newlib/libm/common/s_log2.c

   1 /* @(#)s_log2.c 5.1 93/09/24 */
   2 /* Modification from s_exp10.c Yaakov Selkowitz 2009.  */
   3
   4 /*
   5  * ====================================================
   6  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   7  *
   8  * Developed at SunPro, a Sun Microsystems, Inc. business.
   9  * Permission to use, copy, modify, and distribute this
  10  * software is freely granted, provided that this notice
  11  * is preserved.
  12  * ====================================================
  13  */
  14
  15 /*
  16 FUNCTION
  17         <<log2>>, <<log2f>>---base 2 logarithm
  18 INDEX
  19         log2
  20 INDEX
  21         log2f
  22
  23 SYNOPSIS
  24         #include <math.h>
  25         double log2(double <[x]>);
  26         float log2f(float <[x]>);
  27
  28 DESCRIPTION
  29 The <<log2>> functions compute the base-2 logarithm of <[x]>.  A domain error
  30 occurs if the argument is less than zero.  A range error occurs if the
  31 argument is zero.
  32
  33 The Newlib implementations are not full, intrinisic calculations, but
  34 rather are derivatives based on <<log>>.  (Accuracy might be slightly off from
  35 a direct calculation.)  In addition to functions, they are also implemented as
  36 macros defined in math.h:
  37 . #define log2(x) (log (x) / _M_LN2)
  38 . #define log2f(x) (logf (x) / (float) _M_LN2)
  39 To use the functions instead, just undefine the macros first.
  40
  41 RETURNS
  42 The <<log2>> functions return
  43 @ifnottex
  44 <<log base-2(<[x]>)>>
  45 @end ifnottex
  46 @tex
  47 $log_2(x)$
  48 @end tex
  49 on success.
  50 When <[x]> is zero, the
  51 returned value is <<-HUGE_VAL>> and <<errno>> is set to <<ERANGE>>.
  52 When <[x]> is negative, the returned value is NaN (not a number) and
  53 <<errno>> is set to <<EDOM>>.
  54
  55 PORTABILITY
  56 C99, POSIX, System V Interface Definition (Issue 6).
  57 */
  58
  59 /*
  60  * wrapper log2(x)
  61  */
  62
  63 #include "fdlibm.h"
  64 #if __OBSOLETE_MATH
  65 #include <errno.h>
  66 #include <math.h>
  67 #undef log2
  68
  69 #ifndef _DOUBLE_IS_32BITS
  70
  71 #ifdef __STDC__
  72         double log2(double x)           /* wrapper log2 */
  73 #else
  74         double log2(x)                  /* wrapper log2 */
  75         double x;
  76 #endif
  77 {
  78   return (log(x) / M_LN2);
  79 }
  80
  81 #endif /* defined(_DOUBLE_IS_32BITS) */
  82 #endif /* __OBSOLETE_MATH */