LOGB(3)
 LOGB(3) Linux Programmer's Manual LOGB(3)

# NAME

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 );

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

logb():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or cc -std=c99

logbf(), logbl():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or cc -std=c99

# 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.

# CONFORMING TO

C99, POSIX.1-2001.