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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 ||
       _ISOC99_SOURCE; or cc -std=c99
       logbf(), logbl(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600
       || _ISOC99_SOURCE; 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.

CONFORMING TO

       C99, POSIX.1-2001.

SEE ALSO

       ilogb(3), log(3)

COLOPHON

       This  page  is  part of release 3.24 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/.

                                  2008-08-05