Adding external library into Qt Creator project

Question

The proper way to do this is like this:

LIBS += -L/path/to -lpsapi

This way it will work on all platforms supported by Qt. The idea is that you have to separate the directory from the library name (without the extension and without any 'lib' prefix). Of course, if you are including a Windows specific lib, this really doesn't matter.

In case you want to store your lib files in the project directory, you can reference them with the $$_PRO_FILE_PWD_ variable, e.g.:

LIBS += -L"$$_PRO_FILE_PWD_/3rdparty/libs/" -lpsapi

Are you using qmake projects? If so, you can add an external library using the LIBS variable. E.g:

win32:LIBS += path/to/Psapi.lib

LIBS += C:\Program Files\OpenCV\lib

won't work because you're using white-spaces in Program Files. In this case you have to add quotes, so the result will look like this: LIBS += "C:\Program Files\OpenCV\lib". I recommend placing libraries in non white-space locations ;-)

The error you mean is due to missing additional include path. Try adding it with: INCLUDEPATH += C:\path\to\include\files\ Hope it works. Regards.

And to add multiple library files you can write as below:

INCLUDEPATH *= E:/DebugLibrary/VTK E:/DebugLibrary/VTK/Common E:/DebugLibrary/VTK/Filtering E:/DebugLibrary/VTK/GenericFiltering E:/DebugLibrary/VTK/Graphics E:/DebugLibrary/VTK/GUISupport/Qt E:/DebugLibrary/VTK/Hybrid E:/DebugLibrary/VTK/Imaging E:/DebugLibrary/VTK/IO E:/DebugLibrary/VTK/Parallel E:/DebugLibrary/VTK/Rendering E:/DebugLibrary/VTK/Utilities E:/DebugLibrary/VTK/VolumeRendering E:/DebugLibrary/VTK/Widgets E:/DebugLibrary/VTK/Wrapping

LIBS *= -LE:/DebugLibrary/VTKBin/bin/release -lvtkCommon -lvtksys -lQVTK -lvtkWidgets -lvtkRendering -lvtkGraphics -lvtkImaging -lvtkIO -lvtkFiltering -lvtkDICOMParser -lvtkpng -lvtktiff -lvtkzlib -lvtkjpeg -lvtkexpat -lvtkNetCDF -lvtkexoIIc -lvtkftgl -lvtkfreetype -lvtkHybrid -lvtkVolumeRendering -lQVTKWidgetPlugin -lvtkGenericFiltering

If you want to deploy your application on machines of customers, rather than using your application only yourself, we find that the LIBS+= -Lxxx -lyyy method can lead to confusion if not problems.

We develop applications for Linux, Mac and Windows using Qt. We ship complete, stand-alone applications. So all non-system libraries should be included in the deployment package. We want our customers to be able to run the application from the same USB stick for all OSs. For reasons of platform compatibility the USB stick must then be formatted as FAT32, which does not support (Linux) symlinks.

We found the LIBS+= -Lxxx -lyyy idiom too much of a black box:

1. We do not exactly know what the filepath is of the (static or dynamic) library that has been found by the linker. This is inconvenient. Our Mac linker regularly found libs different from the ones we thought that should be used. This happened several times with OpenSSL libraries where the Mac linker found and used its own - older, incompatible - OpenSSL version rather than our requested version.

2. We cannot afford that the linker uses symlinks to libraries as this would break the deployment package.

3. We want to see from the name of the library whether we link a static or a dynamic library.

So for our particular case we use only absolute filepaths and check whether they exist. We remove all symlinks.

First we find out what operating system we are using and put this in the CONFIG variable. And, for instance for Linux 64bit, then:

linux64 {
    LIBSSL= $$OPENSSLPATH/linux64/lib/libssl.a
    !exists($$LIBSSL): error ("Not existing $$LIBSSL")
    LIBS+= $$LIBSSL
    LIBCRYPTO= $$OPENSSLPATH/linux64/lib/libcrypto.a
    !exists($$LIBCRYPTO): error ("Not existing $$LIBCRYPTO")
    LIBS+= $$LIBCRYPTO
}

All the dependencies can be copied into deployment package as we know their filepaths.

I would like to add for the sake of completeness that you can also add just the LIBRARY PATH where it will look for a dependent library (which may not be directly referenced in your code but a library you use may need it).

For comparison, this would correspond to what LIBPATH environment does but its kind of obscure in Qt Creator and not well documented.

The way i came around this is following:

LIBS += -L"$$_PRO_FILE_PWD_/Path_to_Psapi_lib/"

Essentially if you don't provide the actual library name, it adds the path to where it will search dependent libraries. The difference in syntax is small but this is very useful to supply just the PATH where to look for dependent libraries. It sometime is just a pain to supply each path individual library where you know they are all in certain folder and Qt Creator will pick them up.