Re: [PATCH] windows: fix pread/pwrite

On 5/8/25 5:28 PM, Sitsofe Wheeler wrote:
> 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

Sitsofe, many thanks for taking a look. I think my patch is good enough 
for now but I'm pleased to know that someone who really caress about 
this may be able to eventually improve on it.

Vincent