Re: [PATCH] Initialize IO_STATUS_BLOCK for pread, pwrite

[Mark Geisert <mark at maxrnd dot com>]

Just closing out this email thread...

On Fri, 1 Dec 2017, Mark Geisert wrote:
> On Fri, 1 Dec 2017, Corinna Vinschen wrote:
> > On Dec  1 10:30, Corinna Vinschen wrote:
> > > On Dec  1 00:44, Mark Geisert wrote:
> > > > And then pwrite() creates its own handle (or reuses one (!)) to avoid
> > > > messing up the seek pointer of the fd passed in.
> > >
> > > Wait.  Not "re-use", but "re-open".  If you're more familiar with POSIX
> > > terms, this is along the lines of the fdopen(3) call, just on the NT
> > > API level.  There's an equivalent Win32 function since Windows 2003
> > > called ReOpenFile.
> > >
> > > In terms of pread/pwrite, the new handle shares the same settings with
> > > the original handle.  However, if you use cygwin_attach_handle_to_fd,
> > > there's a chance information got lost.  Nobody actually uses this call ;)
> > >
> > > In terms of FILE_SYNCHRONOUS_IO_NONALERT, this is stored in
> > > fhandler_base::options, utilizing the get_options/set_options methods.
> > > I have a hunch that cygwin_attach_handle_to_fd fails to call set_options,
> > > thus options is 0 when you call pwrite, thus the new handle is opened
> > > without FILE_SYNCHRONOUS_IO_NONALERT and all the other option flags
> > > we use by default.
> >
> > It's more than a hunch.  Of course the info gets lost since
> > none of the functions called by cygwin_attach_handle_to_fd calls
> > NtQueryInformationFile(FileModeInformation) to fetch the options.
>
> Bang.  There it is.  Let me fix the offending program to just use 
> Cygwin-supplied handles and make sure this bug of mine is squashed.  I'll 
> report back but it might be a few days.

Yes, this bug of mine that I was blaming pwrite() internals for, 
doesn't occur when using Cygwin-supplied file descriptors with Cygwin i/o 
calls.  Who new?

Sorry for the noise, but at least what I learned from this discussion is 
very helpful in figuring out what makes sense for a future async i/o 
facility in Cygwin.

I have some further comments but I'll take them to cygwin-developers.
This IO_STATUS_BLOCK patch is withdrawn ;-) .

..mark