LOGB
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
logb, logbf, logbl - get exponent of a floating-point value
SYNOPSIS
#include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
Link with -lm.
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
logb():
-
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
logbf(),
logbl():
-
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTION
These functions extract the exponent from the
internal floating-point representation of
x
and return it as a floating-point value.
The integer constant
FLT_RADIX,
defined in
<float.h>,
indicates the radix used for the system's floating-point representation.
If
FLT_RADIX
is 2,
logb(x)
is equal to
floor(log2(x)),
except that it is probably faster.
If
x
is subnormal,
logb()
returns the exponent
x
would have if it were normalized.
RETURN VALUE
On success, these functions return the exponent of
x.
If
x
is a NaN,
a NaN is returned.
If
x
is zero, then a pole error occurs, and the functions return
-HUGE_VAL,
-HUGE_VALF,
or
-HUGE_VALL,
respectively.
If
x
is negative infinity or positive infinity, then
positive infinity is returned.
ERRORS
See
math_error(7)
for information on how to determine whether an error has occurred
when calling these functions.
The following errors can occur:
- Pole error: x is 0
-
A divide-by-zero floating-point exception
(FE_DIVBYZERO)
is raised.
These functions do not set
errno.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
logb(),
logbf(),
logbl()
| Thread safety | MT-Safe
|
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
SEE ALSO
ilogb(3),
log(3)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- SEE ALSO
-
- COLOPHON
-
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- Running on 
-
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