[Mpi-forum] "BigCount" rendering in PDF

Jeff Hammond jeff.science at gmail.com
Thu Aug 1 05:56:55 CDT 2019

That’s why there will be C90/C99 compatible symbols as well. If you don’t like C11, don’t use it. Nothing will happen. BigCount will still work.

C11 has been the default in GCC and Clang for a while. What compilers are going to limit users to C99 for years to come?


> On Aug 1, 2019, at 3:23 AM, N.M. Maclaren via mpi-forum <mpi-forum at lists.mpi-forum.org> wrote:
>> 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,
> either.
> 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
> reasons.
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