Linking libraries during compilation
Linking libraries is very common in compiling source codes. ld or The GNU Linker is very useful in this regard. But to properly link the libraries to the source codes understanding the linker utility is important and the options available to it.
-l namespec / -l :filename
-l namespec or --library=namespec links a library named namespec. This actually searches for a archied or shared library object file named libnamespec.so or libnamespec.a in available path. If one ‘:’ is provided at the begining then the linker, it searches for filename instead
NOTE: The space between -l and namespec is not mandatory.
-L searchdir
By default the linker searches for the file in the available path but if the file is not available in the path but instead present in a directory named searchdir, one can use -L searchdir or --library-path=searchdir to let the linker know the location of the available file.
Available paths
The available paths to the linker utility can be checked by debugging ld. Run
ld --verbose | grep SEARCH_DIR
Common problem: cannot find -lfftw3
One very common problem we face while linking libraries is, it fails to link the library, while the library is installed properly. It happens because the linker can not find the file in available path or the library is installed with different names. If the file is not available in the default path then its quite easy to provide the path with -L. But if the file is installed with different name usually, with a version name at the end, then its probably best to create a symbolic link of the file at a proper location.
1. Finding the installed files
Before creating the symbolic link, we need to find location and name of the file, installed as a library. If we know then library is fftw3 then the file name must be libfftw3.so or something very similar, like this libfftw3.so.3.5.1, the number at the end being the version number. So, simply search for this file in your system
find / -name libfftw* -type f
and note the location and full name of the file.
2. Creating symbolic link
Once you find the location and name, create a soft symbolic link in a place available to the path.
ln -s /path_to_file/libfftw.so.3.5.1 /path_available_to_linker/libfftw.so
Now the linker utility will easily find the symbolic link and from there the actual library files.