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

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Date:	Mon, 10 Sep 2012 08:19:21 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...i.de>
To:	"J. Bruce Fields" <bfields@...ldses.org>
cc:	linux-kernel@...r.kernel.org, Jim Rees <rees@...ch.edu>
Subject: Re: [PATCH] strings: helper for maximum decimal encoding of an
 unsigned integer

On Tuesday 2012-08-21 23:29, J. Bruce Fields wrote:

>I've seen a couple examples recently where we've gotten this wrong.
>Maybe something like this would help?  Is there some better way?
>(Approximation due to Jim Rees).
>
>+/*
>+ * 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])

But enumerating it (using your ULONG_MAX_STR proposal) should be 
sufficient, since there is only a limited number of types that get e.g. 
sprintf'ed into a buffer.

Something else tho, it seems some don't know that sizeof("") is already 
1, and an extra \0 is often unneeded, like in:

arch/um/drivers/net_user.c:  char addr_buf[sizeof("255.255.255.255\0")];
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