Notes for calling fortran code with openmp in python

python_and_fortran_omp.md

Notes for calling Fortran90 code with OpenMP in python

Dependecies:

• numpy
• gfortran

The function vectormt contain fortran code and openmp calls and will be compiled with gfortran. A similar compilations could be done using hpc-sdk or OpenMP (targets) for GPU support.

File: vectormt.f90

subroutine vectormt (c,n)
      USE OMP_LIB
      implicit none
      integer, intent(in) :: n

      real*8, intent(out), dimension(n) :: c

      PRINT *, "Hello max threads :", OMP_GET_MAX_THREADS()
      PRINT *, "Hello from process:", OMP_GET_THREAD_NUM()

      PRINT *, "vectormt.f90: First element of vector C ", c(1)
end subroutine vectormt

The helper function simple is just an intermediary between python and vectormt and will be compiled by f2py, this is useful when vectormt is too complex to be compiled by f2py, or when the fortran needed by vectormt is not supported by nummpy.f2py.

File: simple.f90

        implicit none

        real*8, allocatable :: c(:)
        real*8 :: alpha
        integer :: n
        integer :: i
        
        n = 1000

        allocate(c(n))
        
        do i = 1, n
                c(i) = 1.0
        enddo

        print *, "simple.f90: First element of vector C ", c(1)
        call vectormt (c,n)

        if (allocated(c)) then
             deallocate(c)
        end if
end subroutine simple

The follow is just a Makefile that summarizes the steps needed to compile the both simple.f90 and vectormt.f90, this will produce the objects, libraries and python modules for this example.

File: Makefile

simpler.so: simple.f90
	f2py -c --fcompiler=gnu95 -m simpler  simple.f90 -L. -lvectormt -lgomp
	ln -s simpler.cpython-36m-x86_64-linux-gnu.so simpler.so

libvectormt.so: vectormt.o
	ld -o libvectormt.so.1 -dy -G -h libvectormt.so.1 vectormt.o
	ln -s libvectormt.so.1 libvectormt.so

vectormt.o: vectormt.f90
	gfortran -shared -fPIC -c -o vectormt.o vectormt.f90

clean:
	rm *.o *.so 

testpython: simpler.so
	python -c "import simpler;simpler.simple()"

With all this files in place the compilations is done calling make simpler.so or with the test case make testpython.

This is should be the output:

 simple.f90: First element of vector C : 1
 Hello max threads :           3
 Hello from process:           0
 vectormt.f90: First element of vector C : 1

NOTES: In case you get : ImportError: libvectormt.so.1: cannot open shared object file: No such file or directory just activate the library to your LD_LIBRARY_PATH path as: export LD_LIBRARY_PATH=pwd:$LD_LIBRARY_PATH