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From: Eric Blake <eblake@redhat.com>
To: cygwin@cygwin.com
Subject: Re: Wrong file position after writing 65537 bytes to block device
Date: Tue, 19 Dec 2017 17:43:00 -0000	[thread overview]
Message-ID: <9f29d418-10af-1dc7-2636-89cae1eb16f4@redhat.com> (raw)
In-Reply-To: <CAKyntiLyTBGcAx3iwhq7dX0wqiE3S685kwmhMg+U2A3huXVzUQ@mail.gmail.com>

On 12/19/2017 09:46 AM, Ivan Kozik wrote:
> Thanks, I can confirm that the 2017-12-18 snapshot fixed the test
> program I posted.
> 
> What about the harder case where the program calls fflush, though?
> 
> #include <stdio.h>
> 
> int main(int argc, char *argv[]) {
>      FILE *f = fopen(argv[1], "w");
>      char x[65536 + 1];
>      fwrite(x, 1, 65536 + 1, f);
>      fflush(f);
>      printf("%ld", ftell(f));

Can block devices report an unaligned offset to lseek()?  If not, then 
when writing an unaligned value to a block device, don't we have to do a 
read-modify-write of the larger aligned cluster, and then put lseek() 
back to the unaligned boundary, and have extra magic in ftell() to track 
that we are at an unaligned position within the block device?  But that 
sounds like a lot of nasty overhead; and that it would be better to make 
sure that block devices can report unaligned lseek() locations (caveat: 
I haven't tested what Linux does in that regards).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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  reply	other threads:[~2017-12-19 16:13 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-12-16  6:13 Ivan Kozik
2017-12-18 16:32 ` Corinna Vinschen
2017-12-19  5:03   ` Steven Penny
2017-12-19 10:20     ` Corinna Vinschen
2017-12-19 16:36       ` Ivan Kozik
2017-12-19 17:43         ` Eric Blake [this message]
2017-12-19 18:18           ` Ivan Kozik
2017-12-19 18:36             ` Corinna Vinschen
2017-12-20  5:47               ` Ivan Kozik
2017-12-20 23:59     ` Kaz Kylheku

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