logb(3) Linux Man Page - System Grab Bag

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 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 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). allbox; lbx lb lb T{ logb()logbf()logbl()

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).