1LOGB(3)                    Linux Programmer's Manual                   LOGB(3)
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NAME

6       logb, logbf, logbl - get exponent of a floating-point value
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SYNOPSIS

9       #include <math.h>
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11       double logb(double x);
12       float logbf(float x);
13       long double logbl(long double x);
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15       Link with -lm.
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17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       logb():
20           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
21               || _XOPEN_SOURCE >= 500
22               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
23               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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25       logbf(), logbl():
26           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
27               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
28               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

31       These  functions  extract the exponent from the internal floating-point
32       representation of x and return it as a floating-point value.  The inte‐
33       ger  constant FLT_RADIX, defined in <float.h>, indicates the radix used
34       for the system's floating-point representation.   If  FLT_RADIX  is  2,
35       logb(x) is equal to floor(log2(x)), except that it is probably faster.
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37       If  x is subnormal, logb() returns the exponent x would have if it were
38       normalized.
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RETURN VALUE

41       On success, these functions return the exponent of x.
42
43       If x is a NaN, a NaN is returned.
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45       If x is zero, then a  pole  error  occurs,  and  the  functions  return
46       -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
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48       If  x is negative infinity or positive infinity, then positive infinity
49       is returned.
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ERRORS

52       See math_error(7) for information on how to determine whether an  error
53       has occurred when calling these functions.
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55       The following errors can occur:
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57       Pole error: x is 0
58              A  divide-by-zero  floating-point  exception  (FE_DIVBYZERO)  is
59              raised.
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61       These functions do not set errno.
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ATTRIBUTES

64       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
65       tributes(7).
66
67       ┌────────────────────────────────────────────┬───────────────┬─────────┐
68       │Interface                                   │ Attribute     │ Value   │
69       ├────────────────────────────────────────────┼───────────────┼─────────┤
70       │logb(), logbf(), logbl()                    │ Thread safety │ MT-Safe │
71       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

74       C99, POSIX.1-2001, POSIX.1-2008.
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SEE ALSO

77       ilogb(3), log(3)
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COLOPHON

80       This  page  is  part of release 5.13 of the Linux man-pages project.  A
81       description of the project, information about reporting bugs,  and  the
82       latest     version     of     this    page,    can    be    found    at
83       https://www.kernel.org/doc/man-pages/.
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87                                  2021-03-22                           LOGB(3)
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