[Interest] Behavior of write() to stdErr in Qt Creator

[Bernhard Lindner]

Hi all!

In a Qt/C++ application I need to use write() to print some text to stderr. I'm doing
something like this multiple times:

  write(stdErrDescriptor, "H", 1);
  write(stdErrDescriptor, "e", 1);
  write(stdErrDescriptor, "l", 1);
  write(stdErrDescriptor, "l", 1);
  write(stdErrDescriptor, "o", 1);
  write(stdErrDescriptor, "\n", 1);

I'm calling that from the main thread and a sub thread. The output of the threads should
be serialized by internal program logic. This means the main thread should not print at
the same time as the sub thread. This seems to be true since in reality I could never see
interleaved lines on the console. Still there is something going on that I do not
understand.

I expected to see something like this:

   Hello
   Hello
   Hello
   Hello
   Hello
   Hello

However I get this:

   Hello
   Hello
   H
   e
   l
   l
   o
   
   H
   e
   l
   l
   o
   
   Hello
   Hello

Please note that lines are not interleaved. Still a line feed is added after each write()
call when executing the code snippet from a sub thread.

The effect is completely reproducible when starting the debug build from Qt Creator under
gdb supervision. However when manually stepping through the code or when running the debug
build without debugger the output looks just fine.

I'm running the code on Kubuntu 20.04 and GCC 12.

What is going on?

-- 
Best Regards,
Bernhard Lindner