The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/libkern/bsearch.c

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    1 /*-
    2  * Copyright (c) 1990, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  * 3. Neither the name of the University nor the names of its contributors
   14  *    may be used to endorse or promote products derived from this software
   15  *    without specific prior written permission.
   16  *
   17  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   27  * SUCH DAMAGE.
   28  *
   29  * @(#)bsearch.c        8.1 (Berkeley) 6/4/93
   30  * $FreeBSD: src/sys/libkern/bsearch.c,v 1.8 2005/01/07 00:24:32 imp Exp $
   31  * $DragonFly: src/sys/libkern/bsearch.c,v 1.1 2007/11/18 17:53:02 pavalos Exp $
   32  */
   33 
   34 #include <sys/param.h>
   35 #include <sys/libkern.h>
   36 
   37 /*
   38  * Perform a binary search.
   39  *
   40  * The code below is a bit sneaky.  After a comparison fails, we
   41  * divide the work in half by moving either left or right. If lim
   42  * is odd, moving left simply involves halving lim: e.g., when lim
   43  * is 5 we look at item 2, so we change lim to 2 so that we will
   44  * look at items 0 & 1.  If lim is even, the same applies.  If lim
   45  * is odd, moving right again involes halving lim, this time moving
   46  * the base up one item past p: e.g., when lim is 5 we change base
   47  * to item 3 and make lim 2 so that we will look at items 3 and 4.
   48  * If lim is even, however, we have to shrink it by one before
   49  * halving: e.g., when lim is 4, we still looked at item 2, so we
   50  * have to make lim 3, then halve, obtaining 1, so that we will only
   51  * look at item 3.
   52  */
   53 void *
   54 kbsearch(const void *key, const void *base0, size_t nmemb, size_t size,
   55         int (*compar)(const void *, const void *))
   56 {
   57         const char *base = base0;
   58         size_t lim;
   59         int cmp;
   60         const void *p;
   61 
   62         for (lim = nmemb; lim != 0; lim >>= 1) {
   63                 p = base + (lim >> 1) * size;
   64                 cmp = (*compar)(key, p);
   65                 if (cmp == 0)
   66                         return ((void *)(uintptr_t)p);
   67                 if (cmp > 0) {  /* key > p: move right */
   68                         base = (const char *)p + size;
   69                         lim--;
   70                 }               /* else move left */
   71         }
   72         return (NULL);
   73 }

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