вернуть несмещенную экспоненту (return an unbiased exponent)
Пролог (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.
Имя (Name)
ilogb, ilogbf, ilogbl — return an unbiased exponent
Синопсис (Synopsis)
#include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
Описание (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.1‐2017 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.
Возвращаемое значение (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-
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.
Ошибки (Error)
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.
Примеры (Examples)
None.
Использование в приложениях (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.
Обоснование (Rationale)
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.
Будущие направления (Future directions)
None.
Смотри также (See also)
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)
