linux-kernel - Re: Write is not atomic?

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Date:	Mon, 15 Oct 2012 16:24:38 -0700
From:	Philippe Troin <phil@...i.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Juliusz Chroboczek <jch@....jussieu.fr>,
	linux-kernel@...r.kernel.org
Subject: Re: Write is not atomic?

On Tue, 2012-10-16 at 10:13 +1100, Dave Chinner wrote:
> On Mon, Oct 15, 2012 at 11:36:15PM +0200, Juliusz Chroboczek 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.
> 
> That's wrong. The file offset update is not synchronised at all with
> the write, and for a shared fd the update will race.

That's what O_APPEND or pread/pwrite are for.

> > 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.
> 
> This is how Linux behaves.
> 
> > The following fragment of code
> > 
> >     int fd;
> >     fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC, 0666);
> >     fork();
> >     write(fd, "Ouille", 6);
> >     close(fd);

can be replaced with:

        int fd;
        fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC | O_APPEND, 0666);
        fork();
        write(fd, "Ouille", 6);
        close(fd);

or:

        int fd;
    fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC, 0666);
    pid_t pid = fork();
    pwrite(fd, "Ouille", 6, strlen("Ouille")*(pid == 0));
    close(fd);

(both code fragments untested)

Phil.

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