f2py with OMP: can't import module, undefined symbol GOMP_*

Question

I was hoping to use openmp to speed up my Fortran code that I run through f2py. However, after compiling succesfully, I can't import the module in Python.

For a Fortran95 module like this:

module test
implicit none
contains
subroutine readygo()
real(kind = 8), dimension(10000) :: q
!$OMP WORKSHARE
q = 7
!$OMP END WORKSHARE
end subroutine
end module

Compiled and imported with these commands:

f2py -m SOmod --fcompiler=gnu95 --f90flags='-march=native -O3 -fopenmp' -c SOtest.f95
python2 -c "import SOmod"

I get an error. The error is for the import - compiling works fine both with f2py or gfortran directly (only get a warning about 'Using deprecated NumPy API').

Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: ./SOmod.so: undefined symbol: GOMP_barrier

I get different GOMP_* errors for different OMP directives. Without directives (but with -openmp flag) it works.

Any help would be greatly appreciated.

Answer 1

I was able to reproduce the error on Mac OS X (10.9.5), with gfortran installed using homebrew, and I was able to fix it by adding -lgomp to the command:

f2py -m SOmod --fcompiler=gnu95 --f90flags='-march=native -O3 -fopenmp' -lgomp -c SOtest.f95

Added by @Mark: Note that -lgomp is an argument for f2py, not gfortran. Although it compiles with just -gomp, both -gomp and -fopenmp are needed for it to be parallel, as described here. GOMP is the GNU openMP implementation.