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/memcchr.c

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    1 /*-
    2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
    3  *
    4  * Copyright (c) 2012 Ed Schouten <ed@FreeBSD.org>
    5  * 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  *
   16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   26  * SUCH DAMAGE.
   27  */
   28 
   29 #include <sys/cdefs.h>
   30 __FBSDID("$FreeBSD: releng/12.0/sys/libkern/memcchr.c 326271 2017-11-27 15:20:12Z pfg $");
   31 
   32 #include <sys/libkern.h>
   33 #include <sys/limits.h>
   34 #include <sys/param.h>
   35 
   36 /*
   37  * memcchr(): find first character in buffer not matching `c'.
   38  *
   39  * This function performs the complement of memchr().  To provide decent
   40  * performance, this function compares data from the buffer one word at
   41  * a time.
   42  *
   43  * This code is inspired by libc's strlen(), written by Xin Li.
   44  */
   45 
   46 #if LONG_BIT != 32 && LONG_BIT != 64
   47 #error Unsupported word size
   48 #endif
   49 
   50 #define LONGPTR_MASK (sizeof(long) - 1)
   51 
   52 #define TESTBYTE                                \
   53         do {                                    \
   54                 if (*p != (unsigned char)c)     \
   55                         goto done;              \
   56                 p++;                            \
   57         } while (0)
   58 
   59 void *
   60 memcchr(const void *begin, int c, size_t n)
   61 {
   62         const unsigned long *lp;
   63         const unsigned char *p, *end;
   64         unsigned long word;
   65 
   66         /* Four or eight repetitions of `c'. */
   67         word = (unsigned char)c;
   68         word |= word << 8;
   69         word |= word << 16;
   70 #if LONG_BIT >= 64
   71         word |= word << 32;
   72 #endif
   73 
   74         /* Don't perform memory I/O when passing a zero-length buffer. */
   75         if (n == 0)
   76                 return (NULL);
   77 
   78         /*
   79          * First determine whether there is a character unequal to `c'
   80          * in the first word.  As this word may contain bytes before
   81          * `begin', we may execute this loop spuriously.
   82          */
   83         lp = (const unsigned long *)((uintptr_t)begin & ~LONGPTR_MASK);
   84         end = (const unsigned char *)begin + n;
   85         if (*lp++ != word)
   86                 for (p = begin; p < (const unsigned char *)lp;)
   87                         TESTBYTE;
   88 
   89         /* Now compare the data one word at a time. */
   90         for (; (const unsigned char *)lp < end; lp++) {
   91                 if (*lp != word) {
   92                         p = (const unsigned char *)lp;
   93                         TESTBYTE;
   94                         TESTBYTE;
   95                         TESTBYTE;
   96 #if LONG_BIT >= 64
   97                         TESTBYTE;
   98                         TESTBYTE;
   99                         TESTBYTE;
  100                         TESTBYTE;
  101 #endif
  102                         goto done;
  103                 }
  104         }
  105 
  106         return (NULL);
  107 
  108 done:
  109         /*
  110          * If the end of the buffer is not word aligned, the previous
  111          * loops may obtain an address that's beyond the end of the
  112          * buffer.
  113          */
  114         if (p < end)
  115                 return (__DECONST(void *, p));
  116         return (NULL);
  117 }

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