linux-kernel - Re: [PATCH] kernel: rounddown helper function

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Date:	Tue, 3 Aug 2010 11:23:54 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov,
	akpm@...ux-foundation.org, sds@...ho.nsa.gov, jmorris@...ei.org
Subject: Re: [PATCH] kernel: rounddown helper function

On Tue, 03 Aug 2010 14:16:07 -0400 Eric Paris wrote:

> The roundup() helper function will round a given value up to a multiple of
> another given value.  aka  roundup(11, 7) would give 14 = 7 * 2.  This new
> function does the opposite.  It will round a given number down to the
> nearest multiple of the second number: rounddown(11, 7) would give 7.
> 
> I need this in some future SELinux code and can carry the macro myself, but
> figured I would put it in the core kernel so others might find and use it
> if need be.
> 
> Signed-off-by: Eric Paris <eparis@...hat.com>
> ---
> 
>  include/linux/kernel.h |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 7d5b10f..d6092fd 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -59,6 +59,7 @@ extern const char linux_proc_banner[];
>  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
>  #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
>  #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> +#define rounddown(x, y) ((x) - ((x) % (y)))
>  #define DIV_ROUND_CLOSEST(x, divisor)(			\
>  {							\
>  	typeof(divisor) __divisor = divisor;		\
> 
> --

I'm more used to seeing it like

#define DIV_ROUND_DOWN(n, d)	(((n) / (d)) * (d))

but since multiply/divide/modulus are usually slower, your (SELinux) way is better,
I suppose.

and the usual caveats apply:  don't use these macros with expressions (nor with y
or d == 0).


---
~Randy
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