ILOGB(3P)                                           POSIX Programmer's Manual                                          ILOGB(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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
       These  functions  shall return the exponent part of their argument x.  Formally, the return value is the integral part of
       log_r|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,  a domain error shall occur, and the value FP_ILOGB0 shall be returned.

       If x is +-Inf,  a domain error shall occur, and the value {INT_MAX} shall be returned.

       If x is a NaN,  a domain error shall occur, and the value FP_ILOGBNAN shall be returned.

       If the correct value is greater than {INT_MAX}, {INT_MAX} shall be returned and a domain error shall occur.

       If the correct value is less than {INT_MIN}, {INT_MIN} shall be returned and a domain error shall occur.

ERRORS
       These functions shall fail if:

       Domain Error
              The x argument is zero, NaN, or +-Inf, or the correct value is not representable as an integer.

       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  sup-
       ported, 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(),  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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     ILOGB(3P)