Re: F_DUPFD_CLOEXEC implementation

[Denys Vlasenko <vda.linux@googlemail.com>]

On Monday 01 October 2007 00:11, Davide Libenzi wrote:
> On Sun, 30 Sep 2007, Denys Vlasenko wrote:
> 
> > Hi Ulrich,
> > 
> > On Friday 28 September 2007 18:34, Ulrich Drepper wrote:
> > > One more small change to extend the availability of creation of
> > > file descriptors with FD_CLOEXEC set.  Adding a new command to
> > > fcntl() requires no new system call and the overall impact on
> > > code size if minimal.
> > 
> > Tangential question: do you have any idea how userspace can
> > safely do nonblocking read or write on a potentially-shared fd?
> > 
> > IIUC, currently it cannot be done without races:
> > 
> > old_flags = fcntl(fd, F_GETFL);
> > ...other process may change flags!...
> > fcntl(fd, F_SETFL, old_flags | O_NONBLOCK);
> > read(fd, ...)
> > ...other process may see flags changed under its feet!...
> > fcntl(fd, F_SETFL, old_flags);
> > 
> > Can this be fixed?
> 
> I'm not sure I understood correctly your use case. But, if you have two 
> processes/threads randomly switching O_NONBLOCK on/off, your problems 
> arise not only the F_SETFL time.

My use case is: I want to do a nonblocking read on descriptor 0 (stdin).
It may be a pipe or a socket.

There may be other processes which share this descriptor with me,
I simply cannot know that. And they, too, may want to do reads on it.

I want to do nonblocking read in such a way that neither those other
processes will ever see fd switching to O_NONBLOCK and back, and
I also want to be safe from other processes doing the same.

I don't see how this can be done using standard unix primitives.
--
vda