memcmp.c

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/*
FUNCTION
	<<memcmp>>---compare two memory areas

INDEX
	memcmp

ANSI_SYNOPSIS
	#include <string.h>
	int memcmp(const void *<[s1]>, const void *<[s2]>, size_t <[n]>);

TRAD_SYNOPSIS
	#include <string.h>
	int memcmp(<[s1]>, <[s2]>, <[n]>)
	void *<[s1]>;
	void *<[s2]>;
	size_t <[n]>;

DESCRIPTION
	This function compares not more than <[n]> characters of the
	object pointed to by <[s1]> with the object pointed to by <[s2]>.


RETURNS
	The function returns an integer greater than, equal to or
	less than zero 	according to whether the object pointed to by
	<[s1]> is greater than, equal to or less than the object
	pointed to by <[s2]>.

PORTABILITY
<<memcmp>> is ANSI C.

<<memcmp>> requires no supporting OS subroutines.

QUICKREF
	memcmp ansi pure
*/

#include <string.h>


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

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

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

int
_DEFUN (memcmp, (m1, m2, n),
	_CONST _PTR m1 _AND
	_CONST _PTR m2 _AND
	size_t n)
{
#if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
  unsigned char *s1 = (unsigned char *) m1;
  unsigned char *s2 = (unsigned char *) m2;

  while (n--)
    {
      if (*s1 != *s2)
	{
	  return *s1 - *s2;
	}
      s1++;
      s2++;
    }
  return 0;
#else  
  unsigned char *s1 = (unsigned char *) m1;
  unsigned char *s2 = (unsigned char *) m2;
  unsigned long *a1;
  unsigned long *a2;

  /* If the size is too small, or either pointer is unaligned,
     then we punt to the byte compare loop.  Hopefully this will
     not turn up in inner loops.  */
  if (!TOO_SMALL(n) && !UNALIGNED(s1,s2))
    {
      /* Otherwise, load and compare the blocks of memory one 
         word at a time.  */
      a1 = (unsigned long*) s1;
      a2 = (unsigned long*) s2;
      while (n >= LBLOCKSIZE)
        {
          if (*a1 != *a2) 
   	    break;
          a1++;
          a2++;
          n -= LBLOCKSIZE;
        }

      /* check m mod LBLOCKSIZE remaining characters */

      s1 = (char*)a1;
      s2 = (char*)a2;
    }

  while (n--)
    {
      if (*s1 != *s2)
	return *s1 - *s2;
      s1++;
      s2++;
    }

  return 0;
#endif /* not PREFER_SIZE_OVER_SPEED */
}