linux-kernel - Re: [PATCH 1/2] lib: Add generic binary search function to the kernel.

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Date:	Thu, 24 Sep 2009 09:41:40 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tim Abbott <tabbott@...lice.com>
Cc:	Alan Jenkins <alan-jenkins@...fmail.co.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-kbuild@...r.kernel.org, linux-modules@...r.kernel.org
Subject: Re: [PATCH 1/2] lib: Add generic binary search function to the kernel.

On Thu, 24 Sep 2009 02:58:45 am Tim Abbott wrote:
> There a large number hand-coded binary searches in the kernel (run
> "git grep search | grep binary" to find many of them).  Since in my
> experience, hand-coding binary searches can be error-prone, it seems
> worth cleaning this up by providing a generic binary search function.
> 
> This generic binary search implementation comes from Ksplice.  It has
> the same basic API as the C library bsearch() function.  Ksplice uses
> it in half a dozen places with 4 different comparison functions, and I
> think our code is substantially cleaner because of this.
> 
> Signed-off-by: Tim Abbott <tabbott@...lice.com>
> ---
>  include/linux/bsearch.h |    9 ++++++++
>  lib/Makefile            |    2 +-
>  lib/bsearch.c           |   53 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 63 insertions(+), 1 deletions(-)
>  create mode 100644 include/linux/bsearch.h
>  create mode 100644 lib/bsearch.c
> 
> diff --git a/include/linux/bsearch.h b/include/linux/bsearch.h
> new file mode 100644
> index 0000000..90b1aa8
> --- /dev/null
> +++ b/include/linux/bsearch.h
> @@ -0,0 +1,9 @@
> +#ifndef _LINUX_BSEARCH_H
> +#define _LINUX_BSEARCH_H
> +
> +#include <linux/types.h>
> +
> +void *bsearch(const void *key, const void *base, size_t num, size_t size,
> +	      int (*cmp)(const void *key, const void *elt));
> +
> +#endif /* _LINUX_BSEARCH_H */
> diff --git a/lib/Makefile b/lib/Makefile
> index 2e78277..fb60af1 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -21,7 +21,7 @@ lib-y	+= kobject.o kref.o klist.o
>  
>  obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
>  	 bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
> -	 string_helpers.o gcd.o
> +	 string_helpers.o gcd.o bsearch.o
>  
>  ifeq ($(CONFIG_DEBUG_KOBJECT),y)
>  CFLAGS_kobject.o += -DDEBUG
> diff --git a/lib/bsearch.c b/lib/bsearch.c
> new file mode 100644
> index 0000000..4297c98
> --- /dev/null
> +++ b/lib/bsearch.c
> @@ -0,0 +1,53 @@
> +/*
> + * A generic implementation of binary search for the Linux kernel
> + *
> + * Copyright (C) 2008-2009 Ksplice, Inc.
> + * Author: Tim Abbott <tabbott@...lice.com>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License as
> + * published by the Free Software Foundation; version 2.
> + */
> +
> +#include <linux/module.h>
> +#include <linux/bsearch.h>
> +
> +/*
> + * bsearch - binary search an array of elements
> + * @key: pointer to item being searched for
> + * @base: pointer to data to sort
> + * @num: number of elements
> + * @size: size of each element
> + * @cmp: pointer to comparison function
> + *
> + * This function does a binary search on the given array.  The
> + * contents of the array should already be in ascending sorted order
> + * under the provided comparison function.
> + *
> + * Note that the key need not have the same type as the elements in
> + * the array, e.g. key could be a string and the comparison function
> + * could compare the string with the struct's name field.  However, if
> + * the key and elements in the array are of the same type, you can use
> + * the same comparison function for both sort() and bsearch().
> + */
> +void *bsearch(const void *key, const void *base, size_t num, size_t size,
> +	      int (*cmp)(const void *key, const void *elt))
> +{
> +	int start = 0, end = num - 1, mid, result;
> +	if (num == 0)
> +		return NULL;
> +
> +	while (start <= end) {

The if (num == 0) line is superfluous.

But I'd be happy to take this as part of Alan's patches.

Thanks!
Rusty.
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