1LOGB(3P)                   POSIX Programmer's Manual                  LOGB(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.
10

NAME

12       logb, logbf, logbl — radix-independent exponent
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SYNOPSIS

15       #include <math.h>
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17       double logb(double x);
18       float logbf(float x);
19       long double logbl(long double x);
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DESCRIPTION

22       The functionality described on this reference page is aligned with  the
23       ISO C  standard.  Any  conflict between the requirements described here
24       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2017
25       defers to the ISO C standard.
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27       These  functions shall compute the exponent of x, which is the integral
28       part of logr |x|, as a signed floating-point  value,  for  non-zero  x,
29       where  r is the radix of the machine's floating-point arithmetic, which
30       is the value of FLT_RADIX defined in the <float.h> header.
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32       If x is subnormal it is treated as though it were normalized; thus  for
33       finite positive x:
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35
36           1 <= x * FLT_RADIX-logb(x) < FLT_RADIX
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38       An  application  wishing to check for error situations should set errno
39       to zero and  call  feclearexcept(FE_ALL_EXCEPT)  before  calling  these
40       functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
41       FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero,  an  error  has
42       occurred.
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RETURN VALUE

45       Upon  successful  completion, these functions shall return the exponent
46       of x.
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48       If x is ±0,  logb(),  logbf(),  and  logbl()  shall  return  -HUGE_VAL,
49       -HUGE_VALF, and -HUGE_VALL, respectively.
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51       On  systems  that  support  the IEC 60559 Floating-Point option, a pole
52       error shall occur;
53       otherwise, a pole error may occur.
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55       If x is NaN, a NaN shall be returned.
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57       If x is ±Inf, +Inf shall be returned.
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ERRORS

60       These functions shall fail if:
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62       Pole Error  The value of x is ±0.
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64                   If the integer expression (math_errhandling  &  MATH_ERRNO)
65                   is  non-zero,  then errno shall be set to [ERANGE].  If the
66                   integer expression (math_errhandling &  MATH_ERREXCEPT)  is
67                   non-zero,  then the divide-by-zero floating-point exception
68                   shall be raised.
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70       These functions may fail if:
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72       Pole Error  The value of x is 0.
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74                   If the integer expression (math_errhandling  &  MATH_ERRNO)
75                   is  non-zero,  then errno shall be set to [ERANGE].  If the
76                   integer expression (math_errhandling &  MATH_ERREXCEPT)  is
77                   non-zero,  then the divide-by-zero floating-point exception
78                   shall be raised.
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80       The following sections are informative.
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EXAMPLES

83       None.
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APPLICATION USAGE

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

91       None.
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FUTURE DIRECTIONS

94       None.
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SEE ALSO

97       feclearexcept(), fetestexcept(), ilogb(), scalbln()
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99       The Base Definitions volume of POSIX.1‐2017, Section 4.20, Treatment of
100       Error Conditions for Mathematical Functions, <float.h>, <math.h>
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103       Portions  of  this text are reprinted and reproduced in electronic form
104       from IEEE Std 1003.1-2017, Standard for Information Technology --  Por‐
105       table  Operating System Interface (POSIX), The Open Group Base Specifi‐
106       cations Issue 7, 2018 Edition, Copyright (C) 2018 by the  Institute  of
107       Electrical  and  Electronics Engineers, Inc and The Open Group.  In the
108       event of any discrepancy between this version and the original IEEE and
109       The  Open Group Standard, the original IEEE and The Open Group Standard
110       is the referee document. The original Standard can be  obtained  online
111       at http://www.opengroup.org/unix/online.html .
112
113       Any  typographical  or  formatting  errors that appear in this page are
114       most likely to have been introduced during the conversion of the source
115       files  to  man page format. To report such errors, see https://www.ker
116       nel.org/doc/man-pages/reporting_bugs.html .
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120IEEE/The Open Group                  2017                             LOGB(3P)
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