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From: Julian Elischer <julian@ref.tfs.com>
Message-Id: <199510170121.SAA16783@ref.tfs.com>
Subject: Re: suggested changes to mbuf routines
To: terry@lambert.org (Terry Lambert)
Date: Mon, 16 Oct 1995 18:21:08 -0700 (PDT)
Cc: matt@lkg.dec.com, julian@freefall.freebsd.org,
        hackers@freefall.freebsd.orgrgrimes@freebsd.org
In-Reply-To: <199510162001.NAA25213@phaeton.artisoft.com> from "Terry Lambert" at Oct 16, 95 01:01:07 pm
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> In  <199510141119.EAA05158@freefall.freebsd.org> , you wrote:
> 
> I find the method of doing the references as done in Digital UNIX
> (aka DEC OSF/1) quite clean.  Basically the m_ext struct gets an
> queue entry added.  When the extended mbuf is first allocated, the
> link queue entry merely points to itself (an empty queue).  As more
> references are made, their queues get linked together.  As references
> are removed, their queues get unlinked.
> 
> To see if there is a non-zero reference count, imply see if the queue
> entry points to itself.

ah I was wondering what that was...
I was looking at that in OSF1/i386 and scratching my head about it..
I guess that means that whatever is externally allocated need not have it's own
reference counts..
(unless they are also used elsewhere than for mbufs)
not sure which I prefer..

rod, is this given in the device driver's book?


> 
> 
> /* description of external storage mapped into mbuf, valid if M_EXT set */
> struct m_ext {
>         caddr_t ext_buf;                /* start of buffer */
>         void    (*ext_free)(caddr_t, u_long, caddr_t);
>         u_int   ext_size;               /* size of buffer, for ext_free */
>         caddr_t ext_arg;                /* additional ext_free argument */
>         struct  ext_refq {              /* reference list */
>                 struct ext_refq *forw, *back;
>         } ext_ref;
> };
> 
> #define MCLREFERENCED(m) \
>         ((m)->m_ext.ext_ref.forw != &((m)->m_ext.ext_ref))