3. Library calls (functions within program libraries)
LOGB
Section: Linux Programmer's Manual (3)Updated: 20170915
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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():

_ISOC99_SOURCE  _POSIX_C_SOURCE >= 200112L
 _XOPEN_SOURCE >= 500
 /* Since glibc 2.19: */ _DEFAULT_SOURCE
 /* Glibc versions <= 2.19: */ _BSD_SOURCE  _SVID_SOURCE
logbf(), logbl():

_ISOC99_SOURCE  _POSIX_C_SOURCE >= 200112L
 /* Since glibc 2.19: */ _DEFAULT_SOURCE
 /* Glibc versions <= 2.19: */ _BSD_SOURCE  _SVID_SOURCE
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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