ilogb(3p) — Linux manual page

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.

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.1‐2017 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         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.1‐2017, Section 4.20,
       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 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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:https://www.opengroup.org/unix/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               2017                         ILOGB(3P)

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