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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
       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  Definitions
       volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Condi-
       tions 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(3P)