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Date: Fri, 21 Jul 1995 09:08:47 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199507202308.JAA10420@godzilla.zeta.org.au>
To: bde@zeta.org.au, terry@cs.weber.edu
Subject: Re: possible ffs_vget() race condition
Cc: bugs@freebsd.org
Sender: bugs-owner@freebsd.org
Precedence: bulk

[Cc list trimmed]

>> The shift method has the advantage of giving the correct result :-).
>> You meant ((~(u_quad_t)0) >> 1).  This depends on u_quad_t being
>> larger than int.

>I don't understand why this would depend on quad being larger than int;
>because the naked 1 is of type int?

I should have said larger or the same size as an int.  ~(foo_t)0
is usually wrong if foo_t is shorter than an int (it is the same as
~0 because of the broken ANSI value-preserving promotion rules),
so using it gives a bad example for beginners to follow.  ((foo_t)(-1))
always works although you may need further casts (e.g., on the i386,
(u_short)(-1) is (int)0xffff, so shifting right 1 works right but
shifting left 16 and then shifting right gives surprising results.

The type of the shift count doesn't affect the type of the result.

Bruce