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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 (consult 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
       The functionality described on this reference page is aligned with  the
       ISO C  standard.  Any  conflict between the requirements described here
       and the ISO C standard is unintentional. This  volume  of  POSIX.1-2008
       defers to the ISO C standard.
       These  functions  shall  return  the exponent part of their argument x.
       Formally, the return value is the integral part of logr|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, the value FP_ILOGB0 shall be  returned.   On  XSI-conformant
       systems, a domain error shall occur;
       otherwise, a domain error may occur.
       If  x  is +-Inf, the value {INT_MAX} shall be returned.  On XSI-confor-
       mant systems, a domain error shall occur;
       otherwise, a domain error may occur.
       If x is a NaN, the value FP_ILOGBNAN shall be returned.  On XSI-confor-
       mant systems, a domain error shall occur;
       otherwise, a domain error may occur.
       If  the  correct  value is greater than {INT_MAX}, a domain error shall
       occur and an unspecified value shall be  returned.   On  XSI-conformant
       systems, a domain error shall occur and {INT_MAX} shall be returned.
       If the correct value is less than {INT_MIN}, a domain error shall occur
       and an unspecified value shall be returned.  On XSI-conformant systems,
       a domain error shall occur and {INT_MIN} shall be returned.
ERRORS
       These functions shall fail if:
       Domain Error
                   The correct value is not representable as an integer.
                   The x argument is zero, NaN, or +-Inf.
                   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.
       These functions may fail if:
       Domain Error
                   The x argument is zero, NaN, or +-Inf.
                   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(), scalbln()
       The Base Definitions volume of POSIX.1-2008, Section 4.19, 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, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            ILOGB(3P)