Re: [PATCH v2 1/1] Fix detection of pwritev support for OSX.

James Hilliard  writes:
> Fixes:
> fd.c:3661:10: warning: 'pwritev' is only available on macOS 11.0 or newer [-Wunguarded-availability-new]

It's still missing preadv, and it still has nonzero chance of breaking
successful detection of pwritev on platforms other than yours, and it's
still really ugly.

But the main reason I don't want to go this way is that I don't think
it'll stop with preadv/pwritev.  If we make it our job to build
successfully even when using the wrong SDK version for the target
platform, we're going to be in for more and more pain with other
kernel APIs.

We could, of course, do what Apple wants us to do and try to build
executables that work across versions.  I do not intend to put up
with the sort of invasive, error-prone source-code-level runtime test
they recommend ... but given that there is weak linking involved here,
I wonder if there is a way to silently sub in src/port/pwritev.c
when executing on a pre-11 macOS, by dint of marking it a weak
symbol?

			regards, tom lane