Re: [PATCH v8 09/11] block: check availablity for preadv/pwritev on mac

[Joelle van Dyne]

Sorry to rephrase this, we would like a single binary to work on both
macOS 10.15 and macOS 11 as well as on both iOS 13 and iOS 14 while
ALSO supporting preadv/pwritev when running on the newer OS.

-j

On Sat, Jan 23, 2021 at 12:00 PM Joelle van Dyne <j@getutm.app> wrote:
>
> The problem here is that we would like to compile on macOS 11 and be
> able to run the built binaries on macOS 10.15 or lower. Without the
> runtime checks, you'll have a crash when calling preadv/pwritev.
>
> -j
>
> On Sat, Jan 23, 2021 at 5:53 AM Peter Maydell <peter.maydell@linaro.org> 
> wrote:
> >
> > On Sat, 23 Jan 2021 at 05:03, Joelle van Dyne <j@getutm.app> wrote:
> > >
> > > macOS 11/iOS 14 added preadv/pwritev APIs. Due to weak linking, configure
> > > will succeed with CONFIG_PREADV even when targeting a lower OS version.
> > > We therefore need to check at run time if we can actually use these APIs.
> >
> > If you make the preadv check be a meson.build has_function()
> > test, I think (but have not tested) that it ought to correctly
> > fail the link in the targeting-lower-OS-version case, because
> > meson.build's has_function() implementation for clang
> > https://github.com/mesonbuild/meson/blob/bad0e95caec4cf49165572df3cf2edc4832280fa/mesonbuild/compilers/mixins/clang.py#L89
> > adds -Wl,-no-weak-imports when it's building the test-case
> > exactly to force that linker visibility obeys the minimum
> > version targets for OSX.
> >
> > If that works I think it would be neater than delaying the
> > check to runtime (and it would also mean another test moved
> > out of configure and into meson.build, which is the direction
> > we're heading anyway).
> >
> > thanks
> > -- PMM