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s_logb.c revision 1.4 
      1 /* @(#)s_logb.c 5.1 93/09/24 */
      2 /*
      3  * ====================================================
      4  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
      5  *
      6  * Developed at SunPro, a Sun Microsystems, Inc. business.
      7  * Permission to use, copy, modify, and distribute this
      8  * software is freely granted, provided that this notice
      9  * is preserved.
     10  * ====================================================
     11  */
     12 
     13 #ifndef lint
     14 static char rcsid[] = "$Id: s_logb.c,v 1.4 1994/03/03 17:04:41 jtc Exp $";
     15 #endif
     16 
     17 /*
     18  * double logb(x)
     19  * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
     20  * Use ilogb instead.
     21  */
     22 
     23 #include <math.h>
     24 #include <machine/endian.h>
     25 
     26 #if BYTE_ORDER == LITTLE_ENDIAN
     27 #define n0	1
     28 #else
     29 #define n0	0
     30 #endif
     31 
     32 #ifdef __STDC__
     33 	double logb(double x)
     34 #else
     35 	double logb(x)
     36 	double x;
     37 #endif
     38 {
     39 	int lx,ix;
     40 	ix = (*(n0+(int*)&x))&0x7fffffff;	/* high |x| */
     41 	lx = *(1-n0+(int*)&x);			/* low x */
     42 	if((ix|lx)==0) return -1.0/fabs(x);
     43 	if(ix>=0x7ff00000) return x*x;
     44 	if((ix>>=20)==0) 			/* IEEE 754 logb */
     45 		return -1022.0;
     46 	else
     47 		return (double) (ix-1023);
     48 }
     49