logbl (3) Linux Manual Page
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 |