Re: [PATCH v5 7/7] block: check availablity for preadv/pwritev on mac

[Joelle van Dyne <j@getutm.app>]

How about this:

static bool preadv_present = true;
static bool preadv_checked = false;

static ssize_t
qemu_preadv(int fd, const struct iovec *iov, int nr_iov, off_t offset)
{
#ifdef CONFIG_DARWIN
    if (!preadv_checked) {
        if (__builtin_available(macOS 11, iOS 14, watchOS 7, tvOS 14, *)) {
            preadv_checked = true;
        } else {
            preadv_present = false;
            return -ENOSYS;
        }
    }
#endif
    return preadv(fd, iov, nr_iov, offset);
}

-j

On Fri, Nov 20, 2020 at 10:21 AM Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> On 11/20/20 4:49 PM, Joelle van Dyne wrote:
> > No, because if you build on a macOS 11 host but try to run it on macOS
> > 10.15 then it will crash.
>
> I'm not asking to move the check to configure/build time,
> but to do it only once at runtime...
>
> >
> > -j
> >
> > On Fri, Nov 20, 2020 at 4:32 AM Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
> >>
> >> On 11/9/20 12:24 AM, Joelle van Dyne 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.
> >>>
> >>> Signed-off-by: Joelle van Dyne <j@getutm.app>
> >>> ---
> >>>  block/file-posix.c | 12 ++++++++++++
> >>>  1 file changed, 12 insertions(+)
> >>>
> >>> diff --git a/block/file-posix.c b/block/file-posix.c
> >>> index d83219df55..a9d69746a0 100644
> >>> --- a/block/file-posix.c
> >>> +++ b/block/file-posix.c
> >>> @@ -1394,12 +1394,24 @@ static bool preadv_present = true;
> >>>  static ssize_t
> >>>  qemu_preadv(int fd, const struct iovec *iov, int nr_iov, off_t offset)
> >>>  {
> >>> +#ifdef CONFIG_DARWIN /* preadv introduced in macOS 11 */
> >>> +    if (!__builtin_available(macOS 11, iOS 14, watchOS 7, tvOS 14, *)) {
> >>
> >> Can we change the CONFIG_PREADV ifdef'ry to run this check once
> >> on macOS 11?
> >>
> >>> +        preadv_present = false;
> >>> +        return -ENOSYS;
> >>> +    } else
> >>> +#endif
> >>>      return preadv(fd, iov, nr_iov, offset);
> >>>  }
> >>>
> >>>  static ssize_t
> >>>  qemu_pwritev(int fd, const struct iovec *iov, int nr_iov, off_t offset)
> >>>  {
> >>> +#ifdef CONFIG_DARWIN /* pwritev introduced in macOS 11 */
> >>> +    if (!__builtin_available(macOS 11, iOS 14, watchOS 7, tvOS 14, *)) {
> >>> +        preadv_present = false;
> >>> +        return -ENOSYS;
> >>> +    } else
> >>> +#endif
> >>>      return pwritev(fd, iov, nr_iov, offset);
> >>>  }
> >>>
> >>>
> >>
> >