Simultaneous read and write on serial port using 2 threads

Question

In Qt, is it possible to simultaneously read from and write to the same serial port from 2 different threads ?

Answer 1

Not directly. Any methods you call on port must be called within port.thread(). Doing otherwise is undefined behavior: you may format your hard drive.

But you can do it indirectly by using the signal-slot mechanism. Instead of calling the methods on the port, let's have an interface class that acts as a thread-safe interface to the port:

struct PortInterface : QObject {
   Q_SIGNAL void writeData(const QByteArray &);
   Q_SIGNAL void hasReadData(const QByteArray &);
   Q_OBJECT
};

int main(int argc, char ** argv) {
  QApplication app(argc, argv);
  PortInterface interface;
  QSerialPort port;

  QObject::connect(&interface, &PortInterface::writeData, &port, [&](const QByteArray &data){
     qDebug() << "writing in thread" << QThread::currentThread();
     Q_ASSERT(QThread::currentThread() == port.thread());
     port.write(data);
  });
  QObject::connect(&port, &QIODevice::readyRead, [&]{
     qDebug() << "reading in thread" << QThread::currentThread();
     Q_ASSERT(QThread::currentThread() == port.thread());
     emit interface.hasReadData(port.readAll());
  });

You can call the writeData method in any thread: Qt's signal-slot mechanism will wrap the call and safely deliver it to the port's thread. Similarly, the hasReadData signal can be called from any thread. The readAll call is done from port's own thread. The code that processes available data should connect to that slot.

Thus, we can have a timer that ticks in a dedicated thread to write some data to the port, and we can have a slot that listens to new data in the main thread:

 QTimer sourceTimer;
  sourceTimer.start(20);
  QObject::connect(&sourceTimer, &QTimer::timeout, [&]{
     qDebug() << "timer tick in thread" << QThread::currentThread();
     interface.writeData(QByteArray(20, 'd'));
  });
  QObject::connect(&interface, &PortInterface::hasReadData, &app, [&](const QByteArray &data){
     qDebug() << "data read in thread" << QThread::currentThread();
     qDebug() << data.toHex();
  });

  QThread sourceThread, portThread;
  QThread::currentThread()->setObjectName("mainThread");
  sourceThread.setObjectName("sourceThread");
  portThread.setObjectName("portThread");;
  sourceTimer.moveToThread(&sourceThread);
  port.moveToThread(&portThread);
  sourceThread.start();
  portThread.start();
  return app.exec();
}

You can have any number of objects attached to the hasReadData signal. These objects can live in any thread. Recall that signal-slot connections are of 1:n kind, where 0<=n.

Similarly, you could have any number of objects call the writeData method of the interface: as long as the data they write is a self-contained packet, you're guaranteed that the packet will be sent on the port as a unit, without interleaving with other packets. The receiver must be able to delineate the packets, though: the packets will need a header or other means of synchronization (e.g. HDLC).

Of course you need to open the port first :)

Answer 2

Not. It is not possible to do read/write from different threads due to its implementation (same as for any of I/O classes of Qt). QSP uses the non-blocking (asynchronous) I/O that allows to use read/write from one thread "simultaneous".