logb(3) - f39

1logb(3)                    Library Functions Manual                    logb(3)
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NAME

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

9       Math library (libm, -lm)
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SYNOPSIS

12       #include <math.h>
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14       double logb(double x);
15       float logbf(float x);
16       long double logbl(long double x);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       logb():
21           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
22               || _XOPEN_SOURCE >= 500
23               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
24               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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26       logbf(), logbl():
27           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
28               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
29               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

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

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

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

65       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
66       tributes(7).
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68       ┌────────────────────────────────────────────┬───────────────┬─────────┐
69       │Interface                                   │ Attribute     │ Value   │
70       ├────────────────────────────────────────────┼───────────────┼─────────┤
71       │logb(), logbf(), logbl()                    │ Thread safety │ MT-Safe │
72       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

75       C11, POSIX.1-2008.
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HISTORY

78       C99, POSIX.1-2001.
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80       logb() 4.3BSD (see IEEE.3 in the 4.3BSD manual).
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