = new QThread;
    threads << thr;
 
    Object *o = new Object;
    o->moveToThread(thr);
    objects << o;
 
    thr->start();
 
  }
  // last object in main thread
  objects << new Object;
  qDebug() << "GUI thread:" << objects.last()->thread();  
  QTimer t;
  t.start(2000);
  foreach(Object *o, objects)
    QObject::connect(&t, SIGNAL(timeout()), o, SLOT(reportThread()));  
  return app.exec();
}

#include <QtCore>

class Object : public QObject {
  Q_OBJECT
public:
  Object(QObject *parent = 0) : QObject(parent) {
  
  }
public slots:
  void reportThread() {
    qDebug() << Q_FUNC_INFO << QThread::currentThread();
  }
};

#include "main.moc"

int main(int argc, char **argv) {
  QCoreApplication app(argc, argv);
  
  QVector<QThread*> threads;
  QVector<Object*> objects;
  for(int i=0;i<4;++i) {
    QThread *thr = new QThread;
    threads << thr;
    
    Object *o = new Object;
    o->moveToThread(thr);
    objects << o;
    
    thr->start();
    
  }
  // last object in main thread
  objects << new Object;
  qDebug() << "GUI thread:" << objects.last()->thread();  
  QTimer t;
  t.start(2000);
  foreach(Object *o, objects)
    QObject::connect(&t, SIGNAL(timeout()), o, SLOT(reportThread()));  
  return app.exec();
}

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You can also move the timer to one of the threads and see if it changes anything.

Oh, one more thing -- you can run this example under a debugger, set a breakpoint in the slot and look at the backtrace. You'll see how it differs for the last added object (the one that lives in the main thread) compared to other objects.

[prof.ebral]

Ok, thanks wysota. I use PyQt and when I look at the current thread inside a method that has been called by a signal it is the main thread. I've yet to see an instance when it is isn't in PyQt.

[wysota]

Ok, thanks wysota. I use PyQt and when I look at the current thread inside a method that has been called by a signal it is the main thread. I've yet to see an instance when it is isn't in PyQt.

Maybe you are using Python threads and not QThreads. I have no idea how Qt will behave then.

I ran this test program:

from PySide import QtCore
import sys
 
class Object(QtCore.QObject):
  def reportThread(self):
    print(QtCore.QThread.currentThread())
 
app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()
 
timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThread()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)
app.exec_()

from PySide import QtCore
import sys

class Object(QtCore.QObject):
  def reportThread(self):
    print(QtCore.QThread.currentThread())

app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()

timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThread()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)
app.exec_()

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and it behaves the same way as I described:

MainThread<PySide.QtCore.QThread object at 0xf6a050>
<PySide.QtCore.QThread object at 0xf6a0e0>
<PySide.QtCore.QThread object at 0xf6a0e0>

[prof.ebral]

This is the output I get

MainThread<PyQt4.QtCore.QThread object at 0x7f16ef83c408>
<PyQt4.QtCore.QThread object at 0x7f16ef83c408>

MainThread<PyQt4.QtCore.QThread object at 0x7f16ef83c408>
<PyQt4.QtCore.QThread object at 0x7f16ef83c408>

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The main thread is located at the same memory allocation as all of the other threads.

edit: By the way, here is the changed code for PyQt

from PyQt4 import QtCore
import sys
 
class Object(QtCore.QObject):
  def reportThread(self):
    print(QtCore.QThread.currentThread())
 
app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()
 
timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThread()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)
app.exec_()

from PyQt4 import QtCore
import sys

class Object(QtCore.QObject):
  def reportThread(self):
    print(QtCore.QThread.currentThread())

app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()

timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThread()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)
app.exec_()

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Not much different.

[wysota]

Maybe that's an issue with PyQt4 (I would rather suspect that it returns an incorrect thread pointer rather than runs the slot in a wrong thread). As you can see I was using PySide.


Edit: LOL... I forgot to move the object to the thread and start the thread

Here is a working version:

from PyQt4 import QtCore
import sys
 
class Object(QtCore.QObject):
  def __init__(self):
    QtCore.QThread.__init__(self)
 
  def reportThread(self):
    print(QtCore.QThread.currentThreadId())
 
app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()
obj.moveToThread(thr)
thr.start()
 
timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThreadId()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)
 
t2 = QtCore.QTimer()
t2.timeout.connect(app.quit)
t2.start(10000)
app.exec_()

from PyQt4 import QtCore
import sys

class Object(QtCore.QObject):
  def __init__(self):
    QtCore.QThread.__init__(self)

  def reportThread(self):
    print(QtCore.QThread.currentThreadId())

app = QtCore.QCoreApplication(sys.argv)
thr = QtCore.QThread()
obj = Object()
obj.moveToThread(thr)
thr.start()

timer = QtCore.QTimer()
print("MainThread"+str(QtCore.QThread.currentThreadId()))
timer.timeout.connect(obj.reportThread)
timer.start(2000)

t2 = QtCore.QTimer()
t2.timeout.connect(app.quit)
t2.start(10000)
app.exec_()

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[prof.ebral]

Maybe that's an issue with PyQt4 (I would rather suspect that it returns an incorrect thread pointer rather than runs the slot in a wrong thread). As you can see I was using PySide.

I think it is a feature of PyQt. I'll have to check over the documentation again, but I think PyQt signals and slots operate 'through' the main thread. Like how I described before. Sorry for adding confusion to the post.

[wysota]

No, it's not a feature of PyQt. Look at my previous post, I just edited it.

[prof.ebral]

That doesn't work for me. I only receive the MainThread print statement in my terminal.

edit: wysota, here is some modified code and the output. Even after the Object class is moved to the Thread class I still get the main thread's ID ... but for me the signal is not being caught by the Object's reportThread method.

from PyQt4 import QtCore
import sys, time
 
class Object(QtCore.QObject):
    def __init__(self):
        QtCore.QObject.__init__(self)
 
    def reportThread(self):
        print self, (QtCore.QThread.currentThreadId())
 
class Thread(QtCore.QThread):
    def __init__(self):
        QtCore.QThread.__init__(self)
 
    def reportThread(self):
        print self, (QtCore.QThread.currentThreadId())
 
app = QtCore.QCoreApplication(sys.argv)
thr = Thread()
obj = Object()
obj.moveToThread(thr)
print obj, obj.thread(), obj.thread().currentThreadId()
thr.start()
 
timer = QtCore.QTimer()
print ( "MainThread "+str(QtCore.QThread.currentThreadId()) )
timer.timeout.connect(obj.reportThread)
timer.timeout.connect(thr.reportThread)
timer.start(2000)
 
t2 = QtCore.QTimer()
t2.timeout.connect(app.quit)
t2.start(10000)
app.exec_()

from PyQt4 import QtCore
import sys, time

class Object(QtCore.QObject):
    def __init__(self):
        QtCore.QObject.__init__(self)

    def reportThread(self):
        print self, (QtCore.QThread.currentThreadId())

class Thread(QtCore.QThread):
    def __init__(self):
        QtCore.QThread.__init__(self)

    def reportThread(self):
        print self, (QtCore.QThread.currentThreadId())

app = QtCore.QCoreApplication(sys.argv)
thr = Thread()
obj = Object()
obj.moveToThread(thr)
print obj, obj.thread(), obj.thread().currentThreadId()
thr.start()

timer = QtCore.QTimer()
print ( "MainThread "+str(QtCore.QThread.currentThreadId()) )
timer.timeout.connect(obj.reportThread)
timer.timeout.connect(thr.reportThread)
timer.start(2000)

t2 = QtCore.QTimer()
t2.timeout.connect(app.quit)
t2.start(10000)
app.exec_()

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output:

<__main__.Object object at 0xabd2f8> <__main__.Thread object at 0xabd270> 140273089054464 MainThread 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464

<__main__.Object object at 0xabd2f8> <__main__.Thread object at 0xabd270> 140273089054464 MainThread 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464
<__main__.Thread object at 0xabd270> 140273089054464

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[wysota]

That doesn't work for me. I only receive the MainThread print statement in my terminal.

edit: wysota, here is some modified code and the output. Even after the Object class is moved to the Thread class I still get the main thread's ID ...

That's because the Thread object lives in the main thread. You only moved the Object instance to the thread, not the Thread instance. There is a distinction between a thread (as in a separate processing flow) and QThread instance.

[prof.ebral]

That's because the Thread object lives in the main thread. You only moved the Object instance to the thread, not the Thread instance. There is a distinction between a thread (as in a separate processing flow) and QThread instance.

>____< I used the code you gave me. "I" did not do that. Anyway, thank you.