ILOGB(3) Library Functions Manual ILOGB(3)

ilogb ilogbf ilogbl — return an unbiased exponent

#include <math.h>

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

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 IEEE Std 1003.1-2001 (“POSIX.1”) 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 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 (FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or (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(3) function and casting the returned value to type int.

If x is 0, [XSI] a domain error shall occur, and the value FP_ILOGB0 shall be returned.

If x is ±Inf, [XSI] a domain error shall occur, and the value {INT_MAX} shall be returned.

If x is a NaN, [XSI] a domain error shall occur, and the value FP_ILOGBNAN shall be returned.

[XSI] 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:

[XSI] 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.

None.

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.

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.

None.

feclearexcept(3) , fetestexcept(3) , logb(3) , scalb(3) , the Base Definitions volume of IEEE Std 1003.1-2001 (“POSIX.1”), xbd_chap04(7), <float.h>, <math.h>

First released in Issue 4, Version 2.

Moved from X/OPEN UNIX extension to BASE.

The ilogb () function is no longer marked as an extension.

The () and () functions are added for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

The RETURN VALUE section is revised for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

XSI extensions are marked.

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December 6, 2001 posix.fail