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

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Message-ID: <1347628693.3813.7.camel@thorin>
Date:	Fri, 14 Sep 2012 15:18:12 +0200
From:	Bernd Petrovitsch <bernd@...rovitsch.priv.at>
To:	Jim Rees <rees@...ch.edu>
Cc:	Jan Engelhardt <jengelh@...i.de>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] strings: helper for maximum decimal encoding of an
 unsigned integer

On Fre, 2012-09-14 at 08:30 -0400, Jim Rees wrote:
> Bernd Petrovitsch wrote:
[...]  
>   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.
> 
> 1. That may give you a signed char on some architectures, which is not what
> you want (although it doesn't matter since the values are all < 128)

And it depends on compiler options BTW.

But we can easily cast it:
#define base10len(i) ((unsigned char)"\x1\x3\x5\x8\xA\xD\xF\x11\x14"[sizeof(i)])

> 2. If you put this in a .h, you'll get multiple copies of the array

That depends on the compiler.

> 3. No bounds checking (but in ninja K&R style you never check bounds)

Well, I assumed that we don't use VLAs as parameter for the sizeof() so
the value is compile time known and the better C compilers can check it.
And then, there is no reason to store the string as such too.

[....]
> Pure K&R:

We can (and should) make it "const" too.

> base10.h:
> extern unsigned char base10len_vals[];
extern const unsigned char base10len_vals[];
> #define base10len(i) (base10len_vals[sizeof(i)])
> 
> base10.c:
> unsigned char base10len_vals[] = {1,3,5,8,10,13,15,17,20};
const unsigned char base10len_vals[] = {1,3,5,8,10,13,15,17,20};
> But I still like my way better.

The 8 wasted bytes probably do not matter ....

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

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