logb, logbf, logbl - get exponent of a floating-point value
Math library (libm
, -lm
)
#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
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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 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.
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.
See math_error(7) for information on how to determine whether an error has occurred when calling these functions.
The following errors can occur:
x
is 0A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.
These functions do not set errno
.
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
Thread safety | MT-Safe |
C11, POSIX.1-2008.
C99, POSIX.1-2001.
4.3BSD (see IEEE.3 in the 4.3BSD manual).