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

DESCRIPTION
These functions extract the exponent of x and return it as a floating-point value. If FLT_RADIX is two, logb(x) is equal to floor(log2(x)), except that it is probably faster.

If x is de-normalized, logb() returns the exponent x would have if it were normalized.

If x is zero, -HUGE_VAL (resp. -HUGE_VALF, -HUGE_VALL) is returned, and a pole error occurs. If x is infinite, plus infinity is returned. If x is NaN, NaN is returned.

ERRORS
In order to check for errors, set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is nonzero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has occurred.

If an error occurs and (math_errhandling & MATH_ERRNO) is nonzero, then errno is set to ERANGE. If an error occurs and (math_errhandling & MATH_ERREXCEPT) is nonzero, then the divide-by-zero floating-point exception is raised.

A pole error occurs when x is zero.

CONFORMING TO
C99

SEE ALSO
log(3), ilogb(3)

COLOPHON
This page is part of release 2.79 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/.