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Subject: Re: Wrong file position after writing 65537 bytes to block device
To: cygwin AT cygwin DOT com
References: <20171218131035 DOT GB11285 AT calimero DOT vinschen DOT de> <5a385ced DOT 195b9d0a DOT d434 DOT 5400 AT mx DOT google DOT com> <20171219091431 DOT GF11285 AT calimero DOT vinschen DOT de> <CAKyntiLyTBGcAx3iwhq7dX0wqiE3S685kwmhMg+U2A3huXVzUQ AT mail DOT gmail DOT com>
From: Eric Blake <eblake AT redhat DOT com>
Message-ID: <9f29d418-10af-1dc7-2636-89cae1eb16f4@redhat.com>
Date: Tue, 19 Dec 2017 10:13:17 -0600
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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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