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 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
2008-08-05