Date: Tue, 9 Dec 1997 05:57:11 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: bakul@torrentnet.com (Bakul Shah) Cc: hackers@freebsd.org Subject: Re: Why FIONREAD has no dual for write ? Message-ID: <199712090457.FAA00305@labinfo.iet.unipi.it> In-Reply-To: <199712090045.TAA06826@chai.torrentnet.com> from "Bakul Shah" at Dec 8, 97 07:44:46 pm
next in thread | previous in thread | raw e-mail | index | archive | help
> > I agree with the above, except that it does not work in all situations.
> > E.g. I am not sure but does select() guarantee that you don't
> > end up writing/reading one byte at a time ? I know in practice things
> > are different, but there is no standard behaviour I think, so the risk
> > is still there. In the audio driver I had to modify the behaviour of
> > select() (block size) using a separate ioctl().
>
> Perhaps one solution is to add an ioctl to set high/low
> watermarks on a device and add new event bits POLLIN_WM and
> POLLLOUT_WM for the poll() syscall.
Good idea, this could solve the blocksize problem rather elegantly
(although with some portability problems...).
> > Plus there are apps which want only to check the status of the queue in
> > a descriptor without actually doing the I/O, for whatever reason they
> > like.
>
> This can be handled by POLL{IN,OUT}_WM.
not sure how... if i get it right your changes can tell a process when
you can transfer x >= WM bytes, but gives no clue about the current status
of the queue. Much like the difference between
Q. Excuse me Sir, do you know what time is it ?
A. Yes.
and
Q. Excuse me Sir, do you know what time is it ?
A. 9:30
Cheers
Luigi
-----------------------------+--------------------------------------
Luigi Rizzo | Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it | Universita' di Pisa
tel: +39-50-568533 | via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522 | http://www.iet.unipi.it/~luigi/
_____________________________|______________________________________
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199712090457.FAA00305>