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);

Link with -lm.

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

logb(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or cc -std=c99
logbf(), logbl(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; 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.

SEE ALSO
ilogb(3), log(3)

COLOPHON
This page is part of release 3.09 of the Linux man-pages project. A description of the project, and information about reporting documentation bugs, can be found at http://www.kernel.org/doc/man-pages/.