linux-ext4 - Re: Regular ext4 error warning with HD in USB dock

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Message-Id: <41DF7E1A-4CF0-44CF-AD8F-29176397B7EF@dilger.ca>
Date:	Mon, 10 Jan 2011 00:45:17 -0700
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Ted Ts'o <tytso@....edu>
Cc:	Rogier Wolff <R.E.Wolff@...wizard.nl>,
	Amir Goldstein <amir73il@...il.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Con Kolivas <kernel@...ivas.org>, linux-ext4@...r.kernel.org
Subject: Re: Regular ext4 error warning with HD in USB dock

On 2011-01-09, at 07:58, Ted Ts'o wrote:
> Defensive programming would be something like
> 
> 	  BUG_ON(sizeof(struct ext4_super_block) != 1024);
> 
> (unfortunately #error sizeof(struct ext4_super_block) != 1024 won't
> work since #error is handled by the preprocessor, and I don't think we
> can trigger a compile-time warning for a structure size issue).

We actually use a compile-time assertion in the Lustre code:

/*
* compile-time assertions. @cond has to be constant expression.
* ISO C Standard:
*
*        6.8.4.2  The switch statement
*
*       ....
*
*       [#3] The expression of each case label shall be an integer
*       constant expression and no two of the case constant expressions
*       in the same switch statement shall have the same value after
*       after conversion...
*/
#define CLASSERT(cond) ({ switch(42) { case (cond): case 0: break; } })

Of course, the "cond" expression must be resolvable at compile time, or
it will turn into a compile error because it is not legal to have a
variable "case".

Cheers, Andreas

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