On Tue, Jan 19, 2021 at 10:29 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> James Hilliard <james.hilliard1@gmail.com> 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.
Setting -Werror=unguarded-availability-new should in theory always
ensure that configure checks fail if the symbol is unavailable or marked
as requiring a target newer than the MACOSX_DEPLOYMENT_TARGET.
>
> 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.
This issue really has nothing to do with the SDK version at all, it's the
MACOSX_DEPLOYMENT_TARGET that matters which must be taken
into account during configure in some way, this is what my patch does
by triggering the pwritev compile test error by setting
-Werror=unguarded-availability-new.
It's expected that MACOSX_DEPLOYMENT_TARGET=10.15 with a
MacOSX11.1.sdk will produce a binary that can run on OSX 10.15.
The MacOSX11.1.sdk is not the wrong SDK for a 10.15 target and
is fully capable of producing 10.15 compatible binaries.
>
> 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?
The check I added is strictly a compile time check still, not runtime.
I also don't think this is a weak symbol.
From the header file it is not have __attribute__((weak_import)):
ssize_t pwritev(int, const struct iovec *, int, off_t)
__DARWIN_NOCANCEL(pwritev) __API_AVAILABLE(macos(11.0), ios(14.0),
watchos(7.0), tvos(14.0));
>
> regards, tom lane
