use of undeclared identifier 'MPI' when using C++ syntax for OpenMPI on MacOS

Question

I have my OpenMPI and gcc installed on my MacOS. C syntax for OpenMPI could run perfectly. However, the c++ syntax for OpenMPI are always not recognized. For example, line with void MPI::Init(int &argc, char**&argv); shows error of use of undeclared identifier 'MPI'.

Could anybody give some detailed procedure to fix this problem?

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

using namespace std;

int main(int argc, char** argv) {


void MPI::Init(int &argc, char**&argv);

// Get the number of processes
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
//int Comm::Get_size() const
// Get the rank of the process
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

// Get the name of the processor
char processor_name[MPI_MAX_PROCESSOR_NAME];
int name_len;
MPI_Get_processor_name(processor_name, &name_len);

// Print off a hello world message
printf("Hello world from processor %s, rank %d out of %d processors\n",
       processor_name, world_rank, world_size);


// Finalize the MPI environment. No more MPI calls can be made after this
MPI_Finalize();

}

Does mpi.h declare a namespace MPI in which there is an Init() function? Or what is MPI? — Rudy Velthuis
– Rudy Velthuis, Commented Jul 31, 2016 at 4:21

wariasr · Accepted Answer · 2017-05-14 21:44:59Z

4

I had the same issue when installing open-mpi using homebrew in Mac OS X (I am a newbie in Mac OS X coming from Debian/Ubuntu Linux). Fortunately, I solved it. As Zulan said, your open-mpi installation was built without C++ bindings. However, I think the best solution is to rebuild it directly from the source code. I downloaded the .tar files directly from its homepage, and if you look closer you will see that the C++ bindings are by default disabled (do the following inside the untar folder):

$ ./configure -help

--enable-mpi-cxx enable C++ MPI bindings (default: disabled)

So, basically I rebuilt open-mpi directly adding that flag, as follow:

$ ./configure --enable-mpi-cxx

Then, just make sure you know where you have installed it (if you used --prefix= or the default) to link it to your makefile properly.

for more information about this issue look here: https://github.com/open-mpi/ompi/issues/3173.

answered May 14, 2017 at 21:44

wariasr

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1 Comment

GunnerFan Over a year ago

I built open-mpi directly from the source using the --enable-mpi-cxx option. However the "mpi.h" file still does not declare a namespace MPI. This leads to "error: use of undeclared identifier 'MPI'".

Zulan · Accepted Answer · 2017-05-15 06:26:19Z

2

It seems your OpenMPI installation was built without C++ bindings. While OpenMPI still contains the C++ bindings, they actually have been removed from the standard years ago. You will probably have to rebuild OpenMPI to enable the C++ bindings, that depends on how you installed it in the first place.

Since they are not part of the standard, the recommendation is to not the C++ bindings in the first place. ~~Instead the most common recommendation is to use Boost.MPI.~~

Unfortunately it appears that Boost.MPI is no longer actively developed (last bug closed 2 years ago), and there are some really nasty open issues, so I cannot recommend Boost.MPI. There are some cool MPI type-traits approaches, but I do not know of a widely accepted / actively developed wrapper.

edited May 15, 2017 at 6:26

answered Jul 31, 2016 at 7:47

Zulan

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2 Comments

Charles Over a year ago

can I understand it as OpenMPI's full functions support C, while C++ is kind of partially supported? Are most OpenMPI application programmers prefer C instead of C++?

Zulan Over a year ago

OpenMPI is an implementation of the MPI standard. Generally you develop having the MPI standard in mind - not a particular implementation. The C++ bindings were removed from the standard years ago. So you should not use them, even if some implementations may still provide them.

ngoldbaum · Accepted Answer · 2017-10-09 16:05:19Z

2

If you installed OpenMPI using homebrew, you need to first uninstall it:

brew uninstall openmpi

And then reinstall it, enabling C++ bindings:

brew install openmpi --with-cxx-bindings

See brew info openmpi for a full list of options. Note that since you're not using the default build of OpenMPI, brew will need to build it from source instead of using a precompiled bottle so it will take longer than usual to install.

answered Oct 9, 2017 at 16:05

ngoldbaum

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1 Comment

GunnerFan Over a year ago

I get this error "Error: invalid option: --with-cxx-bindings". brew info openmpi does not show the available options.

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