How to add a lib to a qt project

[stampede]

You can remove the OTHER_FILES += libpg.lib, because you are going to use Qt plugin for accessing database.
You have the qsqlpsql4.dll, so you should be able to use it, try to copy this file to your applications directory, or another place where windows searches for dlls.
There is a QSqlDatabase::drivers () method, it returns a list of all available drivers.

[WilliamSpiderWeb]

okay, I copied the "qsqlpsql4.dll" and "qsqlpsql.lib" to my project directory and added it in the *.pro file:

LIBS += -L"qsqlpsql4.lib"

LIBS += -L"qsqlpsql4.lib"

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In my *.cpp I asked for available drivers:

QSqlDatabase dbAlex;
QStringList slDrivers = dbAlex.drivers();

QSqlDatabase dbAlex;
QStringList slDrivers = dbAlex.drivers();

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The Result ist slDrivers is:
QSQLITE
QODBC3
QODBC

But no PSQL or something similar.

[stampede]

LIBS += -L"qsqlpsql4.lib"

this line makes no sense - use -L switch when you want to add a directory to linker search path, the same I haven't noticed in your .pro file posted before:

LIBS += -L"D:/Qt-Projekte/SQL_BSP/libpg.lib"

There should be no "-L" here either.
You don't have to add qt sql plugins to pro file, because you want to load them at runtime.
Try to see if the plugin is visible if you call QApplication::addLibraryPath(path) with path=<"directory with qt psql plugin"> after creating QApplication instance.

[WilliamSpiderWeb]

Sorry, but I'm really a beginner in Qt.
I think I need more information, about what I have to do.
Maybe it will help me if there is an easy example for using postgres with qt.

[ChrisW67]

Before you can use the PostgreSQL plugin for Qt you will have to build it. Building the plugin has nothing to do with your project or its PRO file and your project does not need to link with PostgreSQL libraries directly. You only need to link to PostgreSQL libraries directly if your code uses the PostgreSQL API directly, i.e. not through the Qt Sql libraries.

Open Qt Assistant and read the page Sql Database Drivers and particularly the bit about building the PostgreSQL driver on Mac, Linux, or Windows. Just substitute the correct PostgreSQL locations for your machine in the build commands. Once the plugin is built and installed QSqlDatabase::drivers() should show QPSQL.

[stampede]

@ChrisW67:
The thing is that this plugin is probably already built ( qsqlpsql.dll exisits in his Qt installation ).
@WilliamSpiderWeb:
Have you tried with QApplication::addLibraryPath ? I think I've provided you with wrong info, plugins should be located in "sqldrivers" subdirectory, so it should look like:
QApplication::addLibraryPath("some path/plugins"), where "plugins" directory contains "sqldrivers" subdir (and you have psql dll file in this directory).

[WilliamSpiderWeb]

I hope I understood it the right way.
My main() is looking like this now:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    a.addLibraryPath("C:/Qt/2010.05/qt/plugins/sqldrivers");
    MainWindow w;
    w.show();
    return a.exec();
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    a.addLibraryPath("C:/Qt/2010.05/qt/plugins/sqldrivers");
    MainWindow w;
    w.show();
    return a.exec();
}

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But the available drivers are still the same.
The Question is, how should the software know which dll I need. It just knows where the dlls are located.

[stampede]

No, I mean the "parent" directory for plugins:

QApplication::addLibraryPath("C:/Qt/2010.05/qt/plugins"); // without "sqldrivers"

QApplication::addLibraryPath("C:/Qt/2010.05/qt/plugins"); // without "sqldrivers"

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[WilliamSpiderWeb]

sorry, I tried first without "sqldrivers" and after it with "sqldrivers".
But I qoute the wrong codeline to my reply

[stampede]

So it looks like you'll need to build it indeed - its quite easy, just follow the link ChrisW67 gave you in previous post (there is QPSQL section).

[WilliamSpiderWeb]

I tried to build the psql plugin before,
but even there I need some support.

The doc says I have to use these commands

cd %QTDIR%\src\plugins\sqldrivers\psql
 qmake "INCLUDEPATH+=C:\psql\include" "LIBS+=C:\psql\lib\ms\libpq.lib" psql.pro
 nmake

cd %QTDIR%\src\plugins\sqldrivers\psql
 qmake "INCLUDEPATH+=C:\psql\include" "LIBS+=C:\psql\lib\ms\libpq.lib" psql.pro
 nmake

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when I type the second command (yes I used the right path for psql) I get no error but the psql.pro file is still empty.

when I type the nmake my windows tells me it cannot find nmake. And I can't find nmake in the qt directory either.

At this point I have to say, thanks for your effort and patience.

[wysota]

I think the "problem" is a missing PostgreSQL dll, not the Qt driver. The driver also needs the dll.