linux-kernel - Re: Write is not atomic?

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Date:	Tue, 16 Oct 2012 02:21:36 +0400
From:	Max Filippov <jcmvbkbc@...il.com>
To:	Juliusz Chroboczek <jch@....jussieu.fr>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Write is not atomic?

On Tue, Oct 16, 2012 at 1:36 AM, Juliusz Chroboczek <jch@....jussieu.fr> wrote:
> Hi,
>
> The Linux manual page for write(2) says:
>
>     The adjustment of the file offset and the write operation are
>     performed as an atomic step.
>
> This is apparently an extension to POSIX, which says
>
>     This volume of IEEE Std 1003.1-2001 does not specify behavior of
>     concurrent writes to a file from multiple processes. Applications
>     should use some form of concurrency control.
>
> The following fragment of code
>
>     int fd;
>     fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC, 0666);
>     fork();
>     write(fd, "Ouille", 6);

You don't check return code here, does write succeed at all?

>     close(fd);
>
> produces "OuilleOuille", as expected, on ext4 on two machines running
> Linux 3.2 AMD64.  However, over XFS on an old Pentium III at 500 MHz
> running 2.6.32, it produces just "Ouille" roughly once in three times.

Does it ever produce e.g. OuOuilleille (as this is what atomicity is about
here)?

-- 
Thanks.
-- Max
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