logb(3) Library Functions Manual logb(3)
NAME
logb, logbf, logbl - get exponent of a floating-point value
LIBRARY
Math library (libm, -lm)
SYNOPSIS
#include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
logbf(), logbl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 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 inte-
ger 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 similar to floor(log2(fabs(x))), except that the latter may
give an incorrect integer due to intermediate rounding.
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 attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ logb(), logbf(), logbl() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
C11, POSIX.1-2008.
HISTORY
C99, POSIX.1-2001.
logb() 4.3BSD (see IEEE.3 in the 4.3BSD manual).
SEE ALSO
ilogb(3), log(3)
Linux man-pages 6.7 2024-03-12 logb(3)
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