logb, logbf, logbl - get exponent of a floating-point value
#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
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.
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:
- Pole error: x is 0
- A 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 |
logb (), logbf (), logbl () |
Thread safety |
MT-Safe |
C99, POSIX.1-2001, POSIX.1-2008.
ilogb(3),
log(3)