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Date: Tue, 19 Dec 2017 10:14:31 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Wrong file position after writing 65537 bytes to block device
Message-ID: <20171219091431.GF11285@calimero.vinschen.de>
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On Dec 18 16:27, Steven Penny wrote:
> On Mon, 18 Dec 2017 14:10:35, Corinna Vinschen wrote:
> > In general, the writes on disk devices is sector-oriented.  Howewver,
> > in this case ftell should have returned 65536.  The problem here is
> > that the newlib implmentation of ftell/ftello performs an fflush
> > when called on a write stream since about 2008 to adjust for appending
> > streams.  Given your example (thanks for the testcase!) this seems
> > pretty wrong.  Looking further it turns out that neither glibc nor BSD
> > actually calls fflush in this case.  There's only a special case for
> > appending streams, but this calls lseek, not fflush.
> >=20
> > Looks like a patch is required.  Stay tuned.
>=20
> is it though? he says "write 65536 + 1 bytes", but as far as i can tell, =
you
> cant do that. quoting myself:
>=20
> > Seeking, reading and writing must all be done in multiples of sector si=
ze, in
> > my case 512 bytes
>=20
> http://web.archive.org/web/stackoverflow.com/questions/37228874/how-to-fw=
rite-to-removable-volume
>=20
> so it would make sense that the position becomes "65536 + 512"

Neither glibc nor FreeBSD show this behaviour.  Keep in mind that stdio
is designed for buffered I/O.  What should happen, basically, is that a
multiple of the stdio buffersize is written and the remainder is kept in
the stdio buffer:

  fwrite(65537)
  -> write(65536)
  -> store 1 byte in FILE._buf

ftell calls lseek which returns 65536.  It adds the number of bytes
still in the buffer, so it should return 65537.  Further fwrite's
seemlessly append to the bytes already written, as expected.  ftell
calling fflush and thus setting the current file position to the next
sector boundary breaks this expectation.

I pushed a patch yesterday and uploaded new developer snapshots to
https://cygwin.com/snapshots/

Please test.


Thanks,
Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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