PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

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

       The  functionality  described  on  this  reference  page is aligned with the ISO C standard. Any conflict
       between the requirements described  here  and  the  ISO C  standard  is  unintentional.  This  volume  of
       POSIX.1‐2017 defers to the ISO C standard.

       These  functions  shall  compute  the  exponent of x, which is the integral part of logr |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,  logb(),  logbf(),  and  logbl()  shall  return  -HUGE_VAL,  -HUGE_VALF,  and  -HUGE_VALL,
       respectively.

       On systems that support the IEC 60559 Floating-Point option, a pole error shall occur;
       otherwise, a pole error may occur.

       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.

       These functions may 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(), scalbln()

       The Base Definitions volume of POSIX.1‐2017, Section 4.20, 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 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .