How can I terminate a QThread

Question

Recently ,I come across this problem as I memtioned in this Title. I have tried by using QThread::terminate(),but I just can NOT stop the thread ,which is in a dead loop (let's say,while(1)).

thanks a lot.

Answer 1

Terminating the thread is the easy solution to stopping an async operation, but it is usually a bad idea: the thread could be doing a system call or could be in the middle of updating a data structure when it is terminated, which could leave the program or even the OS in an unstable state.

Try to transform your while(1) into while( isAlive() ) and make isAlive() return false when you want the thread to exit.

Answer 2

QThreads can deadlock if they finish "naturally" during termination.

For example in Unix, if the thread is waiting on a "read" call, the termination attempt (a Unix signal) will make the "read" call abort with an error code before the thread is destroyed.

That means that the thread can still reach it's natural exit point while being terminated. When it does so, a deadlock is reached since some internal mutex is already locked by the "terminate" call.

My workaround is to actually make sure that the thread never returns if it was terminated.

while( read(...) > 0 ) {

  // Do stuff...
}

while( wasTerminated )
  sleep(1);

return;

wasTerminated here is actually implemented a bit more complex, using atomic ints:

enum {

  Running, Terminating, Quitting
};

QAtomicInt _state; // Initialized to Running

void myTerminate()
{
  if( _state.testAndSetAquire(Running, Terminating) )
    terminate();
}

void run()
{
  [...]

  while(read(...) > 0 ) {

    [...]
  }

  if( !_state.testAndSetAquire(Running, Quitting) ) {
    for(;;) sleep(1);
  }
}