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  * Copyright (c) 2012 Ed Schouten <ed@FreeBSD.org>
    3  * 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  *
   14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   24  * SUCH DAMAGE.
   25  */
   26 
   27 #include <sys/cdefs.h>
   28 __FBSDID("$FreeBSD: releng/11.2/sys/libkern/memcchr.c 229198 2012-01-01 20:26:11Z ed $");
   29 
   30 #include <sys/libkern.h>
   31 #include <sys/limits.h>
   32 #include <sys/param.h>
   33 
   34 /*
   35  * memcchr(): find first character in buffer not matching `c'.
   36  *
   37  * This function performs the complement of memchr().  To provide decent
   38  * performance, this function compares data from the buffer one word at
   39  * a time.
   40  *
   41  * This code is inspired by libc's strlen(), written by Xin Li.
   42  */
   43 
   44 #if LONG_BIT != 32 && LONG_BIT != 64
   45 #error Unsupported word size
   46 #endif
   47 
   48 #define LONGPTR_MASK (sizeof(long) - 1)
   49 
   50 #define TESTBYTE                                \
   51         do {                                    \
   52                 if (*p != (unsigned char)c)     \
   53                         goto done;              \
   54                 p++;                            \
   55         } while (0)
   56 
   57 void *
   58 memcchr(const void *begin, int c, size_t n)
   59 {
   60         const unsigned long *lp;
   61         const unsigned char *p, *end;
   62         unsigned long word;
   63 
   64         /* Four or eight repetitions of `c'. */
   65         word = (unsigned char)c;
   66         word |= word << 8;
   67         word |= word << 16;
   68 #if LONG_BIT >= 64
   69         word |= word << 32;
   70 #endif
   71 
   72         /* Don't perform memory I/O when passing a zero-length buffer. */
   73         if (n == 0)
   74                 return (NULL);
   75 
   76         /*
   77          * First determine whether there is a character unequal to `c'
   78          * in the first word.  As this word may contain bytes before
   79          * `begin', we may execute this loop spuriously.
   80          */
   81         lp = (const unsigned long *)((uintptr_t)begin & ~LONGPTR_MASK);
   82         end = (const unsigned char *)begin + n;
   83         if (*lp++ != word)
   84                 for (p = begin; p < (const unsigned char *)lp;)
   85                         TESTBYTE;
   86 
   87         /* Now compare the data one word at a time. */
   88         for (; (const unsigned char *)lp < end; lp++) {
   89                 if (*lp != word) {
   90                         p = (const unsigned char *)lp;
   91                         TESTBYTE;
   92                         TESTBYTE;
   93                         TESTBYTE;
   94 #if LONG_BIT >= 64
   95                         TESTBYTE;
   96                         TESTBYTE;
   97                         TESTBYTE;
   98                         TESTBYTE;
   99 #endif
  100                         goto done;
  101                 }
  102         }
  103 
  104         return (NULL);
  105 
  106 done:
  107         /*
  108          * If the end of the buffer is not word aligned, the previous
  109          * loops may obtain an address that's beyond the end of the
  110          * buffer.
  111          */
  112         if (p < end)
  113                 return (__DECONST(void *, p));
  114         return (NULL);
  115 }

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