How to build MPI libraries for 64-bit integers

Before you continue please verify whether you really need 64-bit integers. It is extra work and perhaps not needed.

We are currently aware of two MPI distributions that can either be installed as 64-bit integer MPI version or offer a pre-installed 64-bit integer configuration:

  • Intel MPI (4.0.1 and higher)
  • Open MPI (please check below for more details)

Open MPI with 64-bit integers

Please check the integer type of your currently available Open MPI installation (perhaps it can do 64-bit integers already).

For this, type:

ompi_info -a | grep 'Fort integer size'

if the output is:

Fort integer size: 8

then you have a suitable 64-bit Open MPI installation which uses 64-bit integers by default.

If not and you decided that you want to build one yourself (not difficult), then download the latest stable source from and extract it.

Then enter the directory and configure Open MPI (edit prefix path).

For Intel compilers use:

export F77=ifort
export FC=ifort
export CC=icc
export CXX=icpc
export FFLAGS=-i8
export FCFLAGS=-i8
./configure --prefix=/path/to/openmpi

For GNU compilers use:

export F77=gfortran
export FC=gfortran
export CC=gcc
export CXX=g++
export FFLAGS="-m64 -fdefault-integer-8"
export FCFLAGS="-m64 -fdefault-integer-8"
export CFLAGS=-m64
export CXXFLAGS=-m64
./configure --prefix=/path/to/openmpi

Once the configure step is finished (lot of output, at the end there should be no error), build the library:

make all install

Finally export the path locations (adapt paths) in .bashrc (or in the corresponding file if you don’t use bash):

export PATH=$PATH':/path/to/openmpi/intel/bin'
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH':/path/to/openmpi/intel/lib'

and verify the integer type:

ompi_info -a | grep 'Fort integer size'

if the output is:

Fort integer size: 8

Then you are set. If it is different, then something went wrong. Please double-check and consult Open MPI documentation.