Re: [PATCH v3 1/1] Fix detection of preadv/pwritev support for OSX.

James Hilliard <james(dot)hilliard1(at)gmail(dot)com> writes:
> On Tue, Jul 6, 2021 at 2:34 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> As far as I can tell, the only way to really deal with #2 is to
>> perform a runtime dlsym() probe to see whether pwritev exists, and
>> then fall back to our src/port/ implementation if not. This does
>> not look particularly hard (especially since the lookup code only
>> needs to work on macOS), though I admit I've not tried to code it.

> Maybe we just need to do something like this?:
> https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg06499.html

Hm. I don't trust __builtin_available too much --- does it exist
on really old macOS? dlsym() seems less likely to create problems
in that respect. Of course, if there are scenarios where
__builtin_available knows that the symbol is available but doesn't
work, then we might have to use it.

In any case, I don't like their superstructure around the probe.
What I'd prefer is a function pointer that initially points to
the probe code, and then we change it to point at either pwritev
or the pg_pwritev emulation. Certainly failing with ENOSYS is
exactly what *not* to do.

regards, tom lane