string/strnlen.c

   1 /* Find the length of STRING, but scan at most MAXLEN characters.
   2    Copyright (C) 1991-2022 Free Software Foundation, Inc.
   3
   4    Based on strlen written by Torbjorn Granlund (tege@sics.se),
   5    with help from Dan Sahlin (dan@sics.se);
   6    commentary by Jim Blandy (jimb@ai.mit.edu).
   7
   8    The GNU C Library is free software; you can redistribute it and/or
   9    modify it under the terms of the GNU Lesser General Public License as
  10    published by the Free Software Foundation; either version 2.1 of the
  11    License, or (at your option) any later version.
  12
  13    The GNU C Library is distributed in the hope that it will be useful,
  14    but WITHOUT ANY WARRANTY; without even the implied warranty of
  15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16    Lesser General Public License for more details.
  17
  18    You should have received a copy of the GNU Lesser General Public
  19    License along with the GNU C Library; see the file COPYING.LIB.  If
  20    not, see <https://www.gnu.org/licenses/>.  */
  21
  22 #include <string.h>
  23 #include <stdlib.h>
  24
  25 /* Find the length of S, but scan at most MAXLEN characters.  If no
  26    '\0' terminator is found in that many characters, return MAXLEN.  */
  27
  28 #ifdef STRNLEN
  29 # define __strnlen STRNLEN
  30 #endif
  31
  32 size_t
  33 __strnlen (const char *str, size_t maxlen)
  34 {
  35   const char *char_ptr, *end_ptr = str + maxlen;
  36   const unsigned long int *longword_ptr;
  37   unsigned long int longword, himagic, lomagic;
  38
  39   if (maxlen == 0)
  40     return 0;
  41
  42   if (__glibc_unlikely (end_ptr < str))
  43     end_ptr = (const char *) ~0UL;
  44
  45   /* Handle the first few characters by reading one character at a time.
  46      Do this until CHAR_PTR is aligned on a longword boundary.  */
  47   for (char_ptr = str; ((unsigned long int) char_ptr
  48                         & (sizeof (longword) - 1)) != 0;
  49        ++char_ptr)
  50     if (*char_ptr == '\0')
  51       {
  52         if (char_ptr > end_ptr)
  53           char_ptr = end_ptr;
  54         return char_ptr - str;
  55       }
  56
  57   /* All these elucidatory comments refer to 4-byte longwords,
  58      but the theory applies equally well to 8-byte longwords.  */
  59
  60   longword_ptr = (unsigned long int *) char_ptr;
  61
  62   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  63      the "holes."  Note that there is a hole just to the left of
  64      each byte, with an extra at the end:
  65
  66      bits:  01111110 11111110 11111110 11111111
  67      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  68
  69      The 1-bits make sure that carries propagate to the next 0-bit.
  70      The 0-bits provide holes for carries to fall into.  */
  71   himagic = 0x80808080L;
  72   lomagic = 0x01010101L;
  73   if (sizeof (longword) > 4)
  74     {
  75       /* 64-bit version of the magic.  */
  76       /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
  77       himagic = ((himagic << 16) << 16) | himagic;
  78       lomagic = ((lomagic << 16) << 16) | lomagic;
  79     }
  80   if (sizeof (longword) > 8)
  81     abort ();
  82
  83   /* Instead of the traditional loop which tests each character,
  84      we will test a longword at a time.  The tricky part is testing
  85      if *any of the four* bytes in the longword in question are zero.  */
  86   while (longword_ptr < (unsigned long int *) end_ptr)
  87     {
  88       /* We tentatively exit the loop if adding MAGIC_BITS to
  89          LONGWORD fails to change any of the hole bits of LONGWORD.
  90
  91          1) Is this safe?  Will it catch all the zero bytes?
  92          Suppose there is a byte with all zeros.  Any carry bits
  93          propagating from its left will fall into the hole at its
  94          least significant bit and stop.  Since there will be no
  95          carry from its most significant bit, the LSB of the
  96          byte to the left will be unchanged, and the zero will be
  97          detected.
  98
  99          2) Is this worthwhile?  Will it ignore everything except
 100          zero bytes?  Suppose every byte of LONGWORD has a bit set
 101          somewhere.  There will be a carry into bit 8.  If bit 8
 102          is set, this will carry into bit 16.  If bit 8 is clear,
 103          one of bits 9-15 must be set, so there will be a carry
 104          into bit 16.  Similarly, there will be a carry into bit
 105          24.  If one of bits 24-30 is set, there will be a carry
 106          into bit 31, so all of the hole bits will be changed.
 107
 108          The one misfire occurs when bits 24-30 are clear and bit
 109          31 is set; in this case, the hole at bit 31 is not
 110          changed.  If we had access to the processor carry flag,
 111          we could close this loophole by putting the fourth hole
 112          at bit 32!
 113
 114          So it ignores everything except 128's, when they're aligned
 115          properly.  */
 116
 117       longword = *longword_ptr++;
 118
 119       if ((longword - lomagic) & himagic)
 120         {
 121           /* Which of the bytes was the zero?  If none of them were, it was
 122              a misfire; continue the search.  */
 123
 124           const char *cp = (const char *) (longword_ptr - 1);
 125
 126           char_ptr = cp;
 127           if (cp[0] == 0)
 128             break;
 129           char_ptr = cp + 1;
 130           if (cp[1] == 0)
 131             break;
 132           char_ptr = cp + 2;
 133           if (cp[2] == 0)
 134             break;
 135           char_ptr = cp + 3;
 136           if (cp[3] == 0)
 137             break;
 138           if (sizeof (longword) > 4)
 139             {
 140               char_ptr = cp + 4;
 141               if (cp[4] == 0)
 142                 break;
 143               char_ptr = cp + 5;
 144               if (cp[5] == 0)
 145                 break;
 146               char_ptr = cp + 6;
 147               if (cp[6] == 0)
 148                 break;
 149               char_ptr = cp + 7;
 150               if (cp[7] == 0)
 151                 break;
 152             }
 153         }
 154       char_ptr = end_ptr;
 155     }
 156
 157   if (char_ptr > end_ptr)
 158     char_ptr = end_ptr;
 159   return char_ptr - str;
 160 }
 161 #ifndef STRNLEN
 162 libc_hidden_def (__strnlen)
 163 weak_alias (__strnlen, strnlen)
 164 #endif
 165 libc_hidden_def (strnlen)