How to add a lib to a qt project

[WilliamSpiderWeb]

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       += core gui
 
TARGET = SQL_BSP
TEMPLATE = app
 
LIBS += -lpg
 
SOURCES += main.cpp\
        mainwindow.cpp \
    postgresqldb.cpp
 
HEADERS  += mainwindow.h \
    libpq-fe.h \
    postgres_ext.h \
    postgresqldb.h
 
FORMS    += mainwindow.ui
 
OTHER_FILES += \
    libpq.lib

QT       += core gui
 
TARGET = SQL_BSP
TEMPLATE = app
 
LIBS += -lpg
 
SOURCES += main.cpp\
        mainwindow.cpp \
    postgresqldb.cpp
 
HEADERS  += mainwindow.h \
    libpq-fe.h \
    postgres_ext.h \
    postgresqldb.h
 
FORMS    += mainwindow.ui
 
OTHER_FILES += \
    libpq.lib

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When I start compiling my project I get the following error message:

cannot find -lpg
collect2: ld returned 1 exit status

cannot find -lpg
collect2: ld returned 1 exit status

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I'm waiting for your help.
Thanks,
Alex

[stampede]

Where is this library located ? You need to point the linker to the library directory as well:

LIBS += -L"path/to/library/directory"

LIBS += -L"path/to/library/directory"

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[WilliamSpiderWeb]

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.

LIBS += -lpg
LIBS += -L"D:\Qt-Projekte\SQL_BSP"

LIBS += -lpg
LIBS += -L"D:\Qt-Projekte\SQL_BSP"

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But I still got the same error messages.

[stampede]

Sorry, I guess you want the static linkage:

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

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

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[WilliamSpiderWeb]

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

collect2: ld returned 1 exit status

collect2: ld returned 1 exit status

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I've no idea what that error means.

[stampede]

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.

[WilliamSpiderWeb]

Yes, it's the only error message. In addition I get a warning.

Qmake does not support build directories below the source directory

Qmake does not support build directories below the source directory

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This is my SQL_BSP.pro file.

QT       += core gui
 
TARGET = SQL_BSP
TEMPLATE = app
 
LIBS += -L"D:/Qt-Projekte/SQL_BSP/libpg.lib"
 
SOURCES += main.cpp\
        mainwindow.cpp \
    postgresqldb.cpp
 
HEADERS  += mainwindow.h \
    libpq-fe.h \
    postgres_ext.h \
    postgresqldb.h
 
FORMS    += mainwindow.ui
 
OTHER_FILES += \
    libpq.lib

QT       += core gui

TARGET = SQL_BSP
TEMPLATE = app

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

SOURCES += main.cpp\
        mainwindow.cpp \
    postgresqldb.cpp

HEADERS  += mainwindow.h \
    libpq-fe.h \
    postgres_ext.h \
    postgresqldb.h

FORMS    += mainwindow.ui

OTHER_FILES += \
    libpq.lib

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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.

[stampede]

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.

[WilliamSpiderWeb]

Thank you for your help so far.
My SQL_BSP.pro now looks like this:

QT       += core gui
QT += sql
 
TARGET = SQL_BSP
TEMPLATE = app
 
SOURCES += main.cpp\
        mainwindow.cpp
 
HEADERS  += mainwindow.h
 
FORMS    += mainwindow.ui
 
OTHER_FILES += \
    libpq.lib

QT       += core gui
QT += sql

TARGET = SQL_BSP
TEMPLATE = app

SOURCES += main.cpp\
        mainwindow.cpp

HEADERS  += mainwindow.h

FORMS    += mainwindow.ui

OTHER_FILES += \
    libpq.lib

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I tried to get access to the sql db like this:

#include <QtSql>
#include <QSqlError>
#include <QSqlDriver>
#include <QSqlDriverPlugin>
 
    QSqlDatabase dbAlex = QSqlDatabase::addDatabase("QPSQL");
    dbAlex.setHostName("localhost");
    dbAlex.setPort(5433);
    dbAlex.setUserName("postgres");
    dbAlex.setPassword("***");
    dbAlex.setDatabaseName("QT");
    dbAlex.open("postgres", "***");
 
    if (dbAlex.isOpen() == true)
        m_sConnState = "true";
    else
        m_sConnState = "false";
 
    QSqlError dbError = dbAlex.lastError();
    m_sConnState = dbError.text();
    ui->lineEdit->setText(m_sConnState);

#include <QtSql>
#include <QSqlError>
#include <QSqlDriver>
#include <QSqlDriverPlugin>

    QSqlDatabase dbAlex = QSqlDatabase::addDatabase("QPSQL");
    dbAlex.setHostName("localhost");
    dbAlex.setPort(5433);
    dbAlex.setUserName("postgres");
    dbAlex.setPassword("***");
    dbAlex.setDatabaseName("QT");
    dbAlex.open("postgres", "***");

    if (dbAlex.isOpen() == true)
        m_sConnState = "true";
    else
        m_sConnState = "false";

    QSqlError dbError = dbAlex.lastError();
    m_sConnState = dbError.text();
    ui->lineEdit->setText(m_sConnState);

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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.

[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.