Function mpi::ffi::MPI_Allreduce

pub unsafe extern fn MPI_Allreduce(sendbuf: *const c_void,
                                   recvbuf: *mut c_void,
                                   count: c_int,
                                   datatype: MPI_Datatype,
                                   op: MPI_Op,
                                   comm: MPI_Comm)
                                   -> c_int