# ilogb (3p)

# 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.# NAME

ilogb, ilogbf, ilogbl — return an unbiased exponent# SYNOPSIS

#include <math.h>

int ilogb(doublex); int ilogbf(floatx); int ilogbl(long doublex);

# 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 ${\mathrm{log}}_{r}\left|x\right|$ 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;

*x*is ±Inf, the value {INT_MAX} shall be returned. On XSI-conformant systems, a domain error shall occur;

*x*is a NaN, the value FP_ILOGBNAN shall be returned. On XSI-conformant systems, a domain error shall occur;

# ERRORS

These functions shall fail if:- Domain Error
- The correct value is not representable as an integer.

The

These functions may fail if:
*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.- 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

None.# 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.

# FUTURE DIRECTIONS

None.# SEE ALSO

*,*

*feclearexcept*()*,*

*fetestexcept*()*,*

*logb*()*The Base Definitions volume of POSIX.1‐2008,*

*scalbln*()*Section 4.19*,

*Treatment of Error Conditions for Mathematical Functions*,

*,*

**<float.h>**

**<math.h>**