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

       ilogb, ilogbf, ilogbl — return an unbiased exponent

SYNOPSIS

       #include <math.h>

       int ilogb(double x);
       int ilogbf(float x);
       int ilogbl(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  return the exponent part of their argument x.  Formally, the return value is the
       integral part of logr|x| as a signed integral 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 <float.h>.

       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 part of x as a signed integer
       value. They are equivalent to calling the corresponding logb() function and casting the returned value to
       type int.

       If x is 0, the value FP_ILOGB0 shall be returned.  On XSI-conformant systems, a domain error shall occur;
       otherwise, a domain error may occur.

       If  x  is  ±Inf,  the value {INT_MAX} shall be returned.  On XSI-conformant systems, a domain error shall
       occur;
       otherwise, a domain error may occur.

       If x is a NaN, the value FP_ILOGBNAN shall be returned.  On XSI-conformant systems, a domain error  shall
       occur;
       otherwise, a domain error may occur.

       If the correct value is greater than {INT_MAX}, a domain error shall occur and an unspecified value shall
       be returned.  On XSI-conformant systems, a domain error shall occur and {INT_MAX} shall be returned.

       If the correct value is less than {INT_MIN}, a domain error shall occur and an unspecified value shall be
       returned.  On XSI-conformant systems, a domain error shall occur and {INT_MIN} shall be returned.

ERRORS

       These functions shall fail if:

       Domain Error
                   The correct value is not representable as an integer.

                   The x argument is zero, NaN, or ±Inf.

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

       These functions may fail if:

       Domain Error
                   The x argument is zero, NaN, or ±Inf.

                   If  the  integer  expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
                   set to [EDOM].  If the integer expression (math_errhandling &  MATH_ERREXCEPT)  is  non-zero,
                   then the invalid 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

       The errors come from taking the expected floating-point value and converting  it  to  int,  which  is  an
       invalid operation in IEEE Std 754‐1985 (since overflow, infinity, and NaN are not representable in a type
       int), so should be a domain error.

       There are no known implementations that overflow. For overflow to happen, {INT_MAX}  must  be  less  than
       LDBL_MAX_EXP*log2(FLT_RADIX) or {INT_MIN} must be greater than LDBL_MIN_EXP*log2(FLT_RADIX) if subnormals
       are not supported, or {INT_MIN} must  be  greater  than  (LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX)  if
       subnormals are supported.

FUTURE DIRECTIONS

       None.

SEE ALSO

       feclearexcept(), fetestexcept(), logb(), 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 .