How to make QSerialPort from Qt5.13.1 work?

Question

Problem

QSerialPort from version 5.13.1 of the Qt library does not physically output data under Windows 7 and 10.

Example

In order t

Answer 1

Cause

Searching Qt bug tracker there seem to be multiple bugs about QSerialPort not working on Qt 5.13.1 on Windows. All of them are duplicated with QTBUG-78086 which also contains a link to Gerrit review of the fix.

From the bug description:

The signal readyRead is never emitted, even if data is sent to the serial port from a connected device. The member bytesAvailable returns 0 even if data has been sent to the serial port from a connected device.

Basically, they have tried to emit _q_notify in qwinoverlappedionotifier.cpp only if there's no notification pending. Unfortunatelly

That commit completely breaks the I/O on Windows.

Solution

For now you have the options to downgrade to 5.13.0, wait for Qt 5.13.2 or

Fix the Qt 5.13.1 qserialport yourself:

• open QTDIR\5.13.1\Src\qtserialport\qtserialport.pro with QtCreator
• (optional) you might need to select a kit, e.g. Projects -> Manage kits -> Desktop Qt 5.13.1 MSVC2017 64bit
• in the project tree open src/serialport/serialport-lib/sources/qwinoverlappedionotifier.cpp
• delete QAtomicInt pendingNotifications;

• change

  if (!waiting && pendingNotifications-- == 0)
      emit q->_q_notify();
  

  to

  if (!waiting)
      emit q->_q_notify();
  

• change

  int n = pendingNotifications.fetchAndStoreAcquire(0);
  while (--n >= 0) {
      if (WaitForSingleObject(hSemaphore, 0) == WAIT_OBJECT_0)
          dispatchNextIoResult();
  }
  

  to

  if (WaitForSingleObject(hSemaphore, 0) == WAIT_OBJECT_0)
      dispatchNextIoResult();
  

• In QtCreator go to build -> clean all, then run qmake, then rebuild all

• locate the build folder, then copy and replace Qt5SerialPort.dll and Qt5SerialPortd.dll from build\bin to QTDIR\5.13.1\msvc2017_64\bin

Your code should work now.