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Thread: How to add a lib to a qt project

  1. #1
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    Default How to add a lib to a qt project

    Hello,

    I'm using qt4 on a Windows 7 system and I'm trying to get access to a PostgreSQL 9 DB.

    That's why I'm trying to add libpg.lib (with libpg-fe.h) to my project. But I'm doning something wrong.

    I added "LIBS += -lpg" to my projectfile *.pro.
    Qt Code:
    1. QT += core gui
    2.  
    3. TARGET = SQL_BSP
    4. TEMPLATE = app
    5.  
    6. LIBS += -lpg
    7.  
    8. SOURCES += main.cpp\
    9. mainwindow.cpp \
    10. postgresqldb.cpp
    11.  
    12. HEADERS += mainwindow.h \
    13. libpq-fe.h \
    14. postgres_ext.h \
    15. postgresqldb.h
    16.  
    17. FORMS += mainwindow.ui
    18.  
    19. OTHER_FILES += \
    20. libpq.lib
    To copy to clipboard, switch view to plain text mode 

    When I start compiling my project I get the following error message:
    Qt Code:
    1. cannot find -lpg
    2. collect2: ld returned 1 exit status
    To copy to clipboard, switch view to plain text mode 

    I'm waiting for your help.
    Thanks,
    Alex

  2. #2
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    Default Re: How to add a lib to a qt project

    Where is this library located ? You need to point the linker to the library directory as well:
    Qt Code:
    1. LIBS += -L"path/to/library/directory"
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    Default Re: How to add a lib to a qt project

    Thanks for your reply.

    My libpg.lib is located in the same directory as the whole qt-project.
    Just to be sure, I added information of the whole path of the lib to the *.pro file.

    Qt Code:
    1. LIBS += -lpg
    2. LIBS += -L"D:\Qt-Projekte\SQL_BSP"
    To copy to clipboard, switch view to plain text mode 

    But I still got the same error messages.

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    Default Re: How to add a lib to a qt project

    Sorry, I guess you want the static linkage:
    Qt Code:
    1. LIBS += "D:/Qt-Projekte/SQL_BSP/libpg.lib"
    To copy to clipboard, switch view to plain text mode 

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    Default Re: How to add a lib to a qt project

    If I add the codeline you write and remove my two codelines I still get one errormessage.

    Qt Code:
    1. collect2: ld returned 1 exit status
    To copy to clipboard, switch view to plain text mode 

    I've no idea what that error means.

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    Default Re: How to add a lib to a qt project

    Is it the only error message you get, this single line ? Aren't there any message like "undefined reference to ..." ?
    Are you sure that the path to .lib file is correct ?
    Be sure to remove LIBS += -lpg from the .pro file and add only the LIBS += "path/to/lib/libpg.lib".
    Post the whole .pro file. Try to do a clean build as well.
    Did you built the "libpg.lib" yourself ? If you've downloaded it, are you sure it's good one for your compiler ?
    ---
    if you use QtCreator, try to scroll up in the compiler output window, maybe you just see the last line.
    Last edited by stampede; 27th February 2011 at 22:09. Reason: updated contents

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    Default Re: How to add a lib to a qt project

    Yes, it's the only error message. In addition I get a warning.
    Qt Code:
    1. Qmake does not support build directories below the source directory
    To copy to clipboard, switch view to plain text mode 

    This is my SQL_BSP.pro file.
    Qt Code:
    1. QT += core gui
    2.  
    3. TARGET = SQL_BSP
    4. TEMPLATE = app
    5.  
    6. LIBS += -L"D:/Qt-Projekte/SQL_BSP/libpg.lib"
    7.  
    8. SOURCES += main.cpp\
    9. mainwindow.cpp \
    10. postgresqldb.cpp
    11.  
    12. HEADERS += mainwindow.h \
    13. libpq-fe.h \
    14. postgres_ext.h \
    15. postgresqldb.h
    16.  
    17. FORMS += mainwindow.ui
    18.  
    19. OTHER_FILES += \
    20. libpq.lib
    To copy to clipboard, switch view to plain text mode 

    Yes, I am sure that every file of my project is located in my specified path.
    I got the lib from a friend of mine. He used it to communicate to a postgres db with a Visual C++ project (without MFC).

    If it is not compatible to my compiler, where can I get the right *.lib? I've read about qsqlpsql4.dll and qsqlpsql4.lib in the web. These files are in the plugin folder of qt, but there is no header file existing. So I don't know how to use them.

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    Default Re: How to add a lib to a qt project

    These files are in the plugin folder of qt
    Great, so you can use them - add QT += sql to .pro file and include <QtSql> in order to use Qt database framework. There are many examples how to use it in Qt examples directory, and you have nice demo of sqlbrowser implemented with Qt as well.

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    Default Re: How to add a lib to a qt project

    Thank you for your help so far.
    My SQL_BSP.pro now looks like this:
    Qt Code:
    1. QT += core gui
    2. QT += sql
    3.  
    4. TARGET = SQL_BSP
    5. TEMPLATE = app
    6.  
    7. SOURCES += main.cpp\
    8. mainwindow.cpp
    9.  
    10. HEADERS += mainwindow.h
    11.  
    12. FORMS += mainwindow.ui
    13.  
    14. OTHER_FILES += \
    15. libpq.lib
    To copy to clipboard, switch view to plain text mode 

    I tried to get access to the sql db like this:
    Qt Code:
    1. #include <QtSql>
    2. #include <QSqlError>
    3. #include <QSqlDriver>
    4. #include <QSqlDriverPlugin>
    5.  
    6. QSqlDatabase dbAlex = QSqlDatabase::addDatabase("QPSQL");
    7. dbAlex.setHostName("localhost");
    8. dbAlex.setPort(5433);
    9. dbAlex.setUserName("postgres");
    10. dbAlex.setPassword("***");
    11. dbAlex.setDatabaseName("QT");
    12. dbAlex.open("postgres", "***");
    13.  
    14. if (dbAlex.isOpen() == true)
    15. m_sConnState = "true";
    16. else
    17. m_sConnState = "false";
    18.  
    19. QSqlError dbError = dbAlex.lastError();
    20. m_sConnState = dbError.text();
    21. ui->lineEdit->setText(m_sConnState);
    To copy to clipboard, switch view to plain text mode 

    The message in my lineEdit now is "driver not loaded".

    I now, I habe to work with QSqlDriver and QSqlDriverPlugin, but I have no idea how.

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    Default Re: How to add a lib to a qt project

    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.

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    Default Re: How to add a lib to a qt project

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

    Qt Code:
    1. LIBS += -L"qsqlpsql4.lib"
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    In my *.cpp I asked for available drivers:

    Qt Code:
    1. QSqlDatabase dbAlex;
    2. QStringList slDrivers = dbAlex.drivers();
    To copy to clipboard, switch view to plain text mode 

    The Result ist slDrivers is:
    QSQLITE
    QODBC3
    QODBC

    But no PSQL or something similar.

  12. #12
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    Default Re: How to add a lib to a qt project

    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.

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    Default Re: How to add a lib to a qt project

    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.

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    Default Re: How to add a lib to a qt project

    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.

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    Default Re: How to add a lib to a qt project

    @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).

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    Default Re: How to add a lib to a qt project

    I hope I understood it the right way.
    My main() is looking like this now:
    Qt Code:
    1. int main(int argc, char *argv[])
    2. {
    3. QApplication a(argc, argv);
    4. a.addLibraryPath("C:/Qt/2010.05/qt/plugins/sqldrivers");
    5. MainWindow w;
    6. w.show();
    7. return a.exec();
    8. }
    To copy to clipboard, switch view to plain text mode 

    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.

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    Default Re: How to add a lib to a qt project

    No, I mean the "parent" directory for plugins:
    Qt Code:
    1. QApplication::addLibraryPath("C:/Qt/2010.05/qt/plugins"); // without "sqldrivers"
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    Default Re: How to add a lib to a qt project

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

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    Default Re: How to add a lib to a qt project

    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).

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    Default Re: How to add a lib to a qt project

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

    The doc says I have to use these commands

    Qt Code:
    1. cd %QTDIR%\src\plugins\sqldrivers\psql
    2. qmake "INCLUDEPATH+=C:\psql\include" "LIBS+=C:\psql\lib\ms\libpq.lib" psql.pro
    3. nmake
    To copy to clipboard, switch view to plain text mode 

    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.

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