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ILOGB(3P)                  POSIX Programmer's Manual                 ILOGB(3P)

       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

       ilogb, ilogbf, ilogbl - return an unbiased exponent

       #include <math.h>

       int ilogb(double x);
       int ilogbf(float x);
       int ilogbl(long double x);

       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 feclearex-
       cept(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.

       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.

       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 excep-
       tion shall be raised.

       The following sections are informative.


       On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are  indepen-
       dent of each other, but at least one of them must be non-zero.

       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


       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>

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The original Standard can be
       obtained online at .

IEEE/The Open Group                  2003                            ILOGB(3P)