Qt QCoreApplication addLibraryPath使用

Question

I've been having troubles with the QCoreApplication::addLibraryPath(QString &Path) method on windows.

I've trying to use it to add directories where the application should be looking for dll that i'm loading dynamically with QLibrary.

I soon enough realized that it was not the right way to go. I now use a putenv approach to change directly my environement variables.

Plus, I still don't understand what exactly the addLibraryPath method is supposed to be used for?

Answer 1

I think qt documentation is not clear enough, regarding this topic. There are (at least) 2 sorts of libs/dlls:

• Essential Libs/Dlls das are already needed at program start (like Qt5core.dll).
• "Functionality Libs" like the Qt Plugins and third party stuff, that can be loaded later.

It is not obvious (at least for me) which DLL is of sort 1 or sort 2. This leads to the problem that it may be nasty to find out which you can move into subfolders and point your application to it by addLibraryPath().

For me the following solution worked:

• use windeploy to find out the bigger part of dependencies (my app's executable is in a "bin" folder below the project folder)

  • c:\\Qt\\Qt5.3.2\\5.3\\mingw482_32\\bin\\windeployqt.exe ..\\bin\\myapp.exe --release --force --compiler-runtime -libdir ..\\bin -dir ..\\bin\\plugins
  • this puts the "sort 1" libs into the app folder
  • and the "sort 2" libs into a subfolder plugins

• Additionally, it is needed to let the installer set an environment var "QT_PLUGIN_PATH" in registry to let the app find the plugins. I wasted hours just to find out that setting this path with addLibraryPath() at runtime is just not working. Also a qt.conf file seems not to work. The only alternative for me is setting a environment var in a .bat file, what is essentially the same like the registry setting. Here's the registry key (in inno setup syntax):

  Root: HKLM; Subkey: "SYSTEM\\CurrentControlSet\\Control\\Session Manager\\Environment"; ValueType: string; ValueName: "QT_PLUGIN_PATH"; ValueData: "lib"

Another anoying thing is, that some libs are not identified by windeploy. These are mainly the compiler redistributables, that will vary with the compiler that you use. Others are more depending on the functions that you use, and somehow are not processed by windeploy. This fact is not documented obviously (at least I didn't see it) and is also not easy to understand. For my app, these are the following compiler redists and some database related libs:

libeay32.dll
libgcc_s_dw2-1.dll
libintl.dll
libpq.dll
libstdc++-6.dll
libwinpthread-1.dl

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Dependency Walker is always said as a solution for finding this out. For me that didn't work either. Not all of the mentioned libs were listed, but the app isn't running without. Maybe it's because the libs are just loaded under special circumstances?

Answer 2

addLibraryPath adds a path to the ones that the application will search when dynamically loading libraries.

From the Qt documentation about QCoreApplication::​libraryPaths() :

This list will include the installation directory for plugins if it exists (the default installation directory for plugins is INSTALL/plugins, where INSTALL is the directory where Qt was installed). The directory of the application executable (NOT the working directory) is always added, as well as the colon separated entries of the QT_PLUGIN_PATH environment variable.

Also it's stated in the Qt documentation that :

An application has an applicationDirPath() and an applicationFilePath(). Library paths (see QLibrary) can be retrieved with libraryPaths() and manipulated by setLibraryPaths(), addLibraryPath(), and removeLibraryPath().

So it seems you can add the path for QLibrary with addLibraryPath .