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glibc  2.9
Defines | Functions
rawmemchr.c File Reference
#include <sys/types.h>

Go to the source code of this file.


#define __ptr_t   char *
#define reg_char   char
#define LONG_MAX_32_BITS   2147483647


__ptr_t __rawmemchr (__ptr_t s, int c_in) const

Define Documentation

#define __ptr_t   char *

Definition at line 32 of file rawmemchr.c.


Definition at line 50 of file rawmemchr.c.

#define LONG_MAX_32_BITS   2147483647

Definition at line 47 of file rawmemchr.c.

#define reg_char   char

Definition at line 40 of file rawmemchr.c.

Function Documentation

__ptr_t __rawmemchr ( __ptr_t  s,
int  c_in 
) const

Definition at line 60 of file rawmemchr.c.

  const unsigned char *char_ptr;
  const unsigned long int *longword_ptr;
  unsigned long int longword, magic_bits, charmask;
  unsigned reg_char c;

  c = (unsigned char) c_in;

  /* Handle the first few characters by reading one character at a time.
     Do this until CHAR_PTR is aligned on a longword boundary.  */
  for (char_ptr = (const unsigned char *) s;
       ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
    if (*char_ptr == c)
      return (__ptr_t) char_ptr;

  /* All these elucidatory comments refer to 4-byte longwords,
     but the theory applies equally well to 8-byte longwords.  */

  longword_ptr = (unsigned long int *) char_ptr;

  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
     the "holes."  Note that there is a hole just to the left of
     each byte, with an extra at the end:

     bits:  01111110 11111110 11111110 11111111

     The 1-bits make sure that carries propagate to the next 0-bit.
     The 0-bits provide holes for carries to fall into.  */

  if (sizeof (longword) != 4 && sizeof (longword) != 8)
    abort ();

  magic_bits = 0x7efefeff;
  magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;

  /* Set up a longword, each of whose bytes is C.  */
  charmask = c | (c << 8);
  charmask |= charmask << 16;
  charmask |= charmask << 32;

  /* Instead of the traditional loop which tests each character,
     we will test a longword at a time.  The tricky part is testing
     if *any of the four* bytes in the longword in question are zero.  */
  while (1)
      /* We tentatively exit the loop if adding MAGIC_BITS to
        LONGWORD fails to change any of the hole bits of LONGWORD.

        1) Is this safe?  Will it catch all the zero bytes?
        Suppose there is a byte with all zeros.  Any carry bits
        propagating from its left will fall into the hole at its
        least significant bit and stop.  Since there will be no
        carry from its most significant bit, the LSB of the
        byte to the left will be unchanged, and the zero will be

        2) Is this worthwhile?  Will it ignore everything except
        zero bytes?  Suppose every byte of LONGWORD has a bit set
        somewhere.  There will be a carry into bit 8.  If bit 8
        is set, this will carry into bit 16.  If bit 8 is clear,
        one of bits 9-15 must be set, so there will be a carry
        into bit 16.  Similarly, there will be a carry into bit
        24.  If one of bits 24-30 is set, there will be a carry
        into bit 31, so all of the hole bits will be changed.

        The one misfire occurs when bits 24-30 are clear and bit
        31 is set; in this case, the hole at bit 31 is not
        changed.  If we had access to the processor carry flag,
        we could close this loophole by putting the fourth hole
        at bit 32!

        So it ignores everything except 128's, when they're aligned

        3) But wait!  Aren't we looking for C, not zero?
        Good point.  So what we do is XOR LONGWORD with a longword,
        each of whose bytes is C.  This turns each byte that is C
        into a zero.  */

      longword = *longword_ptr++ ^ charmask;

      /* Add MAGIC_BITS to LONGWORD.  */
      if ((((longword + magic_bits)

           /* Set those bits that were unchanged by the addition.  */
           ^ ~longword)

          /* Look at only the hole bits.  If any of the hole bits
             are unchanged, most likely one of the bytes was a
             zero.  */
          & ~magic_bits) != 0)
         /* Which of the bytes was C?  If none of them were, it was
            a misfire; continue the search.  */

         const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);

         if (cp[0] == c)
           return (__ptr_t) cp;
         if (cp[1] == c)
           return (__ptr_t) &cp[1];
         if (cp[2] == c)
           return (__ptr_t) &cp[2];
         if (cp[3] == c)
           return (__ptr_t) &cp[3];
#if LONG_MAX > 2147483647
         if (cp[4] == c)
           return (__ptr_t) &cp[4];
         if (cp[5] == c)
           return (__ptr_t) &cp[5];
         if (cp[6] == c)
           return (__ptr_t) &cp[6];
         if (cp[7] == c)
           return (__ptr_t) &cp[7];

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