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  * SPDX-License-Identifier: BSD-3-Clause
    3  *
    4  * Copyright (c) 1990, 1993
    5  *      The Regents of the University of California.  All rights reserved.
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  * 3. Neither the name of the University nor the names of its contributors
   16  *    may be used to endorse or promote products derived from this software
   17  *    without specific prior written permission.
   18  *
   19  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   29  * SUCH DAMAGE.
   30  */
   31 
   32 #if defined(LIBC_SCCS) && !defined(lint)
   33 static char sccsid[] = "@(#)bsearch.c   8.1 (Berkeley) 6/4/93";
   34 #endif /* LIBC_SCCS and not lint */
   35 #include <sys/cdefs.h>
   36 __FBSDID("$FreeBSD: releng/12.0/sys/libkern/bsearch.c 326023 2017-11-20 19:43:44Z pfg $");
   37 
   38 #include <sys/param.h>
   39 #include <sys/libkern.h>
   40 
   41 /*
   42  * Perform a binary search.
   43  *
   44  * The code below is a bit sneaky.  After a comparison fails, we
   45  * divide the work in half by moving either left or right. If lim
   46  * is odd, moving left simply involves halving lim: e.g., when lim
   47  * is 5 we look at item 2, so we change lim to 2 so that we will
   48  * look at items 0 & 1.  If lim is even, the same applies.  If lim
   49  * is odd, moving right again involes halving lim, this time moving
   50  * the base up one item past p: e.g., when lim is 5 we change base
   51  * to item 3 and make lim 2 so that we will look at items 3 and 4.
   52  * If lim is even, however, we have to shrink it by one before
   53  * halving: e.g., when lim is 4, we still looked at item 2, so we
   54  * have to make lim 3, then halve, obtaining 1, so that we will only
   55  * look at item 3.
   56  */
   57 void *
   58 bsearch(const void *key, const void *base0, size_t nmemb, size_t size,
   59     int (*compar)(const void *, const void *))
   60 {
   61         const char *base = base0;
   62         size_t lim;
   63         int cmp;
   64         const void *p;
   65 
   66         for (lim = nmemb; lim != 0; lim >>= 1) {
   67                 p = base + (lim >> 1) * size;
   68                 cmp = (*compar)(key, p);
   69                 if (cmp == 0)
   70                         return ((void *)(uintptr_t)p);
   71                 if (cmp > 0) {  /* key > p: move right */
   72                         base = (const char *)p + size;
   73                         lim--;
   74                 }               /* else move left */
   75         }
   76         return (NULL);
   77 }

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