# ilogb man page

## 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.

ilogb, ilogbf, ilogbl — return an unbiased exponent

## Synopsis

```
#include <math.h>
int ilogb(double
```*x*);
int ilogbf(float *x*);
int ilogbl(long double *x*);

## 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.1‐2008 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 ${\text{log}}_{r}|x|$ 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

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

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.*

## 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.

## 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.

## See Also

*feclearexcept()*, *fetestexcept()*, *logb()*, *scalbln()*

The Base Definitions volume of POSIX.1‐2008, *Section 4.19*, *Treatment of Error Conditions for Mathematical Functions*, * <float.h>*,

**<math.h>**## Copyright

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 .