Linux manual pages Section 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);
 
[Note] Note
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
[Note] Note

Link with −lm.

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.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
logb(), logbf(), logbl() Thread safety MT-Safe

CONFORMING TO

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

SEE ALSO

ilogb(3), log(3)

COLOPHON

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