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 .