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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(): _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,  respec-
       tively.

       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 func-
       tions.

       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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2008-08-05                           LOGB(3)