Mutex unlock failure when trying to acquire a QSemaphore - multithreading

Question

I have an application with 2 QThreads which behave like it in the pseudocode below (the semaphores are of type QSemaphore):
Thread1 {
print("about to wait on semaphore 1");
sem1.acquire(1);
print("finished waiting on semaphore 1");
sem2.release(1);
}
Thread2 {
print("signaling semaphore 1");
sem1.release(1);
print("about to wait on semaphore 2");
sem2.acquire(1);
}
The issue is that the first thread does NOT wake up when the first semaphore is signalled, i.e. the application produces the following output:
about to wait on semaphore 1
signaling semaphore 1
about to wait on semaphore 2
And that's it. The first thread no longer wakes up.
Now I change the first thread to do the following:
Thread1 {
print("about to wait on semaphore 1");
while (!sem1.tryAcquire(1, 200));
print("finished waiting on semaphore 1");
sem2.release(1);
}
In this case the first thread sleeps for at most 200ms before it tries to acquire the semaphore again. Now I get the following error:
QWaitCondition::wait(): mutex unlock failure: Invalid argument
No other mutexes or other synchronisation primitives are used by the application. What could be the issue?
Update:
I've removed the Semaphores and replaced each with a QWaitCondition and a QMutex and now it works just fine. I didn't make any other changes and I still don't know why the version with semaphores was incorrect. They were both initialised to 0.

Answer 1

Probably you do something wrong somewhere else (e.g., semaphore initialization code).
The following example compiles and runs (gcc).
threads.h:
#pragma once
#include <QThread>
class Thread1 : public QThread
{
protected:
virtual void run();
};
class Thread2 : public QThread
{
protected:
virtual void run();
};
threads.cpp:
#include "threads.h"
#include <QSemaphore>
#include <iostream>
namespace
{
QSemaphore sem1, sem2;
}
void Thread1::run()
{
std::cout << "about to wait on semaphore 1\n";
sem1.acquire(1);
//while (!sem1.tryAcquire(1, 200)); //works too
std::cout << "finished waiting on semaphore 1\n";
sem2.release(1);
}
void Thread2::run()
{
std::cout << "signaling semaphore 1\n";
sem1.release(1);
std::cout << "about to wait on semaphore 2\n";
sem2.acquire(1);
}
main.cpp:
#include "threads.h"
#include <iostream>
int main(int argc, char *argv[])
{
Thread1 t1;
Thread2 t2;
t1.start();
t2.start();
t1.wait();
t2.wait();
std::cout << "Success\n";
}
possible output:
signaling semaphore 1
about to wait on semaphore 2
about to wait on semaphore 1
finished waiting on semaphore 1
Success