[Mpi-forum] "BigCount" rendering in PDF

N.M. Maclaren nmm1 at cam.ac.uk
Thu Aug 1 05:23:22 CDT 2019

On Jul 30 2019, Jeff Squyres (jsquyres) via mpi-forum wrote:
> B. C11 _Generic polymorphism kinda sucks, *and* we're in a transition 
> period where not all C compilers are C11-capable. Hence, we're exposing 
> up to *3* C bindings per MPI procedure to applications (including 
> explicitly exposing the "_x" variant where relevant).

This following point may have already been debated, so please excuse me if
has.  Let's skip the politics, for the sake of all our blood pressures.

The executive summary is that the failure mode pointed out by Joseph
Schuchart is going to be a very, very serious problem for BigCount users,
and continue for the forseeable future.  I would guess until at least
2025, and possibly long after that.

Speaking as someone who may be teaching MPI programming again, with an
emphasis on reliability and portability, I would almost certainly add
warnings that would be, roughly: "Don't touch BigCount if you can find a
way round it; and be paranoid about it if you use it."  I already do that
about I/O attributes, for similar reasons.  That isn't good.

I don't know how you would document that, but the MPI standard already
has gotchas that aren't easy to find, and adding another one isn't good,

The explanation:

C99 was not received favourably by most of the C-using community (I don't
mean compilers here).  I tracked a dozen important, active projects, and
it was 2010/11 (yes, over a decade) before even half of them converted
from C90 to C99 as a base.  I last checked a few years ago, but quite a
few C99 features were still not reliably available in compilers; I know
that many of the ones I found still aren't.  Courses are another problem,
because they rarely include warnings about gotchas caused by standards
differences (and there are lots between C90 and C99).

I haven't tracked C11 as carefully, but the evidence I have seen is that it
received even less interest and acceptance than C99, so people are going
to be using C99 compilers for a LONG time yet.  There is also the problem
that C is not a language that is upwards compatible between versions, but
C++ takes more notice, so C++ compilers' C support (which is arguably more
important than direct C support, because people call MPI using C++'s C
interface) is often in conflict.  This is almost certainly a case where
that will be true, but it may not affect these interfaces - I can't say.

The result is that C code (and, worse, libraries) often require specific
versions (i.e. not just 'any standard later than'). I agree that it looks
likely that generic interfaces are going to be one of the more widely
implemented parts of C99, but don't discount the problem of people using
multiple libraries where other ones constrain the C version for other

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