3. Library calls (functions within program libraries)
LOGB
Section: Linux Programmer's Manual (3)Updated: 20150419
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NAME
logb, logbf, logbl  get exponent of a floatingpoint valueSYNOPSIS
#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 floatingpoint representation of x and return it as a floatingpoint value. The integer constant FLT_RADIX, defined in <float.h>, indicates the radix used for the system's floatingpoint 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 dividebyzero floatingpoint 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  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.SEE ALSO
ilogb(3), log(3)COLOPHON
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