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 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() , 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, 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 .