should MPI_ABI_VERSION and MPI_ABI_SUBVERSION be defined in non-ABI mpi.h include files

[hppritcha]

Problem

It is not clear from the wording in the ABI chapter of the MPI-5 standard whether or not it is permissible for an MPI implementation to define MPI_ABI_(SUB)VERSION) in their implementation specific mpi.h include file.

It may help application developers as they prepare to port to using the ABI approach to know whether or not these macros are present in a non-ABI mpi.h. It would be good to have this behavior standardized across MPI implementations that support the ABI.

See discussions at

open-mpi/ompi#13280 (review)

Proposal

needs discussion

Changes to the Text

needs discusson

Impact on Implementations

may cause minor issues supporting macros in both abi-mpi.h and non-abi mpi.h

Impact on Users

varied

References and Pull Requests

none yet

[jeffhammond]

MPI_ABI_GET_VERSION produces the standard ABI version, if supported. Other-
wise, the values of the major and minor version are set to −1. The ABI version is independent of the MPI specification version. The major and minor version of the ABI associated with MPI-5.0 are 1 and 0.

The ABI version macros MPI_ABI_VERSION and MPI_ABI_SUBVERSION are present in
the MPI header and modules so that applications can check for consistency between the compilation environment and the properties of the implementation at runtime.

While not explicit, it is natural to infer from this that the header of a non-ABI implementation should define the ABI versions to -1, although I suppose it's also legal to not define them.

[jeffhammond]

Is there a use case where it matters whether they are undefined or defined to -1? It seems like a user needs to handle the undefined case anyways if they want to support MPI implementations where MPI_VERSION < 5.

I guess what I'm saying is, what is the meaningful difference between an implementation of MPI 5.0 that doesn't support the standard ABI, and an implementation of MPI 4.1?

[jsquyres]

FWIW, MPICH 5.0.0b1 sets MPI_ABI_[SUB]VERSION to 1 / 0, respectively, in the installed mpi.h if built with --enable-mpi-abi, but does not define MPI_ABI_[SUB]VERSION in the installed mpi.h if not built with that switch.

So it would probably be good to be explicit about the desired behavior in the spec.

[hppritcha]

actually I'm confused about the MPICH support for the abi. When I do an install of MPICH at dcfab13774 I'm seeing this in the install include dir:

mpi_abi.h  mpicxx.h  mpif.h  mpi.h  mpi_proto.h

with the MPI_ABI_VERSION etc. only in mpi_abi.h, but no such macro in mpi.h But i'm confused about the mpi_abi.h. In the MPI 5 standard in section 20.2.1 it is stated that the ABI mpi header file is named mpi.h. I hope the intent with mpich is not that those using the ABI have to modify their codes to include mpi_abi.h rather than mpi.h.

For OMPI our implementation has an mpicc_abi that points the app to an include file named mpi.h, just in a different location from the OMPI native mpi.h.

[hppritcha]

oh never mind I should have looked at the contents of the mpi.h.