Question

I’m trying to link to a static library, libcovis.a. Everything looks fine but I still have

undefined reference to `CoViG_PublicDemo::MoInS::reset()’

I checked that the name exists in the library

$nm libcovis.a | grep reset

…

_ZN16CoViG_PublicDemo5MoInS5resetEv

…

I’m using linking arguments -L/path/to/libcovis.a -lcovis

What am I doing wrong ?

Edit:
The error might be something else, if do

gcc main.cpp -I/usr/include/opencv -I/usr/include/cairo -I../../Source -o slam -rdynamic -lGLU -lGL -lSM -lICE -lX11 -lXext -lglut -lXi -lxml2 -lboost_filesystem-mt -llapack -lblas -lcv -lcxcore -lcvaux -lhighgui -lcairo ../../Source/libcovis.a ../../Source/contrib/gnuplot_i/libcovis_contrib_gnuplot_i.a -lgl2ps

It works !

But when I’m in KDevelop using cmake, I doesn’t work anymore. I use

CMAKE_EXE_LINKER_FLAGS:STRING=-rdynamic -lGLU -lGL -lSM -lICE -lX11 -lXext -lglut -lXi -lxml2 -lboost_filesystem-mt -llapack -lblas -lcv -lcxcore -lcvaux -lhighgui -lcairo /usr/local/src/CoViS-0.0.0-1/Source/libcovis.a /usr/local/src/CoViS-0.0.0-1/Source/contrib/gnuplot_i/libcovis_contrib_gnuplot_i.a -lgl2ps

CMAKE_CXX_FLAGS:STRING=-I/usr/local/src/CoViS-0.0.0-1/Source -I/usr/include/opencv -I/usr/include/cairo

The only difference I can see is that the paths are absolute and not relative, but if he couldn’t find the libs, he would say it…

Answer 1

There are two different issues there, the first of which is the simplest, you have used the wrong compiler options. The -L option tells the linker to also look in the directory when looking for a library. The -l tells it to link the specific library. To link you would then use:

g++ -o test test.o -L/path/to -lcovis

or

g++ -o test test.o -l/path/to/libcovis.a

To force static linking if the same library is present as a dynamic library in the same directory.

The second potential issue is that the order of static libraries in the linker command line does matter, so that might also be an issue if there is a dependency on different static libs.

g++ -o test tests.o -ldependent -lprovider

The linker will process the libraries in order as they are in the command line, and from each static lib it will only pull those symbols that are required (with as much information as the linker has at that time). In the command line above, the linker will extract from dependent the symbols it needs for test.o, and that might in turn add new undefined symbols to the program (the dependencies of dependent). When it processes provider it will fill in those symbols. If the order was reversed in the command line, the symbols that are required by dependent but not by test.o would not be added to the executable, as the linker does not know that those symbols will be needed when processing provider.