This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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 .

IEEE/The Open Group                                   2017                                         ILOGB(3POSIX)

       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.

       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.

       Thefollowingsectionsareinformative.

       None.

       None.

       ilogb, ilogbf, ilogbl — return an unbiased exponent

       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.

       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.

       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.

feclearexcept(), fetestexcept(), logb(), scalbln()

       The Base Definitions volume of POSIX.1‐2017, Section4.20, TreatmentofErrorConditionsforMathematicalFunctions, <float.h>, <math.h>

       #include <math.h>

       int ilogb(double x);
       int ilogbf(float x);
       int ilogbl(long double x);