Is it possible to force MPI to always block on send?

Is there a way to force MPI to always block on send? This might be useful when looking for deadlocks in a distributed algorithm which otherwise depends on the buffering MPI might choose to do on send.

For example, the following program (run with 2 processes) works without problems on my machine:

// C++
#include <iostream>
#include <thread>

// Boost
#include <boost/mpi.hpp>
namespace mpi = boost::mpi;

int main() {
    using namespace std::chrono_literals;

    mpi::environment env;
    mpi::communicator world;
    auto me = world.rank();
    auto other = 1 - me;

    char buffer[10] = {0};

    while (true) {
        world.send(other, 0, buffer);
        world.recv(other, 0, buffer);
        std::cout << "Node " << me << " received" << std::endl;

        std::this_thread::sleep_for(200ms);
    }
}

But if I change the size of the buffer to 10000 it blocks indefinitely.

For pure MPI codes, what you describe is exactly what MPI_Ssend() gives you. However, here, you are not using pure MPI, you are using boost::mpi. And unfortunately, according to boost::mpi's documentation, MPI_Ssend() isn't supported.

That said, maybe boost::mpi offers another way, but I doubt it.