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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():
           _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 floating-point
       representation of x and return it as a floating-point value.  The inte-
       ger  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.
ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).
       +-------------------------+---------------+---------+
       |Interface                | Attribute     | Value   |
       +-------------------------+---------------+---------+
       |logb(), logbf(), logbl() | Thread safety | MT-Safe |
       +-------------------------+---------------+---------+
CONFORMING TO
       C99, POSIX.1-2001, POSIX.1-2008.
SEE ALSO
       ilogb(3), log(3)
COLOPHON
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.
                                  2017-09-15                           LOGB(3)