PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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.

       delim $$

NAME         top

       ilogb, ilogbf, ilogbl — return an unbiased exponent

SYNOPSIS         top

       #include <math.h>

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

DESCRIPTION         top

       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 $log sub{r}|x|$ 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         top

       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-
       conformant 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-
       conformant 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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       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         top

       None.

SEE ALSO         top

       feclearexcept(3p), fetestexcept(3p), logb(3p), scalbln(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment
       of Error Conditions for Mathematical Functions, float.h(0p),
       math.h(0p)

COPYRIGHT         top

       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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           ILOGB(3P)

Pages that refer to this page: math.h(0p)expm1(3p)logb(3p)