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

[Daniel Eischen]

On Mon, 22 Nov 2004, D Richard Felker III wrote:

> On Sun, Nov 21, 2004 at 08:21:58PM -0800, Linus Torvalds wrote:
>
> hi linus.
>
> > On Mon, 22 Nov 2004, Ivan Kalvachev wrote:
> > >
> > > You are very wrong at this point. A full write WILL block if it writes
> > > an PIPE_SIZE+1 bytes. And it have nothing to do with select()!
> >
> > Don't bother arguing when you don't know what you're talking about.
>
> i think some of us do know what we're talking about... if you think
> otherwise please quote the relevant parts of posix.
>
> > 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
> immediately and return that number. to my knowledge, many programs
> assume this behavior (which i expect is correct, but i don't have
> posix in front of me...) and will have strange problems if the system
> behaves otherwise.

I hope you are only referring to non-blocking I/O above.  For
blocking I/O, the write request must be fully satisfied before
returning, otherwise the caller blocks.  When you call select(),
the kernel has no idea how much you want to write.  If there's
room for a byte to be written, but no more, select() can
return with the fd ready for writing.

Please see:

  http://www.opengroup.org/onlinepubs/009695399/functions/write.html