NAME

       logb, logbf, logbl - radix-independent exponent

SYNOPSIS

       #include <math.h>

       double logb(double x);
       float logbf(float x);
       long double logbl(long double x);

DESCRIPTION

       These  functions  shall  compute  the exponent of x, which is the integral part of log_r |x|, as a signed
       floating-point value, for non-zero x, where r is the radix of the  machine's  floating-point  arithmetic,
       which is the value of FLT_RADIX defined in the <float.h> header.

       If x is subnormal it is treated as though it were normalized; thus for finite positive x:

              1 <= x * FLT_RADIX**-logb(x) < FLT_RADIX

       An   application   wishing   to   check   for  error  situations  should  set  errno  to  zero  and  call
       feclearexcept(FE_ALL_EXCEPT) before calling  these  functions.   On  return,  if  errno  is  non-zero  or
       fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

RETURN VALUE

       Upon successful completion, these functions shall return the exponent of x.

       If x is ±0, a pole error shall occur and logb(), logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF,
       and -HUGE_VALL, respectively.

       If x is NaN, a NaN shall be returned.

       If x is ±Inf, +Inf shall be returned.

ERRORS

       These functions shall fail if:

       Pole Error
              The value of x is ±0.

       If  the  integer  expression  (math_errhandling  &  MATH_ERRNO)  is  non-zero, then errno shall be set to
       [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the  divide-by-
       zero floating-point exception shall be raised.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       On  error,  the  expressions  (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are
       independent of each other, but at least one of them must be non-zero.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       feclearexcept()  ,  fetestexcept()  ,  ilogb()   ,   scalb()   ,   the   Base   Definitions   volume   of
       IEEE Std 1003.1-2001,  Section 4.18, Treatment of Error Conditions for Mathematical Functions, <float.h>,
       <math.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc  and  The  Open Group. In the event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                               LOGB(P)