Path: utzoo!attcan!uunet!mcsun!sunic!dkuug!freja!skinfaxe!thorinn
From: thorinn@skinfaxe.diku.dk (Lars Henrik Mathiesen)
Newsgroups: comp.unix.wizards
Subject: Re: why limit of 16 iovec's in readv/writev?
Keywords: iovec readv writev
Message-ID: <4996@freja.diku.dk>
Date: 23 Nov 89 19:01:58 GMT
References: <473@siswat.UUCP>
Sender: news@freja.diku.dk
Lines: 19

buck@siswat.UUCP (A. Lester Buck) writes:
>I ran across the limit on 16 iovec's in each readv(2) or writev(2) call.

The limit is there because the iovecs are copied into an array on the
kernel stack; the further processing uses a pointer to this array and
a count (contained in a structure which also has total and residual
byte counts and some flags).

>This limits the maximum transfer per system call to about 24KB.

In my opinion you should not use the readv/writev mechanism to provide
packet delimiters, even for debugging. Unless you are on a machine
where memory is tight, I would suggest that your driver treat the user
buffer as an array of (e.g.) 2K subbuffers, one for each packet. The
last word in each subbuffer could then be a bytecount.

--
Lars Mathiesen, DIKU, U of Copenhagen, Denmark      [uunet!]mcvax!diku!thorinn
Institute of Datalogy -- we're scientists, not engineers.      thorinn@diku.dk