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/bcmp.c

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    1 /*-
    2  * Copyright (c) 1987, 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  * 4. 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 
   30 #include <sys/cdefs.h>
   31 __FBSDID("$FreeBSD: releng/6.1/sys/libkern/bcmp.c 139815 2005-01-07 00:24:33Z imp $");
   32 
   33 #include <sys/libkern.h>
   34 #include <machine/endian.h>
   35 
   36 typedef const void      *cvp;
   37 typedef const unsigned char     *ustring;
   38 typedef unsigned long   ul;
   39 typedef const unsigned long     *culp;
   40 
   41 /*
   42  * bcmp -- vax cmpc3 instruction
   43  */
   44 int
   45 bcmp(b1, b2, length)
   46         const void *b1, *b2;
   47         register size_t length;
   48 {
   49 #if BYTE_ORDER == LITTLE_ENDIAN
   50         /*
   51          * The following code is endian specific.  Changing it from
   52          * little-endian to big-endian is fairly trivial, but making
   53          * it do both is more difficult.
   54          *
   55          * Note that this code will reference the entire longword which
   56          * includes the final byte to compare.  I don't believe this is
   57          * a problem since AFAIK, objects are not protected at smaller
   58          * than longword boundaries.
   59          */
   60         int     shl, shr, len = length;
   61         ustring p1 = b1, p2 = b2;
   62         ul      va, vb;
   63 
   64         if (len == 0)
   65                 return (0);
   66 
   67         /*
   68          * align p1 to a longword boundary
   69          */
   70         while ((long)p1 & (sizeof(long) - 1)) {
   71                 if (*p1++ != *p2++)
   72                         return (1);
   73                 if (--len <= 0)
   74                         return (0);
   75         }
   76 
   77         /*
   78          * align p2 to longword boundary and calculate the shift required to
   79          * align p1 and p2
   80          */
   81         shr = (long)p2 & (sizeof(long) - 1);
   82         if (shr != 0) {
   83                 p2 -= shr;                      /* p2 now longword aligned */
   84                 shr <<= 3;                      /* offset in bits */
   85                 shl = (sizeof(long) << 3) - shr;
   86 
   87                 va = *(culp)p2;
   88                 p2 += sizeof(long);
   89 
   90                 while ((len -= sizeof(long)) >= 0) {
   91                         vb = *(culp)p2;
   92                         p2 += sizeof(long);
   93                         if (*(culp)p1 != (va >> shr | vb << shl))
   94                                 return (1);
   95                         p1 += sizeof(long);
   96                         va = vb;
   97                 }
   98                 /*
   99                  * At this point, len is between -sizeof(long) and -1,
  100                  * representing 0 .. sizeof(long)-1 bytes remaining.
  101                  */
  102                 if (!(len += sizeof(long)))
  103                         return (0);
  104 
  105                 len <<= 3;              /* remaining length in bits */
  106                 /*
  107                  * The following is similar to the `if' condition
  108                  * inside the above while loop.  The ?: is necessary
  109                  * to avoid accessing the longword after the longword
  110                  * containing the last byte to be compared.
  111                  */
  112                 return ((((va >> shr | ((shl < len) ? *(culp)p2 << shl : 0)) ^
  113                     *(culp)p1) & ((1L << len) - 1)) != 0);
  114         } else {
  115                 /* p1 and p2 have common alignment so no shifting needed */
  116                 while ((len -= sizeof(long)) >= 0) {
  117                         if (*(culp)p1 != *(culp)p2)
  118                                 return (1);
  119                         p1 += sizeof(long);
  120                         p2 += sizeof(long);
  121                 }
  122 
  123                 /*
  124                  * At this point, len is between -sizeof(long) and -1,
  125                  * representing 0 .. sizeof(long)-1 bytes remaining.
  126                  */
  127                 if (!(len += sizeof(long)))
  128                         return (0);
  129 
  130                 return (((*(culp)p1 ^ *(culp)p2)
  131                          & ((1L << (len << 3)) - 1)) != 0);
  132         }
  133 #else
  134         const char *p1, *p2;
  135 
  136         if (length == 0)
  137                 return(0);
  138         p1 = b1;
  139         p2 = b2;
  140         do
  141                 if (*p1++ != *p2++)
  142                         break;
  143         while (--length);
  144         return(length);
  145 #endif
  146 }

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