linux-kernel - Re: [PATCH] strings: helper for maximum decimal encoding of an unsigned integer

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Date:	Fri, 14 Sep 2012 11:17:54 +0200
From:	Bernd Petrovitsch <bernd@...rovitsch.priv.at>
To:	Jan Engelhardt <jengelh@...i.de>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-kernel@...r.kernel.org, Jim Rees <rees@...ch.edu>
Subject: Re: [PATCH] strings: helper for maximum decimal encoding of an
 unsigned integer

On Mon, 2012-09-10 at 08:19 +0200, Jan Engelhardt wrote:
> On Tuesday 2012-08-21 23:29, J. Bruce Fields wrote:
[...]
> >+/*
> >+ * length of the decimal representation of an unsigned integer.  Just an
> >+ * approximation, but it's right for types of size 1 to 36 bytes:
> >+ */
> >+#define base10len(i) (sizeof(i) * 24 / 10 + 1)
> 
> gcc provides... "interesting" features at times.
> 
> /* for unsigned "i"s */
> #define base10len(i) ((const int[]){1,3,5,8,10,13,15,17,20}[i])

Shouldn't that have been
----  snip ----
#define base10len(i) ((const int[]){1,3,5,8,10,13,15,17,20}[sizeof(i)])
----  snip ----
?

A pure K&R-C version would use a string:
----  snip ----
#define base10len(i) "\0x1\0x3\0x5\0x8\0x0A\0x0D\0x0F\0x11\0x14"[sizeof(i)]
----  snip ----
(if I converted them properly into hexadecimal) and that gives a "char"
which is happily promoted to whatever one needs in that place.

Kind regards,
	Bernd
-- 
Bernd Petrovitsch                  Email : bernd@...rovitsch.priv.at
                     LUGA : http://www.luga.at


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