Calling conventions

All operations in kamping::v2 follow the same argument order: the communicator comes last and is always required, followed only by an optional MPI_Status*. Arguments with an obvious default can be left out in front of it, by passing fewer arguments:

v2::send(buf, dest, comm);           v2::send(buf, dest, tag, comm);
v2::recv(buf, comm);                 v2::recv(buf, source, comm);
v2::recv(buf, source, tag, comm);
v2::bcast(buf, comm);                v2::bcast(buf, root, comm);
v2::reduce(s, r, comm);              v2::reduce(s, r, op, comm);
v2::reduce(s, r, op, root, comm);

A left-out argument takes the obvious value:

Argument	Default
send tag	DEFAULT_SEND_TAG (0)
source of a receive	MPI_ANY_SOURCE
tag of a receive	MPI_ANY_TAG
root	0
reduction operation	std::plus<>

Rooted collectives also have a shorthand for non-root processes, without the buffer that only the root uses: gather(sbuf, [root,] comm), scatter(rbuf, [root,] comm), scatterv(rbuf, [root,] comm) and reduce(sbuf, [op, [root,]] comm). gatherv has none, since what a non-root process passes depends on whether the root’s counts are deferred.

A request you manage yourself is passed as the first argument of a non-blocking operation: v2::isend(request, buf, dest, comm). The send mode, if not standard, is also passed first: v2::send(v2::send_mode::sync, buf, dest, comm).

Why not default arguments?

On some MPI implementations, MPI_Comm, MPI_Request, MPI_Op and the other handles are plain ints, just like ranks and tags. With default arguments in the middle of the list, a communicator could silently be taken for a tag. With the rules above, the forms of an operation differ in their number of arguments or in an argument that is never an int, so every call is unambiguous. For the same reason there is no reduce(s, r, root, comm): a root cannot be told apart from an MPI_Op.

Return values

Operations return the buffers they were given (see Non-blocking operations): a single buffer directly, a send and a receive buffer as a result that can be unpacked with a structured binding, and non-blocking operations an iresult.