Re: Write is not atomic?

From: Philippe Troin
Date: Mon Oct 15 2012 - 19:34:37 EST


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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/