MySQL Driver not loaded on PC without Qt

[wysota]

The problem is that my Application only searches in that path, no matter what, for plugins.

How do you know it doesn't search elsewhere?

I could add thousand additional paths and put the needed dll in every single one of them, but only the one lies in the qt/plugins-path will be used.
And this is not that good, if I want to use my application on a PC without Qt :/

Apparently you are missing some library that is required by the driver. Run dependency walker on the plugin and see if anything is missing and what is required for it to work.

[KTvsPeacock]

It was just a guess, because by removing every library path the application will still look in qt/plugins/sqldrivers.

I used Dependency Walker on App/sqldrivers/qsqlmysql4.dll:
QtCore4.dll and QtSql4.dll were missing, but they were in the /App-folder, so I copied them also into that folder.
-> Available Drivers: ("QSQLITE", "QMYSQL3", "QMYSQL", "QODBC3", "QODBC") , but still QSqlError(-1, "Driver not loaded", "Driver not loaded")

Dependencies on qsqlmysql4.dll:
LIBMYSQL.DLL
KERNEL32.DLL
MINGWM10.DLL
MSVCRT.DLL
LIBGCC_S_DW2-1.DLL
QTCORE4.DLL
QTSQL4.DLL

[wysota]

Obviously your driver is seen by the application. Show us your code.

[KTvsPeacock]

I tried to pick the important ones:

main.cpp:

#include <QtGui/QApplication>
#include <QObject>
#include <QDebug>
#include "dialog.h"
#include "mainwindow.h"
#include "datenbank.h"
 
int main(int argc, char *argv[])
{
    qDebug()<<"["<<(Datenbank::iMethodenZaehler++)<<"]"<<"enter main";
    QApplication a(argc, argv);
    Dialog *d = new Dialog;
    MainWindow *w = new MainWindow;
    QObject::connect(d,SIGNAL(signalUser(QString)),w,SLOT(slotUser(QString)));
    if(d->exec()){
    w->show();
    }else{
    delete w;
    delete d;
    return 1;
    }
    delete d;
    int i = a.exec();
    qDebug()<<"["<<(--Datenbank::iMethodenZaehler)<<"]"<<"exit main";
    return i;
}

#include <QtGui/QApplication>
#include <QObject>
#include <QDebug>
#include "dialog.h"
#include "mainwindow.h"
#include "datenbank.h"

int main(int argc, char *argv[])
{
    qDebug()<<"["<<(Datenbank::iMethodenZaehler++)<<"]"<<"enter main";
    QApplication a(argc, argv);
    Dialog *d = new Dialog;
    MainWindow *w = new MainWindow;
    QObject::connect(d,SIGNAL(signalUser(QString)),w,SLOT(slotUser(QString)));
    if(d->exec()){
    w->show();
    }else{
    delete w;
    delete d;
    return 1;
    }
    delete d;
    int i = a.exec();
    qDebug()<<"["<<(--Datenbank::iMethodenZaehler)<<"]"<<"exit main";
    return i;
}

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datenbank.cpp:
(every method in the datenbank class is static)

#include "datenbank.h"
 
QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL");
 
bool Datenbank::initDB(QString qsDB,QString qsHost,QString qsNutzer, QString qsKW, int iPort, DB_MODES mode){
    myDB.setDatabaseName(qsDB);
    myDB.setHostName(qsHost);
    myDB.setUserName(qsNutzer);
    myDB.setPassword(qsKW);
    myDB.setPort(iPort);
    qDebug()<<myDB.drivers();
    bool b = false;
    checkOpenConnection(mode);
    if(myDB.isOpen()){
	b=true;
	checkCloseConnection(mode);
    }else{
	qDebug()<<myDB.lastError();
    }
    return b;
}

#include "datenbank.h"

QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL");

bool Datenbank::initDB(QString qsDB,QString qsHost,QString qsNutzer, QString qsKW, int iPort, DB_MODES mode){
    myDB.setDatabaseName(qsDB);
    myDB.setHostName(qsHost);
    myDB.setUserName(qsNutzer);
    myDB.setPassword(qsKW);
    myDB.setPort(iPort);
    qDebug()<<myDB.drivers();
    bool b = false;
    checkOpenConnection(mode);
    if(myDB.isOpen()){
	b=true;
	checkCloseConnection(mode);
    }else{
	qDebug()<<myDB.lastError();
    }
    return b;
}

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includes in datenbank.h:

#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QStringList>
#include <QVariant>
#include <QDebug>
#include <QDateTime>

#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QStringList>
#include <QVariant>
#include <QDebug>
#include <QDateTime>

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Hope this helps.

[wysota]

Here we are...

Your addDatabase() call is made as an initialization of a static variable which places it before the execution of main(). Now in main() the QApplication object is being initialized and during its initialization it loads plugins. Therefore you are trying to add a database before any plugins are actually loaded. This can't work regardless of whether Qt is preinstalled on a particular machine or not.

Now to fix it all... QSqlDatabase::addDatabase() and QSqlDatabase::database() are static calls. The point is you don't have to assign their return values to any persistent variables because you can always retrieve the connection later. You can also simply move the initialization of the database object to main() or any other place after QApplication is already created.

When talking about main(), you can simplify it to:

int main(int argc, char *argv[])
{
    qDebug()<<"["<<(Datenbank::iMethodenZaehler++)<<"]"<<"enter main";
    QApplication a(argc, argv);
    Dialog d;
    MainWindow w;
    QObject::connect(&d,SIGNAL(signalUser(QString)),&w,SLOT(slotUser(QString)));
    if(d.exec()){
      w.show();
    }else{
      return 1;
    }
    int i = a.exec();
    qDebug()<<"["<<(--Datenbank::iMethodenZaehler)<<"]"<<"exit main";
    return i;
}

int main(int argc, char *argv[])
{
    qDebug()<<"["<<(Datenbank::iMethodenZaehler++)<<"]"<<"enter main";
    QApplication a(argc, argv);
    Dialog d;
    MainWindow w;
    QObject::connect(&d,SIGNAL(signalUser(QString)),&w,SLOT(slotUser(QString)));
    if(d.exec()){
      w.show();
    }else{
      return 1;
    }
    int i = a.exec();
    qDebug()<<"["<<(--Datenbank::iMethodenZaehler)<<"]"<<"exit main";
    return i;
}

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Also have a look at this snippet regarding debugging:
http://blog.wysota.eu.org/index.php/...ugging-helper/

[KTvsPeacock]

Strange, BUT addDatabase() does work, if qsqlmysql4.dll lies in qt/plugins/sqldrivers :/

Finally I could solve this mistery with your help.
I added Datenbank::myDB.addDataBase("myConnection") to my main.cpp return the connection to myDB in the initDB-method using myDB = QSqlDatabase::database("myConnection");

Now I have to write an e-mail to my teacher, which taught me to define a database connection this way

[wysota]

Strange, BUT addDatabase() does work, if qsqlmysql4.dll lies in qt/plugins/sqldrivers :/

I don't think it is possible. Maybe you didn't rebuild some code or you have a static compilation of your application or something like that. addDatabase() cannot work before Q(Core)Application is constructed.

[KTvsPeacock]

It worked like this in many other applications of me and about 60 other people I know
Maybe it's an error of Qt.

[wysota]

It worked like this in many other applications of me and about 60 other people I know
Maybe it's an error of Qt.

It would rather be a miracle (or a badly written compiler) as I can assure you plugins are not loaded before QApplication constructor is executed.

[KTvsPeacock]

You can try it on your own, if you want to.
Just:
- download the Qt SDK from the website
- install MySQL
- use the generic settings
- write a class like my datenbank.h/cpp
- compile

Like I said: I was told by my teacher to use QSqlDatabase this way and it worked for me and each other pupil of him in the last 3 years

[wysota]

This crashes on me immediately (on Linux):

#include <QtCore>
#include <QtSql>
 
class DBClass {
 
private:
  static QSqlDatabase db;
};
 
QSqlDatabase DBClass::db = QSqlDatabase::addDatabase("QSQLITE");
 
int main(int argc, char **argv){
  QCoreApplication app(argc, argv);
  return 0;
};

#include <QtCore>
#include <QtSql>

class DBClass {

private:
  static QSqlDatabase db;
};

QSqlDatabase DBClass::db = QSqlDatabase::addDatabase("QSQLITE");

int main(int argc, char **argv){
  QCoreApplication app(argc, argv);
  return 0;
};

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[KTvsPeacock]

This setting "worked" for me:
database.pro:

QT       += core gui
 
TARGET = database
TEMPLATE = app
 
 
SOURCES += main.cpp\
    datenbank.cpp
 
HEADERS  += \
    datenbank.h
 
QT += sql

QT       += core gui

TARGET = database
TEMPLATE = app


SOURCES += main.cpp\
    datenbank.cpp

HEADERS  += \
    datenbank.h

QT += sql

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datenbank.h:

#ifndef DATENBANK_H
#define DATENBANK_H
 
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QStringList>
#include <QVariant>
#include <QDebug>
 
class Datenbank
{
 
public:
    static QSqlDatabase myDB;
    Datenbank();
};
 
#endif // DATENBANK_H

#ifndef DATENBANK_H
#define DATENBANK_H

#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QStringList>
#include <QVariant>
#include <QDebug>

class Datenbank
{

public:
    static QSqlDatabase myDB;
    Datenbank();
};

#endif // DATENBANK_H

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datenbank.cpp:

#include "datenbank.h"
 
QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL"); //works also with QSQLITE
 
Datenbank::Datenbank()
{
}

#include "datenbank.h"

QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL"); //works also with QSQLITE

Datenbank::Datenbank()
{
}

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main.cpp:

#include <QtGui/QApplication>
#include "datenbank.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qDebug()<<Datenbank::myDB;
    return a.exec();
}

#include <QtGui/QApplication>
#include "datenbank.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qDebug()<<Datenbank::myDB;
    return a.exec();
}

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Using QCoreApplication won't do the trick.

[wysota]

What if you move the qDebug() statement before the application constructor?

[KTvsPeacock]

Still works, but only with QApplication.
QCoreApplication doesn't like it that way

[wysota]

What if you remove the application object completly and just leave the debug statement?

[KTvsPeacock]

It works even without QApplication, but it has to be included.
Else I wont get any message.

[wysota]

And what message do you get?

[KTvsPeacock]

If I added the setUser, etc.-methods and now I'm able to open a database.
QSqlDatabase(driver=""QMYSQL"", database=""notenuebersicht_10_11"", host=""192.168.0.112"", port=3306, user=""gast"", open=true)

main.cpp:

#include <QApplication>
#include "datenbank.h"
 
 
int main(int argc, char *argv[])
{
    Datenbank::initDB();
    qDebug()<<Datenbank::myDB;
}

#include <QApplication>
#include "datenbank.h"


int main(int argc, char *argv[])
{
    Datenbank::initDB();
    qDebug()<<Datenbank::myDB;
}

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datenbank.h:

#ifndef DATENBANK_H
#define DATENBANK_H
 
#include <QSqlDatabase>
#include <QDebug>
 
class Datenbank
{
 
public:
    static QSqlDatabase myDB;
    Datenbank();
    static void initDB();
};
 
#endif // DATENBANK_H

#ifndef DATENBANK_H
#define DATENBANK_H

#include <QSqlDatabase>
#include <QDebug>

class Datenbank
{

public:
    static QSqlDatabase myDB;
    Datenbank();
    static void initDB();
};

#endif // DATENBANK_H

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datenbank.cpp:

#include "datenbank.h"
 
QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL");
 
Datenbank::Datenbank()
{
}
 
void Datenbank::initDB(){
   myDB.setHostName("192.168.0.112");
   myDB.setDatabaseName("notenuebersicht_10_11");
   myDB.setUserName("gast");
   myDB.setPassword("gast");
   myDB.setPort(3306);
   myDB.open();
}

#include "datenbank.h"

QSqlDatabase Datenbank::myDB = QSqlDatabase::addDatabase("QMYSQL");

Datenbank::Datenbank()
{
}

void Datenbank::initDB(){
   myDB.setHostName("192.168.0.112");
   myDB.setDatabaseName("notenuebersicht_10_11");
   myDB.setUserName("gast");
   myDB.setPassword("gast");
   myDB.setPort(3306);
   myDB.open();
}

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[wysota]

Could you please check what QCoreApplication::startingUp() returns?

I just started thinking... maybe they changed QSqlDatabase::addDatabase() to load plugins if they are not loaded? I'll have a look at the source code...

Edit: No, they didn't...

[KTvsPeacock]

startingUp() returns true.