trusty (3) ilogb.3posix.gz

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 .