Download the tarball src.tar.gz from the dist directory.

Unpack the tarball and create a shell/environment variable CPPLIBS that contains the absolute path to the src/libs directory.

qmake can access an environment variable such as CPPLIBS from inside a project file using the syntax $$(CPPLIBS).

qmake can also include other project file (fragments).

For example, the project file in Example 7.1 includes the file common.pri, for the common application build settings you saw earlier in Example 1.6.

# include common qmake settings
include (../../common.pri)

# this project depends on libdataobjects:
LIBS += -ldataobjects

# this directory contains the libraries:
LIBS += -L$$(CPPLIBS)

# Search here for headers:
INCLUDEPATH += . $$(CPPLIBS)/dataobjects

QT += xml gui

CONFIG += console
TEMPLATE = app

SOURCES += main.cpp slacker.cpp domwalker.cpp xmltreemodel.cpp
HEADERS += slacker.h domwalker.h xmltreemodel.h

In addition, the project adds some values to the LIBS and INCLUDEPATH qmake variables so the project can find dependent libraries and headers.

The command

qmake -project

overwrites the project file, and generates one that contains information based only on the contents of the current working directory.

If a project depends on an external library, the project file contains customizations to variables such as INCLUDEPATH and LIBS.

Rerunning qmake -project clobbers those customizations, so don't.

For example, suppose you develop an application that uses our dataobjects library.

The header files are in $CPPLIBS/dataobjects

the lib shared object files are in $CPPLIBS.

Then you must add the following lines to the project file.

Assignments to the LIBS variable generally contain two kinds of linker switches that are passed directly to the compiler and the linker.