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 4.16 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)