1LOGB(3)                    Linux Programmer's Manual                   LOGB(3)
2
3
4

NAME

6       logb, logbf, logbl - get exponent of a floating-point value
7

SYNOPSIS

9       #include <math.h>
10
11       double logb(double x);
12       float logbf(float x);
13       long double logbl(long double x);
14
15       Link with -lm.
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18
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
24
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
29

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.
36
37       If  x is subnormal, logb() returns the exponent x would have if it were
38       normalized.
39

RETURN VALUE

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

ERRORS

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

ATTRIBUTES

64       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
65       tributes(7).
66
67       ┌────────────────────────────────────────────┬───────────────┬─────────┐
68Interface                                   Attribute     Value   
69       ├────────────────────────────────────────────┼───────────────┼─────────┤
70logb(), logbf(), logbl()                    │ Thread safety │ MT-Safe │
71       └────────────────────────────────────────────┴───────────────┴─────────┘
72

CONFORMING TO

74       C99, POSIX.1-2001, POSIX.1-2008.
75

SEE ALSO

77       ilogb(3), log(3)
78

COLOPHON

80       This  page  is  part of release 5.12 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/.
84
85
86
87                                  2021-03-22                           LOGB(3)
Impressum