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():
           _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 floating-point representation of  x
       and  return  it  as  a  floating-point  value.  The integer 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
   Multithreading (see pthreads(7))
       The logb(), logbf(), and logbl() functions are thread-safe.

CONFORMING TO
       C99, POSIX.1-2001.

SEE ALSO
       ilogb(3), log(3)

COLOPHON
       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



                                            2014-02-28                                    LOGB(3)