[MPlayer-dev-eng] Re: kernel: to do_select(pipe) or not to do

[D Richard Felker III]

On Mon, Nov 22, 2004 at 06:26:16PM +0100, Gábor Lénárt wrote:
> On Mon, Nov 22, 2004 at 10:35:52AM -0500, D Richard Felker III wrote:
> > > As I already mentioned, "select()" usage ONLY MAKES SENSE FOR NONBLOCKING 
> > > FILE DESCRIPTORS.
> > > 
> > > Of _course_ a write() will block if the fd is blocking. But if you have a 
> > > nonblocking fd it will NOT block.
> > 
> > a write should never block if select indicated that the fd was
> > writeable. it should write however many bytes can be written
> 
> Maybe I'm wrong at this point but what about if you got "fd is
> writeable" from select but before reaching write() another
> thread got control and writes something?

this is not a problem unless you have more than one process/thread
writing the same fd at the same time, in which case you're either a
total idiot or you've already carefully thought out what should
happen. more likely the former if you're using threads to begin
with... :)

> I mean: use select() _THEN_
> write() is not an atomic operation of course so race condition
> can be experienced here. I'm assuming this because of Linux
> specific clone() system call (CLONE_FILES flag) which is/can be
> used to create threads.
> 
> But this whole blocking/non-blocking issue from the point of
> view of the kernel is quite unknown for me, so maybe I'm wrong.

you're just asking about something entirely unrelated to the topic at
hand.

rich