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

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Message-ID: <20120914123014.GB29160@umich.edu>
Date:	Fri, 14 Sep 2012 08:30:14 -0400
From:	Jim Rees <rees@...ch.edu>
To:	Bernd Petrovitsch <bernd@...rovitsch.priv.at>
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

Bernd Petrovitsch wrote:

  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.

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)

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

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

4. Unreadable.

Pure K&R:

base10.h:
extern 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};

But I still like my way better.
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