Name
logb, logbf, logbl - get exponent of a floating-point valueLibrary
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 integer constant FLT_RADIX ,defined in<float.h>
, indicates the radix used for the system's floating-point representation. If FLT_RADIX is 2, is equal to 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 ofx
. 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 |
T} | 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).