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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 .