Re: [PATCH] windows: fix pread/pwrite

Hi Vincent,

On Thu, 8 May 2025 at 18:33, Vincent Fu <[email protected]> wrote:
>
> On Thu, May 8, 2025 at 1:18 PM Jens Axboe <[email protected]> wrote:
> >
> > On 5/8/25 11:14 AM, Vincent Fu wrote:
> > > The pread and pwrite functions for Windows posix emulation never actually seek
> > > to the requested offset. Fix this so that the psync ioengine works correctly on
> > > Windows.

Oof! Good catch!

[...]

> > lol, how broken is that... Does windows have any offset read/write
> > variants, or is lseek required?
>
> I could not find a better way to do this but I am hoping someone who
> knows more about Windows than I do can weigh in. Sitsofe?

Sadly (?) I'm not a Windows expert (even if I've occasionally played one) but
according to an answer on
https://stackoverflow.com/questions/3852758/equivalent-to-pread-pwrite-in-msvc
the only thing that can take an offset with the request is ReadFile/WriteFile
and this is done by passing an OVERLAPPED structure
(https://learn.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-overlapped
) which in turn can specify an offset even for synchronous I/O (see
https://learn.microsoft.com/en-gb/windows/win32/api/fileapi/nf-fileapi-writefile?redirectedfrom=MSDN#synchronization-and-file-position
). Unfortunately ReadFile/WriteFile only work with Windows handles but I think
you can get one from a file descriptor via _get_osfhandle (see
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170).

--
Sitsofe