How to pass MPI information to ctypes in python

Question

My motivation is to pass MPI information effectively from python to C functions invoked through ctypes. I used mpi4py for MPI bindings in python. I would like to learn it through a simple example MPI code written in C and invoked through ctypes in python. I have detailed the steps and the error that I get while running below.

C code [passMpi4Py.c]

#include <stdio.h>
#include <mpi.h>

void sayhello(MPI_Comm comm)
{
  int size, rank;
  MPI_Comm_size(comm, &size);
  MPI_Comm_rank(comm, &rank);
  printf("Hello, World! "
         "I am process %d of %d.\n",
         rank, size);
}

I compiled the above c code with gcc/openmpi-1.6 as follows:

mpicc -shared -Wl,-soname,passMpi4Py -o passMpi4Py.so -fPIC passMpi4Py.c

Python Wrapper [passMpi4PyWrapper.py]

import ctypes
from mpi4py import MPI
testlib = ctypes.CDLL('path-to-file/passMpi4Py/passMpi4Py.so')

testlib.sayhello(MPI.COMM_WORLD)

When i try to run the above code by using

mpirun -np 4 python passMpi4PyWrapper.py

I get the following error

Traceback (most recent call last):
Traceback (most recent call last):
  File "passMpi4PyWrapper.py", line 5, in <module>
Traceback (most recent call last):
  File "passMpi4PyWrapper.py", line 5, in <module>
  File "passMpi4PyWrapper.py", line 5, in <module>
Traceback (most recent call last):
    testlib.sayhello(MPI.COMM_WORLD)
ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1
    testlib.sayhello(MPI.COMM_WORLD)
ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1
    testlib.sayhello(MPI.COMM_WORLD)
ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1
  File "passMpi4PyWrapper.py", line 5, in <module>
    testlib.sayhello(MPI.COMM_WORLD)
ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1

Update:

Using *MPI_COMM_WORLD* instead of comm in the C program MPI functions helps me to remove the error. However I still would like to know if this is the best possible way to pass MPI information to a C program.

Have you tried it without the communicator? e.g. hardcoding MPI_COMM_WORLD in sayhello? — mgilson
I just tried but what is the data type i must give for MPI_COMM_WORLD in sayhello without which it gives me an error while compiling C code — rambalachandran
No, I mean make sayhello a function which accepts 0 arguments. e.g. void sayhello(){ ...; MPI_Comm_size(MPI_COMM_WORLD,&size);... } — mgilson
One problem here is that the type of MPI_COMM_WORLD is of type MPI_Comm which could be just about anything. It could be typedefed to int,long,long long ... or it could even be some sort of struct (the standard doesn't say)`. Likely, it is an 4 byte integer on most systems to be compatible with the fortran bindings, but that's definitely not guaranteed. — mgilson
Cool. The MPI_COMM_WORLD that you suggested worked now. Thank you. But is this the best way to send MPI information from python wrapper to C code? Do you know of any books or online tutorials which deal with this in detail? — rambalachandran

Greg Inozemtsev Greg Inozemtsev · Accepted Answer · 2012-09-18T05:47:12

You are missing a way to map Python's MPI.COMM_WORLD (which is an instance of the mpi4py's Comm class) to MPI_COMM_WORLD (which is an int handle). This can be done by generating a wrapper with SWIG. The mpi4py tutorial has basically the same example as you have, but with the SWIG interface file added.

If you'd rather not use SWIG, you can perform the conversion in the C code. If you look at the file mpi4py.i that the SWIG example is importing, you can see that conversion is done with PyMPIComm_Get. mpi4py source comes with an example that does not use SWIG.

How to pass MPI information to ctypes in python

1 Answers