New to QMutex

[durbrak]

Hello there,

I'm new to the whole Multithreaded programming and I'm having issues.

functions.h

namespace Functions
{
    bool existsInDB(int id);
}

namespace Functions
{
    bool existsInDB(int id);
}

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functions.cpp

#include "functions.h"
#include <QMutex>
#include [...]
 
QMutex mutex(QMutex::Recursive);
 
bool Functions::existsInDB(int id)
{
    static QList<int> cache;
 
    QMutexLocker lock(&mutex);
 
    if(cached.contains(id))
        return true;
    else
    {
        cached.append(id);
 
        QSqlQuery query;
        query.prepare("SELECT COUNT(id) FROM customers WHERE id=?;");
        query.addBindValue(id);
        query.exec();
        query.next();
 
        return (query.value(0).toInt() > 0)
    }
}

#include "functions.h"
#include <QMutex>
#include [...]

QMutex mutex(QMutex::Recursive);

bool Functions::existsInDB(int id)
{
    static QList<int> cache;

    QMutexLocker lock(&mutex);

    if(cached.contains(id))
        return true;
    else
    {
        cached.append(id);

        QSqlQuery query;
        query.prepare("SELECT COUNT(id) FROM customers WHERE id=?;");
        query.addBindValue(id);
        query.exec();
        query.next();

        return (query.value(0).toInt() > 0)
    }
}

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Now I have a subclass of QThread:

void myThread::run()
{
    bool exists = Functions::existsInDB(data->id);
    if(exists) {...}
}

void myThread::run()
{
    bool exists = Functions::existsInDB(data->id);
    if(exists) {...}
}

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As you can see it calls existsInDB(). Now in the main-thread (GUI-Thread) there a sometimes also calls to existsInDB() which causes the following errors to occur in the console (can't remember the exact words):

QSqlQuery::value not positioned on a valid row

If I remove the existsInDB() call in the GUI thread the error disappears. If I remove the existsInDB() call in myThread the error also disappears. So I guess the problem is that both threads are trying to access that function at the same time or something but I can't wrap my head around that whole QMutex story and how to make it work? Can someone point me in the right direction?

Thank you.