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

Rusty Russell
2009-09-24T00:11:39Z

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@ksplice.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@ksplice.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.