1LOG2(3P)                   POSIX Programmer's Manual                  LOG2(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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11

NAME

13       log2, log2f, log2l — compute base 2 logarithm functions
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SYNOPSIS

16       #include <math.h>
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18       double log2(double x);
19       float log2f(float x);
20       long double log2l(long double x);
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DESCRIPTION

23       The functionality described on this reference page is aligned with  the
24       ISO C  standard.  Any  conflict between the requirements described here
25       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2008
26       defers to the ISO C standard.
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28       These functions shall compute the base 2 logarithm of their argument x,
29       log2(x).
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31       An application wishing to check for error situations should  set  errno
32       to  zero  and  call  feclearexcept(FE_ALL_EXCEPT)  before calling these
33       functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
34       FE_DIVBYZERO  |  FE_OVERFLOW  | FE_UNDERFLOW) is non-zero, an error has
35       occurred.
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RETURN VALUE

38       Upon successful completion, these functions shall  return  the  base  2
39       logarithm of x.
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41       If  x  is ±0, a pole error shall occur and log2(), log2f(), and log2l()
42       shall return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively.
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44       For finite values of x that are less than 0, or if x is −Inf, a  domain
45       error  shall  occur, and either a NaN (if supported), or an implementa‐
46       tion-defined value shall be returned.
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48       If x is NaN, a NaN shall be returned.
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50       If x is 1, +0 shall be returned.
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52       If x is +Inf, x shall be returned.
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ERRORS

55       These functions shall fail if:
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57       Domain Error
58                   The finite value of x is less than zero, or x is −Inf.
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60                   If the integer expression (math_errhandling  &  MATH_ERRNO)
61                   is  non-zero,  then  errno  shall be set to [EDOM].  If the
62                   integer expression (math_errhandling &  MATH_ERREXCEPT)  is
63                   non-zero,  then  the invalid floating-point exception shall
64                   be raised.
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66       Pole Error  The value of x is zero.
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68                   If the integer expression (math_errhandling  &  MATH_ERRNO)
69                   is  non-zero,  then errno shall be set to [ERANGE].  If the
70                   integer expression (math_errhandling &  MATH_ERREXCEPT)  is
71                   non-zero,  then the divide-by-zero floating-point exception
72                   shall be raised.
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74       The following sections are informative.
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EXAMPLES

77       None.
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APPLICATION USAGE

80       On  error,  the  expressions  (math_errhandling   &   MATH_ERRNO)   and
81       (math_errhandling  & MATH_ERREXCEPT) are independent of each other, but
82       at least one of them must be non-zero.
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RATIONALE

85       None.
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FUTURE DIRECTIONS

88       None.
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SEE ALSO

91       feclearexcept(), fetestexcept(), log()
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93       The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of
94       Error Conditions for Mathematical Functions, <math.h>
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97       Portions  of  this text are reprinted and reproduced in electronic form
98       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
99       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
100       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
101       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
102       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  In  the
103       event of any discrepancy between this version and the original IEEE and
104       The Open Group Standard, the original IEEE and The Open Group  Standard
105       is  the  referee document. The original Standard can be obtained online
106       at http://www.unix.org/online.html .
107
108       Any typographical or formatting errors that appear  in  this  page  are
109       most likely to have been introduced during the conversion of the source
110       files to man page format. To report such errors,  see  https://www.ker
111       nel.org/doc/man-pages/reporting_bugs.html .
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115IEEE/The Open Group                  2013                             LOG2(3P)
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