Setting RPATH order in QMake

Question

I have a Linux Qt program. I'd like it to preferentially use the (dynamic) Qt libraries in the executable's directory if they exist, otherwise use the system's Qt libs. RPATH to the rescue.

I add this line to the qmake's .pro file:

QMAKE_LFLAGS    += '-Wl,-rpath,\'\$$ORIGIN\''

and looking at the resulting executable with readelf I see:

0x000000000000000f (RPATH)              Library rpath: [$ORIGIN:/usr/local/Trolltech/Qt-5.2.0/lib]
0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:/usr/local/Trolltech/Qt-5.2.0/lib]

Seems right, but ldd shows it's using the system version:

libQt5Core.so.5 => /usr/local/Trolltech/Qt-5.2.0/lib/libQt5Core.so.5 (0x00007f2d2fe09000)

If I manually edit qmake's resulting Makefile to swap the order of the two rpaths, so $ORIGIN comes after /usr/local/..., I get the right behavior:

0x000000000000000f (RPATH)              Library rpath: [/usr/local/Trolltech/Qt-5.2.0/lib:$ORIGIN]
0x000000000000001d (RUNPATH)            Library runpath: [/usr/local/Trolltech/Qt-5.2.0/lib:$ORIGIN]

libQt5Core.so.5 => ./libQt5Core.so.5 (0x00007fb92aba9000)

My problem is with how qmake constructs the final LFLAGS variable. I can't figure out how to make it put my addition ($ORIGIN) after the system library. Any ideas?

Answer 1

You can add the following to your .pro file to force the dynamic linker to look in the same directory as your Qt application at runtime in Linux :

unix:{
    # suppress the default RPATH if you wish
    QMAKE_LFLAGS_RPATH=
    # add your own with quoting gyrations to make sure $ORIGIN gets to the command line unexpanded
    QMAKE_LFLAGS += "-Wl,-rpath,\'\$$ORIGIN\'"
}

If you want it to look in a subdirectory of the executable path, you can use :

QMAKE_LFLAGS += "-Wl,-rpath,\'\$$ORIGIN/libs\'"

Note that you should have the .so files with the exact same name in your application directory. For example you should copy libQt5Core.so.5.2.0 to your application directory with the name libQt5Core.so.5. Now the ldd shows the directory of the application.

You can also have libQt5Core.so.5.2.0 and a link to it with the name libQt5Core.so.5 in the application directory.