External library throws undefined reference errors in Qt Creator

Question

I have some problems with using an external library/SDK. I am using Qt Creator under Ubuntu with the Oculus Rift SDK. The Oculus Rift SDK consists of a platform dependent lib file and some c++ source code. Note that I highly doubt my problem is in any way specific to the Oculus SDK or Xinerama.

I used the wizard and added the SDK as an external library. This was the result:

unix:!macx: LIBS += -L$$PWD/../OculusSDK/LibOVR/Lib/Linux/Release/x86_64/ -lovr
INCLUDEPATH += $$PWD/../OculusSDK/LibOVR/Include
DEPENDPATH += $$PWD/../OculusSDK/LibOVR/Include
unix:!macx: PRE_TARGETDEPS += $$PWD/../OculusSDK/LibOVR/Lib/Linux/Release/x86_64/libovr.a

Now I should be able to call methods from the SDK. However, this is one of the errors I get while compiling:

/home/me/OculusSDK/LibOVR/Lib/Linux/Release/x86_64/libovr.a(OVR_Linux_HMDDevice.o):-1: In function `OVR::Linux::HMDDeviceFactory::EnumerateDevices(OVR::DeviceFactory::EnumerateVisitor&)':
OVR_Linux_HMDDevice.cpp:-1: error: undefined reference to `XineramaIsActive'

I already tried adding this line to my .pro file but nothing changed:

unix:LIBS += -lXinerama

If I include the very same header file as OVR_Linux_HMDDevice.cpp does,

#include <X11/extensions/Xinerama.h>

I can successfully call XineramaIsActive from my code. So apparently, it's just the external source code that has issues accessing the method.

Any ideas what changes I have to make to the build process for this to work?

Community · Accepted Answer · 2017-05-23 12:31:52Z

4

Link order is important here. You need to make sure the -lXinerama option is added after the -lovr option. qmake should preserve the order of libraries as you add them in your .pro file.

See this answer for more information.

edited May 23, 2017 at 12:31

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answered Jan 13, 2014 at 16:08

rubenvb

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3 Comments

derp Over a year ago

I indeed added -lXinerama before -lovr. After moving it down and adding another library, it finally compiled. Thank you so much! But I realize I am lacking some very basic understanding here: Intuition tells me it should come first, so why is it the other way round?

rubenvb Over a year ago

Dependencies are resolved left to right. So if you do -la -lb -lc, libb will be used to resolve undefined symbols in liba, and libc will be used to resolve dependencies in liba and libb. If, say, liba depends on libb, libb depends on libc, and libc depends on liba... you will have to "link" liba twice: -la -lb -lc -la. This leads to funny situations. Note that GNU binutils has the options --begin-group and --end-group which do search circularly until dependencies are resolved. In principle though, this circular search could lead to a deadlock.

derp Over a year ago

Thank you again! Thinking about it, this makes a lot more sense.

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