Re: [PATCH v2 9/9] block: check availablity for preadv/pwritev on mac

[Joelle van Dyne]

On Mon, Oct 19, 2020 at 1:27 AM Thomas Huth <thuth@redhat.com> wrote:
>
> On 19/10/2020 03.39, Joelle van Dyne wrote:
> > From: osy <osy86@users.noreply.github.com>
>
> That "From:" line looks wrong ... could you please fix the "Author" of your
> patches / your git config?
osy wrote the original changes. I joined the UTM project to help bring
the changes upstream with permission. However, they have agreed that
if required that we can use my name as the author.

>
> > 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.
>
> That sounds like the wrong approach to me ... could you please try to fix
> the check in "configure" instead? E.g. by running compile_prog with
> "-Werror", so that the test fails if there is no valid prototype available?
It's not that simple. Xcode 11 and below (supporting macOS 10.15 and
below, iOS 13 and below, etc) does not have preadv/pwritev symbols
defined and would fail to compile. Xcode 12 (supporting macOS 11 and
below, iOS 14 and below, etc) have preadv/pwritev weakly defined so if
it runs on, for example, 10.15, it would abort. There is no way to
determine at compile time if you can use preadv/pwritev or not when
building with Xcode 12. The availability checks are Apple's preferred
way to handle this kind of situation (they discourage directly
checking if an API exists on a system).

-j

>
>  Thomas
>