From: David Gay <dgay@EECS.Berkeley.EDU>
Message-Id: <200010092256.PAA16357@barnowl.CS.berkeley.EDU>
Subject: Re: ick, ick -- grossness in linux kernel
Date: Mon, 09 Oct 2000 15:56:54 -0700
> do { __asm__ __volatile__("pushfl ; popl %0 ; cli":"=g" (
> flags ): :"memory") ; spin_lock( &tqueue_lock ); } while (0) ;
>
> Currently my lock checker complains about this code, because it thinks
> that the spin_lock may get executed multiple times (which would be a Bad
> Thing).
>
> Anyone know why this code gets wrapped in a useless do loop? I assume
> it's to fix some bug in some version of gcc.
No, it's a standard C hack to get a macro to baheve more like a real
function: if it was just
#define FOO() { stuff; }
then
if (x) FOO();
else baz();
would not behave as expected (you would get a syntax error).
With
#define FOO() do { stuff; } while (0)
Note that the other standard form for this is something like
#define FOO() if (1) { stuff; } else
I think you might want to constant fold if and while conditions to avoid
surprises like this.
David
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