string/strchrnul.c

   1 /* Copyright (C) 1991-2013 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   4    with help from Dan Sahlin (dan@sics.se) and
   5    bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
   6    adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   7    and implemented by Roland McGrath (roland@ai.mit.edu).
   8
   9    The GNU C Library is free software; you can redistribute it and/or
  10    modify it under the terms of the GNU Lesser General Public
  11    License as published by the Free Software Foundation; either
  12    version 2.1 of the License, or (at your option) any later version.
  13
  14    The GNU C Library is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17    Lesser General Public License for more details.
  18
  19    You should have received a copy of the GNU Lesser General Public
  20    License along with the GNU C Library; if not, see
  21    <http://www.gnu.org/licenses/>.  */
  22
  23 #include <string.h>
  24 #include <memcopy.h>
  25 #include <stdlib.h>
  26
  27 #undef __strchrnul
  28 #undef strchrnul
  29
  30 /* Find the first occurrence of C in S or the final NUL byte.  */
  31 char *
  32 __strchrnul (s, c_in)
  33      const char *s;
  34      int c_in;
  35 {
  36   const unsigned char *char_ptr;
  37   const unsigned long int *longword_ptr;
  38   unsigned long int longword, magic_bits, charmask;
  39   unsigned char c;
  40
  41   c = (unsigned char) c_in;
  42
  43   /* Handle the first few characters by reading one character at a time.
  44      Do this until CHAR_PTR is aligned on a longword boundary.  */
  45   for (char_ptr = (const unsigned char *) s;
  46        ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
  47        ++char_ptr)
  48     if (*char_ptr == c || *char_ptr == '\0')
  49       return (void *) char_ptr;
  50
  51   /* All these elucidatory comments refer to 4-byte longwords,
  52      but the theory applies equally well to 8-byte longwords.  */
  53
  54   longword_ptr = (unsigned long int *) char_ptr;
  55
  56   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  57      the "holes."  Note that there is a hole just to the left of
  58      each byte, with an extra at the end:
  59
  60      bits:  01111110 11111110 11111110 11111111
  61      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  62
  63      The 1-bits make sure that carries propagate to the next 0-bit.
  64      The 0-bits provide holes for carries to fall into.  */
  65   switch (sizeof (longword))
  66     {
  67     case 4: magic_bits = 0x7efefeffL; break;
  68     case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;
  69     default:
  70       abort ();
  71     }
  72
  73   /* Set up a longword, each of whose bytes is C.  */
  74   charmask = c | (c << 8);
  75   charmask |= charmask << 16;
  76   if (sizeof (longword) > 4)
  77     /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
  78     charmask |= (charmask << 16) << 16;
  79   if (sizeof (longword) > 8)
  80     abort ();
  81
  82   /* Instead of the traditional loop which tests each character,
  83      we will test a longword at a time.  The tricky part is testing
  84      if *any of the four* bytes in the longword in question are zero.  */
  85   for (;;)
  86     {
  87       /* We tentatively exit the loop if adding MAGIC_BITS to
  88          LONGWORD fails to change any of the hole bits of LONGWORD.
  89
  90          1) Is this safe?  Will it catch all the zero bytes?
  91          Suppose there is a byte with all zeros.  Any carry bits
  92          propagating from its left will fall into the hole at its
  93          least significant bit and stop.  Since there will be no
  94          carry from its most significant bit, the LSB of the
  95          byte to the left will be unchanged, and the zero will be
  96          detected.
  97
  98          2) Is this worthwhile?  Will it ignore everything except
  99          zero bytes?  Suppose every byte of LONGWORD has a bit set
 100          somewhere.  There will be a carry into bit 8.  If bit 8
 101          is set, this will carry into bit 16.  If bit 8 is clear,
 102          one of bits 9-15 must be set, so there will be a carry
 103          into bit 16.  Similarly, there will be a carry into bit
 104          24.  If one of bits 24-30 is set, there will be a carry
 105          into bit 31, so all of the hole bits will be changed.
 106
 107          The one misfire occurs when bits 24-30 are clear and bit
 108          31 is set; in this case, the hole at bit 31 is not
 109          changed.  If we had access to the processor carry flag,
 110          we could close this loophole by putting the fourth hole
 111          at bit 32!
 112
 113          So it ignores everything except 128's, when they're aligned
 114          properly.
 115
 116          3) But wait!  Aren't we looking for C as well as zero?
 117          Good point.  So what we do is XOR LONGWORD with a longword,
 118          each of whose bytes is C.  This turns each byte that is C
 119          into a zero.  */
 120
 121       longword = *longword_ptr++;
 122
 123       /* Add MAGIC_BITS to LONGWORD.  */
 124       if ((((longword + magic_bits)
 125
 126             /* Set those bits that were unchanged by the addition.  */
 127             ^ ~longword)
 128
 129            /* Look at only the hole bits.  If any of the hole bits
 130               are unchanged, most likely one of the bytes was a
 131               zero.  */
 132            & ~magic_bits) != 0 ||
 133
 134           /* That caught zeroes.  Now test for C.  */
 135           ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
 136            & ~magic_bits) != 0)
 137         {
 138           /* Which of the bytes was C or zero?
 139              If none of them were, it was a misfire; continue the search.  */
 140
 141           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 142
 143           if (*cp == c || *cp == '\0')
 144             return (char *) cp;
 145           if (*++cp == c || *cp == '\0')
 146             return (char *) cp;
 147           if (*++cp == c || *cp == '\0')
 148             return (char *) cp;
 149           if (*++cp == c || *cp == '\0')
 150             return (char *) cp;
 151           if (sizeof (longword) > 4)
 152             {
 153               if (*++cp == c || *cp == '\0')
 154                 return (char *) cp;
 155               if (*++cp == c || *cp == '\0')
 156                 return (char *) cp;
 157               if (*++cp == c || *cp == '\0')
 158                 return (char *) cp;
 159               if (*++cp == c || *cp == '\0')
 160                 return (char *) cp;
 161             }
 162         }
 163     }
 164
 165   /* This should never happen.  */
 166   return NULL;
 167 }
 168
 169 weak_alias (__strchrnul, strchrnul)