QThread::wait() causing crash

[Olliebrown]

Greetings!

I'm having a problem with my QThread classes. I based them on the Qt Example Mandlebrot (I'm also doing an long rendering process). I extracted the core mutex and waitCondition syncronization that was in the renderThread into it's own class (which I call InfiniteThread) and then I sub-class this to specialize what task that thread will perform.

Problem is, whenever an instance of InfiniteThread is destroyed and the destructor gets called I get a SIGABRT and the following console output:

pure virtual method called
terminate called without an active exception
The program has unexpectedly finished.

Here's the top of the stack at the time:

0	__semwait_signal					0	0x00007fff87ecdeb6	
1	_pthread_cond_wait					0	0x00007fff87ed1cd1	
2	QWaitCondition::wait					0	0x000000010103bfa0	
3	QThread::wait						0	0x00000001010a419e	
4	InfiniteThread::~InfiniteThread	infinitethread.cpp	17	0x000000010001022c	
5	EMSampleThread::~EMSampleThread	emsamplethread.cpp	18	0x000000010000fbfd	
6	RFViewer::~RFViewer		rfviewer.cpp		50	0x0000000100008a8f	
7	main				main.cpp		10	0x000000010000602a

0	__semwait_signal					0	0x00007fff87ecdeb6	
1	_pthread_cond_wait					0	0x00007fff87ed1cd1	
2	QWaitCondition::wait					0	0x000000010103bfa0	
3	QThread::wait						0	0x00000001010a419e	
4	InfiniteThread::~InfiniteThread	infinitethread.cpp	17	0x000000010001022c	
5	EMSampleThread::~EMSampleThread	emsamplethread.cpp	18	0x000000010000fbfd	
6	RFViewer::~RFViewer		rfviewer.cpp		50	0x0000000100008a8f	
7	main				main.cpp		10	0x000000010000602a

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So the SIGABRT seems to be coming from QThread::wait(), which is called in ~InfiniteThread(). Code for InfiniteThread is included below. Note that InfiniteThread::centralTask() is a pure-virtual member that is re-implemented to specialize the thread for a specific task.

Like I mentioned before, InfiniteThread is based entirely on renderthread.cpp in the Mandlebrot example and it does not have this problem. Any help is much appreciated. I'm new to threading but I am pretty comfortable with Qt.

Thanks!
Seth

InfiniteThread.h:

#ifndef INFINITETHREAD_H
#define INFINITETHREAD_H
 
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
 
class InfiniteThread : public QThread
{
    Q_OBJECT
 
public:
    explicit InfiniteThread(QObject *parent = 0);
    ~InfiniteThread();
 
signals:
    void reportProgress(int pProg, QString message);
    void taskDone(bool pSuccess);
 
protected:
    void startTask();
    void run();
 
    // Override this in the child class
    virtual bool centralTask() = 0;
 
    QMutex mutex;
    QWaitCondition condition;
    bool restart;
    bool abort;
};
 
#endif // INFINITETHREAD_H

#ifndef INFINITETHREAD_H
#define INFINITETHREAD_H

#include <QThread>
#include <QMutex>
#include <QWaitCondition>

class InfiniteThread : public QThread
{
    Q_OBJECT

public:
    explicit InfiniteThread(QObject *parent = 0);
    ~InfiniteThread();

signals:
    void reportProgress(int pProg, QString message);
    void taskDone(bool pSuccess);

protected:
    void startTask();
    void run();

    // Override this in the child class
    virtual bool centralTask() = 0;

    QMutex mutex;
    QWaitCondition condition;
    bool restart;
    bool abort;
};

#endif // INFINITETHREAD_H

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InfiniteThread.cpp:

#include "infinitethread.h"
 
InfiniteThread::InfiniteThread(QObject *parent) :
    QThread(parent)
{    
    restart = false;
    abort = false;
}
 
InfiniteThread::~InfiniteThread()
{
    mutex.lock();
    abort = true;
    condition.wakeOne();
    mutex.unlock();
 
    wait();
}
 
void InfiniteThread::startTask()
{
    QMutexLocker locker(&mutex);
 
    if (!isRunning()) {
        start(LowPriority);
    } else {
        restart = true;
        condition.wakeOne();
    }
}
 
void InfiniteThread::run()
{
    forever {
        // Perform central task
        bool result = centralTask();
        if(!restart) emit taskDone(result);
 
        // Abort thread if requested
        if(abort) return;
 
        // Restart when new central task is given
        mutex.lock();
        if(!restart) condition.wait(&mutex);
        restart = false;
        mutex.unlock();
    }
}

#include "infinitethread.h"

InfiniteThread::InfiniteThread(QObject *parent) :
    QThread(parent)
{    
    restart = false;
    abort = false;
}

InfiniteThread::~InfiniteThread()
{
    mutex.lock();
    abort = true;
    condition.wakeOne();
    mutex.unlock();

    wait();
}

void InfiniteThread::startTask()
{
    QMutexLocker locker(&mutex);

    if (!isRunning()) {
        start(LowPriority);
    } else {
        restart = true;
        condition.wakeOne();
    }
}

void InfiniteThread::run()
{
    forever {
        // Perform central task
        bool result = centralTask();
        if(!restart) emit taskDone(result);

        // Abort thread if requested
        if(abort) return;

        // Restart when new central task is given
        mutex.lock();
        if(!restart) condition.wait(&mutex);
        restart = false;
        mutex.unlock();
    }
}

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[tbscope]

The error says it all:

pure virtual method called

Your problem is this line:

virtual bool centralTask() = 0;

virtual bool centralTask() = 0;

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I guess you implemented this in a subclass?
Looking at the trace, I guess the subclass gets deleted first (obviously)?

Edit: possible solution, move the run() code from your base class to the subclass.

[Olliebrown]

Ahhh! Yup, that makes sense. I was thoroughly confused by the error as the only pure virtual function was CentralTask() and I was re-implementing it properly. It never occurred to me that if the sub-class get's deleted then this will no longer be the case. Good eye!

Thanks!
Seth

[Olliebrown]

That did the trick! I worked around the deletion order by moving the code that's in the destructor into a function called stopTask(). Then, I call this from the child's destructor so the run loop will get stopped BEFORE CentralTask() becomes a pure virtual function again. Worked like a charm.

Here's the updated code:

Seth

#ifndef INFINITETHREAD_H
#define INFINITETHREAD_H
 
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
 
class InfiniteThread : public QThread
{
    Q_OBJECT
 
public:
    explicit InfiniteThread(QObject *parent = 0);
    ~InfiniteThread();
 
    void startTask();
    void stopTask();
 
signals:
    void reportProgress(int pProg, QString message);
    void taskDone(bool pSuccess);
 
protected:
    void run();
 
    // Override this in the child class
    virtual bool centralTask() = 0;
 
    QMutex mutex;
    QWaitCondition condition;
    bool restart;
    bool abort;
};
 
#endif // INFINITETHREAD_H

#ifndef INFINITETHREAD_H
#define INFINITETHREAD_H

#include <QThread>
#include <QMutex>
#include <QWaitCondition>

class InfiniteThread : public QThread
{
    Q_OBJECT

public:
    explicit InfiniteThread(QObject *parent = 0);
    ~InfiniteThread();

    void startTask();
    void stopTask();

signals:
    void reportProgress(int pProg, QString message);
    void taskDone(bool pSuccess);

protected:
    void run();

    // Override this in the child class
    virtual bool centralTask() = 0;

    QMutex mutex;
    QWaitCondition condition;
    bool restart;
    bool abort;
};

#endif // INFINITETHREAD_H

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#include "infinitethread.h"
 
InfiniteThread::InfiniteThread(QObject *parent) :
    QThread(parent)
{    
    restart = false;
    abort = false;
}
 
InfiniteThread::~InfiniteThread()
{
}
 
void InfiniteThread::stopTask()
{
    mutex.lock();
    abort = true;
    condition.wakeOne();
    mutex.unlock();
 
    wait();
}
 
void InfiniteThread::startTask()
{
    QMutexLocker locker(&mutex);
 
    if (!isRunning()) {
        start(LowPriority);
    } else {
        restart = true;
        condition.wakeOne();
    }
}
 
void InfiniteThread::run()
{
    forever {
        // Perform central task
        bool result = centralTask();
        if(!restart) emit taskDone(result);
 
        // Abort thread if requested
        if(abort) return;
 
        // Restart when new central task is given
        mutex.lock();
        if(!restart) condition.wait(&mutex);
        restart = false;
        mutex.unlock();
    }
}

#include "infinitethread.h"

InfiniteThread::InfiniteThread(QObject *parent) :
    QThread(parent)
{    
    restart = false;
    abort = false;
}

InfiniteThread::~InfiniteThread()
{
}

void InfiniteThread::stopTask()
{
    mutex.lock();
    abort = true;
    condition.wakeOne();
    mutex.unlock();

    wait();
}

void InfiniteThread::startTask()
{
    QMutexLocker locker(&mutex);

    if (!isRunning()) {
        start(LowPriority);
    } else {
        restart = true;
        condition.wakeOne();
    }
}

void InfiniteThread::run()
{
    forever {
        // Perform central task
        bool result = centralTask();
        if(!restart) emit taskDone(result);

        // Abort thread if requested
        if(abort) return;

        // Restart when new central task is given
        mutex.lock();
        if(!restart) condition.wait(&mutex);
        restart = false;
        mutex.unlock();
    }
}

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