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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