Linking errors when trying to compile Fubi

Question

This problem is not specific to Fubi, but a general linker issue. These past few days (read as 5) have been full of linking errors, but I've managed to narrow it down to just a handful.

I'm trying to compile Fubi (Full Body Interaction framework) under the Linux environment. It has only been tested on Windows 7, and the web is lacking resources for compiling on a *nix platform.

Now, like I mentioned above, I had a plethora of linking problems that dealt mostly with incorrect g++ flags. Fubi requires OpenNI and NITE ( as well as OpenCV, if you want ) in order to provide it's basic functionality. I've been able to successfully compile both samples from the OpenNI and NITE frameworks.

As far as I understand, Fubi is a framework, thus I would need to compile a shared library and not a binary file.

When I try to compile it as a binary file using the following command

g++ *.cpp -lglut -lGL -lGLU -lOpenNI -lXnVNite_1_5_2 -I/usr/include/nite -I/usr/include/ni -I/usr/include/GL -I./GestureRecognizer/ -o FubiBin

and I get the output located here . (It's kind of long and I did not want to ruin the format)

If I instead compile into object files (-c flag), no errors appear and it builds the object files successfully. Note, I'm using the following command:

g++ -c *.cpp -lglut -lGL -lGLU -lOpenNI -lXnVNite_1_5_2 -I/usr/include/nite -I/usr/include/ni -I/usr/include/GL -I./GestureRecognizer/ 

I then am able to use the ar command to generate a statically linked library. No error [probably] occurs (this is only a guess on my end) because it has not run through the linker yet, so those errors won't appear.

Thanks for being patient and reading all of that. Finally, question time:

1) Is the first error regarding the undefined reference to main normal when trying to compile to a binary file? I searched all of the files within that folder and not a single main function exists.

2) The rest of the undefined reference errors complain that they cannot find the functions mentioned. All of these functions are located in .cpp and .h files in the subdirectory GestureRecognizer/ which is a subdirectory of the path I'm compiling in. So wouldn't the parameter -I./GestureRecognizer/ take care of this issue?

I want to be sure that when I do create the shared library that I won't have any linking issues during run-time. Would all of these errors disappear when trying to compile to a binary file if they were initially linked properly?

Answer 1

You are telling the compiler to create an executable in the first invocation and an executable needs a main() function, which it can't find. So no, the error is not normal. In order to create a shared library, use GCC's "-shared" option for that. Trying some test code here, on my system it also wants "-fPIC" when compiling, but that might differ. Best idea is to dissect the compiler and linker command lines of a few other libraries that build correctly on your system.

In order to add the missing symbols from the subdirs, you have to compile those, too: g++ *.cpp ./GestureRecognizer/*.cpp ... . The "-I..." only tells the compiler where to search when it finds an #include ... . I wouldn't be surprised if this wasn't even necessary, many projects use #include "GestureRecognizer/Foo.h" to achieve that directly.

BTW:

• Consider activating warnings when running the compiler ("-W...").
• You can split between compiling ("-c") and linking. In both cases, use "g++" though. This should decrease your turnaround time when testing different linker settings.