When data is written by QSerialPort, there is still data in buffer or not complete to write data even QSerialPort::bytesToWrite is 0 after calling QSerialPort::write.
 
Reproducible steps:
 

• Using cross cable.

  /dev/ttyUSB0 /dev/ttyUSB1
  (FT232R 1)    (FT232R 2)
  TXD ----------- RXD
  RXD ----------- TXD
  GND ----------- GND
  

• Create data

  $ base64 qtserialport-everywhere-src-5.12.3.tar.xz > INPUT
  $ wc -c INPUT
  408968 INPUT
  

• Using creadersync and cwritersync which are contained examples for Qt SerialPort.

  $ PATH/TO/examples/serialport/creadersync/creadersync ttyUSB1 9600 | sed -e '1 d' > OUTPUT &
  [1] 00000 00000
  $ cat INPUT | PATH/TO/examples/serialport/cwritersync/cwritersync ttyUSB0 9600
  Data successfully sent to port ttyUSB0
  $
  [1] + 00000 done ./creadersync/creadersync ttyUSB1 | 00000 done sed -e '1 d' > ./OUTPUT
  $
  

   
  Actual: cwritersync is terminated even though it has not completed writing, then read data by creadersync is dropped.

  $ wc -c OUTPUT
  5381 OUTPUT
  

  Expected: Get actual timing to complete writing data.