src/system/libroot/posix/string/strnlen.cpp

   1 /*
   2 ** Copyright 2001, Travis Geiselbrecht. All rights reserved.
   3 ** Distributed under the terms of the NewOS License.
   4 */
   5
   6 #include <string.h>
   7 #include <SupportDefs.h>
   8
   9
  10 /* From Bit twiddling hacks:
  11         http://graphics.stanford.edu/~seander/bithacks.html */
  12 #define LACKS_ZERO_BYTE(value) \
  13         (((value - 0x01010101) & ~value & 0x80808080) == 0)
  14
  15
  16 size_t
  17 strnlen(const char* string, size_t count)
  18 {
  19         size_t length = 0;
  20         /* Align access for four byte reads */
  21         for (; (((addr_t)string + length) & 3) != 0; length++) {
  22                 if (length == count || string[length] == '\0')
  23                         return length;
  24         }
  25
  26         /* Check four bytes for zero char */
  27         const uint32* kMaxScanPosition = (uint32*)(string + count - 4);
  28         uint32* valuePointer = (uint32*)(string + length);
  29         for (; valuePointer <= kMaxScanPosition && LACKS_ZERO_BYTE(*valuePointer);
  30                 valuePointer++)
  31                 ;
  32
  33         /* Find the exact length */
  34         for (length = ((char*)valuePointer) - string; length < count
  35                 && string[length] != '\0'; length++)
  36                 ;
  37
  38         return length;
  39 }