QThread::start: Thread termination error

Question

I'm using an OpenSSL library in multi-threading application.
For various reasons I'm using blocking SSL connection. And there is a situation when client hangs on

SSL_connect

function.
I moved connection procedure to another thread and created timer. On timeout connection thread is terminated using:

QThread::terminate()

The thread is terminable, but on the next attempt to start thread I get:

QThread::start: Thread termination error:

I checked the "max thread issue" and that's not the case.
I'm working on CentOS 6.0 with QT 4.5, OpenSSL 1.0
The question is how to completely terminate a thread.

Possible duplicate of http://stackoverflow.com/questions/14261067/. — bn., Jan 10 '13 at 15:15
Are you taking care of the OpenSSL multithreading requirements??: http://stackoverflow.com/questions/3417706/openssl-and-multi-threads — Jon Ander Ortiz Durántez, Jan 10 '13 at 15:39

score 3 · Answer 1 · answered Jan 10 '13 at 15:43

The Qt Documentation about terminate() tells:

The thread may or may not be terminated immediately, depending on the operating systems scheduling policies. Use QThread::wait() after terminate() for synchronous termination.

but also:

Warning: This function is dangerous and its use is discouraged. The thread can be terminated at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to clean up after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.

Assuming you didn't reimplement QThread::run() (which is usually not necessary) - or if you actually reimplemented run and called exec() yourself, the usual way to stop a thread would be:

_thread->quit();
_thread->wait();

The first line tells the thread asynchronously to stop execution which usually means the thread will finish whatever it is currently doing and then return from it's event loop. However, quit() always instantly returns which is why you need to call wait() so the main thread is blocked until _thread was actually ended. After that, you can safely start() the thread again.

If you really want to get rid of the thread as quickly as possible, you can also call wait() after terminate() or at least before you call start() again

I reimplemented QThread::run() actually. Anyway thanks. As far as I see - the only way to overcome my issue is to implement usage of non-blocking openSSL functions. — don_pardon, Jan 11 '13 at 15:45

QThread::start: Thread termination error

1 Answers1