memchr.c

用于嵌入式Linux系统的标准C的库函数

/*
FUNCTION
	<<memchr>>---find character in memory

INDEX
	memchr

ANSI_SYNOPSIS
	#include <string.h>
	void *memchr(const void *<[src]>, int <[c]>, size_t <[length]>);

TRAD_SYNOPSIS
	#include <string.h>
	void *memchr(<[src]>, <[c]>, <[length]>)
	void *<[src]>;
	void *<[c]>;
	size_t <[length]>;

DESCRIPTION
	This function searches memory starting at <<*<[src]>>> for the
	character <[c]>.  The search only ends with the first
	occurrence of <[c]>, or after <[length]> characters; in
	particular, <<NULL>> does not terminate the search.

RETURNS
	If the character <[c]> is found within <[length]> characters
	of <<*<[src]>>>, a pointer to the character is returned. If
	<[c]> is not found, then <<NULL>> is returned.

PORTABILITY
<<memchr>>> is ANSI C.

<<memchr>>  requires no supporting OS subroutines.

QUICKREF
	memchr ansi pure
*/

#include <_ansi.h>
#include <string.h>
#include <limits.h>

/* Nonzero if either X or Y is not aligned on a "long" boundary.  */
#define UNALIGNED(X) ((long)X & (sizeof (long) - 1))

/* How many bytes are loaded each iteration of the word copy loop.  */
#define LBLOCKSIZE (sizeof (long))

/* Threshhold for punting to the bytewise iterator.  */
#define TOO_SMALL(LEN)  ((LEN) < LBLOCKSIZE)

#if LONG_MAX == 2147483647L
#define DETECTNULL(X) (((X) - 0x01010101) & ~(X) & 0x80808080)
#else
#if LONG_MAX == 9223372036854775807L
/* Nonzero if X (a long int) contains a NULL byte. */
#define DETECTNULL(X) (((X) - 0x0101010101010101) & ~(X) & 0x8080808080808080)
#else
#error long int is not a 32bit or 64bit type.
#endif
#endif

#ifndef DETECTNULL
#error long int is not a 32bit or 64bit byte
#endif


_PTR
_DEFUN (memchr, (src_void, c, length),
	_CONST _PTR src_void _AND
	int c _AND
	size_t length)
{
#if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
  _CONST unsigned char *src = (_CONST unsigned char *) src_void;

  c &= 0xff;
  
  while (length--)
    {
      if (*src == c)
	return (char *) src;
      src++;
    }
  return NULL;
#else
  _CONST unsigned char *src = (_CONST unsigned char *) src_void;
  unsigned long *asrc;
  unsigned long  buffer;
  unsigned long  mask;
  int i, j;

  c &= 0xff;
  
  /* If the size is small, or src is unaligned, then 
     use the bytewise loop.  We can hope this is rare.  */
  if (!TOO_SMALL (length) && !UNALIGNED (src)) 
    {
      /* The fast code reads the ASCII one word at a time and only 
         performs the bytewise search on word-sized segments if they
         contain the search character, which is detected by XORing 
         the word-sized segment with a word-sized block of the search
         character and then detecting for the presence of NULL in the
         result.  */
      asrc = (unsigned long*) src;
      mask = 0;
      for (i = 0; i < LBLOCKSIZE; i++)
        mask = (mask << 8) + c;

      while (length >= LBLOCKSIZE)
        {
          buffer = *asrc;
          buffer ^=  mask;
          if (DETECTNULL (buffer))
	    {
              src = (unsigned char*) asrc;
  	      for ( j = 0; j < LBLOCKSIZE; j++ )
	        {
                  if (*src == c)
                    return (char*) src;
                  src++;
	        }
   	    }
          length -= LBLOCKSIZE;
          asrc++;
        }
  
      /* If there are fewer than LBLOCKSIZE characters left,
         then we resort to the bytewise loop.  */

      src = (unsigned char*) asrc;
    }

  while (length--)
    { 
      if (*src == c)
        return (char*) src;
      src++;
    } 

  return NULL;
#endif /* not PREFER_SIZE_OVER_SPEED */
}