| 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 | You can use the (non-ANSI) function <<matherr>> to specify error |
|---|
| 42 | handling for these functions, indirectly through the respective <<log>> |
|---|
| 43 | function. |
|---|
| 44 | |
|---|
| 45 | RETURNS |
|---|
| 46 | The <<log2>> functions return |
|---|
| 47 | @ifnottex |
|---|
| 48 | <<log base-2(<[x]>)>> |
|---|
| 49 | @end ifnottex |
|---|
| 50 | @tex |
|---|
| 51 | $log_2(x)$ |
|---|
| 52 | @end tex |
|---|
| 53 | on success. |
|---|
| 54 | When <[x]> is zero, the |
|---|
| 55 | returned value is <<-HUGE_VAL>> and <<errno>> is set to <<ERANGE>>. |
|---|
| 56 | When <[x]> is negative, the returned value is NaN (not a number) and |
|---|
| 57 | <<errno>> is set to <<EDOM>>. You can control the error behavior via |
|---|
| 58 | <<matherr>>. |
|---|
| 59 | |
|---|
| 60 | PORTABILITY |
|---|
| 61 | C99, POSIX, System V Interface Definition (Issue 6). |
|---|
| 62 | */ |
|---|
| 63 | |
|---|
| 64 | /* |
|---|
| 65 | * wrapper log2(x) |
|---|
| 66 | */ |
|---|
| 67 | |
|---|
| 68 | #include "fdlibm.h" |
|---|
| 69 | #include <errno.h> |
|---|
| 70 | #include <math.h> |
|---|
| 71 | #undef log2 |
|---|
| 72 | |
|---|
| 73 | #ifndef _DOUBLE_IS_32BITS |
|---|
| 74 | |
|---|
| 75 | #ifdef __STDC__ |
|---|
| 76 | double log2(double x) /* wrapper log2 */ |
|---|
| 77 | #else |
|---|
| 78 | double log2(x) /* wrapper log2 */ |
|---|
| 79 | double x; |
|---|
| 80 | #endif |
|---|
| 81 | { |
|---|
| 82 | return (log(x) / M_LN2); |
|---|
| 83 | } |
|---|
| 84 | |
|---|
| 85 | #endif /* defined(_DOUBLE_IS_32BITS) */ |
|---|
