Re: [PATCH] windows: fix pread/pwrite

[Jens Axboe <axboe@kernel.dk>]

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