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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);
 
Link with -lm.
 

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.

SEE ALSO

ilogb(3), log(3)

COLOPHON

This page is part of release 3.44 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/.
2010-09-20