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

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 con‐
       verting 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  hap‐
       pen,  {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  subnor‐
       mals   are   not	  supported,   or   {INT_MIN}  must  be	 greater  than
       (LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX)  if	 subnormals  are  sup‐
       ported.

FUTURE DIRECTIONS
       None.

SEE ALSO
       feclearexcept()	, fetestexcept() , logb() , scalb() , the Base Defini‐
       tions 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, Copyright (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.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003			      ILOGB(P)
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