logb - Online Linux Manual Page

Section : 3
Updated : 2023-02-05
Source : Linux man-pages 6.03

NAME

logb, logbf, logbl − get exponent of a floating-point value

LIBRARY

Math library

(

libm

−lm

)

SYNOPSIS

#include <math.h>

double logb(double

);

float logbf(float

);

long double logbl(long double

);

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

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(

)

is equal to floor(log2(

)

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

STANDARDS

C99, POSIX.1-2001, POSIX.1-2008.