Man page - logb(3)

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Manual

logb

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
STANDARDS
HISTORY
SEE ALSO

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, logb( x ) is similar to floor(log2(fabs( x ))) , except that the latter may give an incorrect integer due to intermediate rounding.

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).

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STANDARDS

C11, POSIX.1-2008.

HISTORY

C99, POSIX.1-2001.

logb ()

4.3BSD (see IEEE.3 in the 4.3BSD manual).

SEE ALSO

ilogb (3), log (3)