From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <ee9e417a0512081804ua34babr42d2735d1c81bf0f@mail.gmail.com>
Date: Thu,  8 Dec 2005 21:04:51 -0500
From: Russ Cox <rsc@swtch.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] const
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	<cd82d31866c1bdb93bf534489df22cf2@collyer.net>
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> At a quick read, it appears that `const' and `volatile' are treated as
> `garbage type qualifiers' and ignored.

Const is ignored.  Volatile is not ignored anymore.
/sys/src/cmd/cc/dcl.c:768 or so inserts USED(&x)
if you declare volatile int x;, presumably so that
the compiler will not registerize x.  This is occasionally
necessary in the kernel because of waserror().
For example:

    x =3D 0;
    if(waserror()){
        print("x=3D%d\n", x);
        nexterror();
    }
    x =3D 1;
    x =3D f(x);

If f errors, then the waserror might print 0 if the compiler
has chosen not to store the 1 (it's not used after the call
to f -- it will be overwritten once f returns).

This happens in namec in /sys/src/9/port/chan.c.  Rob defined
an empty function saveregisters() and called it to force the
compiler to write the new value of x to memory before calling f
(in this case, devtab[c->type]->open).  It could be replaced
by making c volatile now, though I'd rather not.

Russ