sysdeps/ieee754/logb.c

   1 /* Copyright (C) 1992, 1995 Free Software Foundation, Inc.
   2 This file is part of the GNU C Library.
   3
   4 The GNU C Library is free software; you can redistribute it and/or
   5 modify it under the terms of the GNU Library General Public License as
   6 published by the Free Software Foundation; either version 2 of the
   7 License, or (at your option) any later version.
   8
   9 The GNU C Library is distributed in the hope that it will be useful,
  10 but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12 Library General Public License for more details.
  13
  14 You should have received a copy of the GNU Library General Public
  15 License along with the GNU C Library; see the file COPYING.LIB.  If
  16 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  17 Cambridge, MA 02139, USA.  */
  18
  19 #include <ansidecl.h>
  20 #include <math.h>
  21 #include <float.h>
  22 #include "ieee754.h"
  23
  24 /* Return the base 2 signed integral exponent of X.  */
  25 double
  26 DEFUN(__logb, (x), double x)
  27 {
  28   union ieee754_double u;
  29
  30   if (__isnan (x))
  31     return x;
  32   else if (__isinf (x))
  33     return HUGE_VAL;
  34   else if (x == 0.0)
  35     return - HUGE_VAL;
  36
  37   u.d = x;
  38
  39   if (u.ieee.exponent == 0)
  40     /* A denormalized number.
  41        Multiplying by 2 ** DBL_MANT_DIG normalizes it;
  42        we then subtract the DBL_MANT_DIG we added to the exponent.  */
  43     return (__logb (x * ldexp (1.0, DBL_MANT_DIG)) - DBL_MANT_DIG);
  44
  45   return (int) u.ieee.exponent - (DBL_MAX_EXP - 1);
  46 }
  47
  48 weak_alias (__logb, logb)