Date: Tue, 13 Aug 2002 16:34:59 -0700 (PDT) From: Hiten Pandya <hitmaster2k@yahoo.com> To: Terry Lambert <tlambert2@mindspring.com> Cc: freebsd-hackers@FreeBSD.org Subject: Re: SysV IPC related question Message-ID: <20020813233459.25838.qmail@web21102.mail.yahoo.com> In-Reply-To: <3D5994D7.E047294C@mindspring.com>
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--- Terry Lambert <tlambert2@mindspring.com> wrote:
> Hiten Pandya wrote:
> > I was wondering why we have a struct mymsg in <sys/msg.h>, when many
> > utilities defined their own version of it. I am curious about this
> > because our stock version of struct mymsg:
> >
> > struct mymsg {
> > long mtype; /* message type */
> > char mtext[1]; /* message body */
> > };
> >
> > Why do we have a value of [1] in the mtext array? Are we meant to
> > define a struct mymsg at all!?
>
> This is the message contents. It is an overlay structure. The
> [1] is the same thing that, in the current ANSI C standard, you
> would define in terms of [0].
>
> The point is that you have a structure that sonsists of a long
> followed by an indeterminate number of bytes. You cast the
> combination to a pointer to a structure of this type, and you
> can reference the long as mymsgp->mtype, and the contents as
> mymsgp->mtype.
>
> Please leave it alone. 8-).
OK. One more question; so, is there any particular reason why our cousin
NetBSD doesnt use this "overlay" structure? Also, the NetBSD SysV Msg
regression tool defines its own struct mymsg; and doesnt have one standard
in the <sys/msg.h> header file.
Also, if possible, could you outline some situations where this would be
used? Help will be very appreicated.
Thanks.
-- Hiten
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