Re: [PATCH] windows: fix pread/pwrite

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.
> >
> > Signed-off-by: Vincent Fu <[email protected]>
> > ---
> >  os/windows/posix.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/os/windows/posix.c b/os/windows/posix.c
> > index e3abf383..3e48c3ff 100644
> > --- a/os/windows/posix.c
> > +++ b/os/windows/posix.c
> > @@ -830,18 +830,24 @@ ssize_t pwrite(int fildes, const void *buf, size_t nbyte,
> >               off_t offset)
> >  {
> >       int64_t pos = _telli64(fildes);
> > -     ssize_t len = _write(fildes, buf, nbyte);
> > +     ssize_t len;
> >
> > +     _lseeki64(fildes, offset, SEEK_SET);
> > +     len = _write(fildes, buf, nbyte);
> >       _lseeki64(fildes, pos, SEEK_SET);
> > +
> >       return len;
> >  }
> >
> >  ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset)
> >  {
> >       int64_t pos = _telli64(fildes);
> > -     ssize_t len = read(fildes, buf, nbyte);
> > +     ssize_t len;
> >
> > +     _lseeki64(fildes, offset, SEEK_SET);
> > +     len = read(fildes, buf, nbyte);
> >       _lseeki64(fildes, pos, SEEK_SET);
> > +
> >       return len;
> >  }
>
> lol, how broken is that... Does windows have any offset read/write
> variants, or is lseek required?
>
> --
> Jens Axboe
>

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?

Vincent