s_ilogb.c

标准c库代码,可以应用于各个系统提供了大量的基本函数

/* @(#)s_ilogb.c 5.1 93/09/24 */
/*
 * ====================================================
 * 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
       <<ilogb>>, <<ilogbf>>---get exponent of floating point number
INDEX
	ilogb
INDEX
	ilogbf

ANSI_SYNOPSIS
	#include <math.h>
        int ilogb(double <[val]>);
        int ilogbf(float <[val]>);

TRAD_SYNOPSIS
	#include <math.h>
        int ilogb(<[val]>)
        double <[val]>;

        int ilogbf(<[val]>)
        float <[val]>;


DESCRIPTION

	All non zero, normal numbers can be described as <[m]> *
	2**<[p]>.  <<ilogb>> and <<ilogbf>> examine the argument
	<[val]>, and return <[p]>.  The functions <<frexp>> and
	<<frexpf>> are similar to <<ilogb>> and <<ilogbf>>, but also
	return <[m]>.

RETURNS

<<ilogb>> and <<ilogbf>> return the power of two used to form the
floating point argument.  If <[val]> is <<0>>, they return <<-
INT_MAX>> (<<INT_MAX>> is defined in limits.h).  If <[val]> is
infinite, or NaN, they return <<INT_MAX>>.

PORTABILITY
	Neither <<ilogb>> nor <<ilogbf>> is required by ANSI C or by
	the System V Interface Definition (Issue 2).  */

/* ilogb(double x)
 * return the binary exponent of non-zero x
 * ilogb(0) = 0x80000001
 * ilogb(inf/NaN) = 0x7fffffff (no signal is raised)
 */

#include "fdlibm.h"
#include <limits.h>

#ifdef __STDC__
	int ilogb(double x)
#else
	int ilogb(x)
	double x;
#endif
{
#ifndef _DOUBLE_IS_32BITS
	__int32_t hx,lx,ix;

	EXTRACT_WORDS(hx,lx,x);
	hx &= 0x7fffffff;
	if(hx<0x00100000) {
	    if((hx|lx)==0) 
		return - INT_MAX;	/* ilogb(0) = 0x80000001 */
	    else			/* subnormal x */
		if(hx==0) {
		    for (ix = -1043; lx>0; lx<<=1) ix -=1;
		} else {
		    for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1;
		}
	    return ix;
	}
	else if (hx<0x7ff00000) return (hx>>20)-1023;
	else return INT_MAX;
#else /* defined (_DOUBLE_IS_32BITS) */
	return ilogbf ((float) x);
#endif /* defined (_DOUBLE_IS_32BITS) */
}