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);

[CX] 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 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, the value FP_ILOGB0 shall be returned. [XSI] On XSI-conformant systems, a domain error shall occur; otherwise, a [CX] domain error may occur.

If x is ±Inf, the value {INT_MAX} shall be returned. [XSI] On XSI-conformant systems, a domain error shall occur; otherwise, a [CX] domain error may occur.

If x is a NaN, the value FP_ILOGBNAN shall be returned. [XSI] On XSI-conformant systems, a domain error shall occur; otherwise, a [CX] domain error may occur.

If the correct value is greater than {INT_MAX}, [MX] a domain error shall occur and an unspecified value shall be returned. [XSI] On XSI-conformant systems, a domain error shall occur and {INT_MAX} shall be returned.

If the correct value is less than {INT_MIN}, [MX] a domain error shall occur and an unspecified value shall be returned. [XSI] On XSI-conformant systems, a domain error shall occur and {INT_MIN} shall be returned.

These functions shall fail if:

[XSI|MX] The correct value is not representable as an integer.

[XSI] 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:

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.

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), scalbln(3)

XBD V1_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.

Functionality relating to the XSI option is marked.

ISO/IEC 9899:1999 (“ISO C99”) standard, Technical Corrigendum 2 #48 (SD5-XSH-ERN-71), #49, and #79 (SD5-XSH-ERN-72) are applied.

January 31, 2018 posix.fail