Re: [PATCH] windows: fix pread/pwrite

[Jens Axboe <axboe@kernel.dk>]

On 5/8/25 11:31 AM, Vincent Fu wrote:
> On Thu, May 8, 2025 at 1:18?PM Jens Axboe <axboe@kernel.dk> 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 <vincent.fu@samsung.com>
>>> ---
>>>  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?

It's probably not a big deal. I did a quick search and doesn't seem like
one exists for that.

-- 
Jens Axboe