Subversion Repositories openrisc_2011-10-31

/* @(#)s_log2.c 5.1 93/09/24 */

/* Modification from s_exp10.c Yaakov Selkowitz 2009.  */

/*

 * ====================================================

 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.

 *

 * Developed at SunPro, a Sun Microsystems, Inc. business.

 * Permission to use, copy, modify, and distribute this

 * software is freely granted, provided that this notice

 * is preserved.

 * ====================================================

 */

/*

FUNCTION

        <<log2>>, <<log2f>>--base 2 logarithm

INDEX

        log2

INDEX

        log2f

ANSI_SYNOPSIS

        #include <math.h>

        double log2(double <[x]>);

        float log2f(float <[x]>);

DESCRIPTION

The <<log2>> functions compute the base-2 logarithm of <[x]>.  A domain error

occurs if the argument is less than zero.  A range error occurs if the

argument is zero.

The Newlib implementations are not full, intrinisic calculations, but

rather are derivatives based on <<log>>.  (Accuracy might be slightly off from

a direct calculation.)  In addition to functions, they are also implemented as

macros defined in math.h:

. #define log2(x) (log (x) / _M_LOG2_E)

. #define log2f(x) (logf (x) / (float) _M_LOG2_E)

To use the functions instead, just undefine the macros first.

You can use the (non-ANSI) function <<matherr>> to specify error

handling for these functions, indirectly through the respective <<log>>

function.

RETURNS

The <<log2>> functions return

@ifnottex

<<log base-2(<[x]>)>>

@end ifnottex

@tex

$log_2(x)$

@end tex

on success.

When <[x]> is zero, the

returned value is <<-HUGE_VAL>> and <<errno>> is set to <<ERANGE>>.

When <[x]> is negative, the returned value is NaN (not a number) and

<<errno>> is set to <<EDOM>>.  You can control the error behavior via

<<matherr>>.

PORTABILITY

C99, POSIX, System V Interface Definition (Issue 6).

*/

/*

 * wrapper log2(x)

 */

#include "fdlibm.h"

#include <errno.h>

#include <math.h>

#undef log2

#ifndef _DOUBLE_IS_32BITS

#ifdef __STDC__

        double log2(double x)           /* wrapper log2 */

#else

        double log2(x)                  /* wrapper log2 */

        double x;

#endif

{

  return (log(x) / M_LOG2_E);

}

#endif /* defined(_DOUBLE_IS_32BITS) */