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

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Message-ID: <20120914131457.GA19666@fieldses.org>
Date:	Fri, 14 Sep 2012 09:14:57 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jim Rees <rees@...ch.edu>
Cc:	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Jan Engelhardt <jengelh@...i.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] strings: helper for maximum decimal encoding of an
 unsigned integer

On Fri, Sep 14, 2012 at 08:30:14AM -0400, Jim Rees wrote:
> But I still like my way better.

Yeah.  It looks less mysterious once you realize it's just a
straightforward application of the usual coincidence

	2^10 = 1024 ~ 1000 = 10^3

How about this?  Also with some more defines because I like typing

	char buf[ULONG_STR_MAX + 1];

better than

	char bug[base10len(unsigned long) + 1];

--b.

commit 59a620640cd05d3d29e678ff893cfe266091fba7
Author: J. Bruce Fields <bfields@...hat.com>
Date:   Wed Aug 15 17:41:47 2012 -0400

    strings: helper for maximum decimal encoding of an unsigned integer
    
    I've seen a couple examples recently where we've gotten this wrong.
    Maybe something like this would help?
    
    Suggested-by: Jim Rees <rees@...ch.edu>
    Signed-off-by: J. Bruce Fields <bfields@...hat.com>

diff --git a/include/linux/string.h b/include/linux/string.h
index ffe0442..38da7a0 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -126,6 +126,27 @@ extern void argv_free(char **argv);
 extern bool sysfs_streq(const char *s1, const char *s2);
 extern int strtobool(const char *s, bool *res);
 
+/*
+ * length of the decimal representation of an integer, not including any
+ * sign or null termination.  Just an approximation, but it's right for
+ * unsigned types of size 1 to 26 bytes:
+ */
+#define base10len(i) (sizeof(i) * 8 * 3 / 10 + 1)
+
+/* Actually a slight overestimate in the signed 64-bit case: */
+
+#define UCHAR_STR_MAX	base10len(char)
+#define CHAR_STR_MAX	base10len(char) + 1
+#define UINT_STR_MAX	base10len(int)
+#define INT_STR_MAX	base10len(int) + 1
+#define ULONG_STR_MAX	base10len(long)
+#define LONG_STR_MAX	base10len(long) + 1
+
+#define U32_STR_MAX	base10len(u32)
+#define S32_STR_MAX	base10len(u32) + 1
+#define U64_STR_MAX	base10len(u64)
+#define S64_STR_MAX	base10len(u64) + 1
+
 #ifdef CONFIG_BINARY_PRINTF
 int vbin_printf(u32 *bin_buf, size_t size, const char *fmt, va_list args);
 int bstr_printf(char *buf, size_t size, const char *fmt, const u32 *bin_buf);
diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index 2afd2a8..d80d482 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -1409,7 +1409,7 @@ static ssize_t read_flush(struct file *file, char __user *buf,
 			  size_t count, loff_t *ppos,
 			  struct cache_detail *cd)
 {
-	char tbuf[20];
+	char tbuf[ULONG_STR_MAX + 1];
 	unsigned long p = *ppos;
 	size_t len;
 
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