Wolfgang Tichy

Important libraries

BLAS and LAPACK

BLAS contains functions for common linear algebra operations such as vector addition, scalar multiplication, dot products, linear combinations, and matrix multiplication.

LAPACK relies on BLAS to provide more complicated linear algebra routines for e.g. solving systems of linear equations and linear least squares, eigenvalue problems, or matrix factorizations such as LU.

Many different implementations of BLAS and LAPACK exist. A commonly used one is OpenBLAS, which contains both BLAS and LAPACK in one library file. I.e. if one uses OpenBLAS it suffices to add

-lopenblas

to link in support for both BLAS and LAPACK functions.

In other implementations BLAS and LAPACK are in separate libraries and then one usually adds

-lblas -llapack

to link in support for BLAS and LAPACK functions.

Using libraries on supercomputers

As explained in the Compile Notes one often also needs to specify the exact directories where the libraries and their include files reside (i.e. the -L and -I options). If there are several versions of each library, this can get tedious very quickly. For this reason many supercomputers have a module system, that can be used to set up the default search locations for the compiler. E.g. in order to link with OpenBLAS one would simply type

module load openblas

before compiling. Then the -L and -I options do not need to be given, and configurations such as MyConfig simplify.

This works because the module system will set the environment variables LIBRARY_PATH and CPATH that the gcc compiler uses (if no -L or -I are given) to search for libraries and include files. You can verify that they have been set with:

echo $LIBRARY_PATH
echo $CPATH

The module system should also set the environment variable LD_LIBRARY_PATH. LD_LIBRARY_PATH is not directly used by the compiler. Rather it is used at program start to find all shared libraries that the program needs. Check it with:

echo $LD_LIBRARY_PATH

If only LD_LIBRARY_PATH is wrong, the compilation will succeed, but when you start the compiled program, you may get "error while loading shared libraries".

Unfortunately, on FAU's Athene supercomputer some of these modules are broken in the sense that they do not set all required environment variables, so that one still may need to specify some -L and -I options anyway. For some libraries we can find out these options using pkg-config. E.g.

pkg-config --libs openblas
pkg-config --cflags openblas

should tell us the locations for openblas. When pkg-config works on Athene, we can even use

SPECIALLIBS += $(shell pkg-config --libs openblas)

directly in the MyConfig. But on Athene you never know if a module sets up the environment variables that pkg-config needs, so that pkg-config may fail as well.