Correctly linking GLX library in Ubuntu

Question

I'm trying to compile one of these mixes of X11 + OpenGL, but I'm not having luck with the compiler. In particular, I get:

  undefined symbol: glXMakeCurrent

I have tried

   -lX11 -lGLU -lGL -lXext

as arguments to the linker, and some permutations of them, with no luck so far.

I'm running Ubuntu 12.04, and I have installed all the development packages related to opengl that I had a fuzzy idea could be related. I'm also developing in C++, something that could cause problems if the opengl headers are not prepared for it... but they are right?

I even looked for the symbol explicitly with an fgrep in /usr/lib/x86_64-linux-gnu/, but it is not there, and furthermore, `nm' says that there are no symbols.

So, what's the correct way of linking with glx?

EDIT: It is linking problem, the error is produced when python tries to load the compiled (and incorrectly linked) module. Not at compilation time.

EDIT: Here is the compilation log

    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    g++ -o build/debug/objects/alve/layouter/flowing_data.os -c -std=c++0x -g -I/usr     /include/python2.7 -fPIC -I/opt/cairo_new/include/cairo/ -I/opt/boost_1_48_0/include -DMIC_RT_SPEED_BACKS -Icsrc csrc/alve/layouter/flowing_data.cpp
    g++ -o build/debug/objects/alve/layouter/liblayouter.so -L/opt/cairo_new/lib -L/opt/boost_1_48_0/lib -shared build/debug/objects/alve/layouter/flowing_data.os build/debug/objects/alve/layouter/show_network.os -Lbuild/debug/lib -Llibdeps
    Install file: "build/debug/objects/alve/layouter/liblayouter.so" as "build/debug/lib/liblayouter.so"
    g++ -o build/debug/objects/alve/layouter/liblayouter_mod.so -L/opt/cairo_new/lib -L/opt/boost_1_48_0/lib -shared build/debug/objects/alve/layouter/module.os  Lbuild/debug/lib -Llibdeps -lboost_python build/debug/objects/alve/layouter/liblayouter.so -lcairo -lX11 -lGL -lGLU -lXext
    scons: done building targets.

and here is how the function is called:

glXMakeCurrent (dpy, win, ctx);

Answer 1

The message "undefined symbol" indicates that it's not a linker, but a compilation unit problem: The compiler does not know the symbol glXMakeCurrent because it has been neither declared, nor defined, but you use it.

Probably the GLX header has not been included.

Add

#include <GL/glx.h>

As it turns out OPs problem was related to the fact, that the build consisted of cascading shared objects forming a Python module. One shared object implements the actual OpenGL operations, while the other does the interfacing to the Python interpreter.

Now shared objects (.so) are fully qualified ELF binaries themself, each with their own import and export symbol table. A shared object can be configured to expose all the symbols of other shared object it links to. However a shared object will not see any symbols of the compilation units they're linked into (if you think about it, this is to be expected, as a shared object can not and should not make any assumptions about the environment it's going to be linked into).

Hence, when compiling and linking multiple shared object in a larger build it's important to individually link each shared object to any libraries it will need at runtime.

Answer 2

As @datenwolf ways, special precautions are needed for linking. Which are them is a mystery for me, but using ldd helps. So basically what I did was to use ldd in both the final and the intermediate shared objects. Despite the command line arguments, my library didn't get linked with libGL and its dependencies until I also included the '-lGL -lGLU -lX11' in the intermediate step (production of 'liblayouter.so').