The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> 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

> [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.1-2008 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 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.

#### 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. [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.

#### ERRORS

> These functions shall fail if:

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

>   [XSI] The x argument is zero, NaN, or ±Inf.

>   [XSI|MX] 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.

#### 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, scalbln

> XBD Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>

#### CHANGE HISTORY

> First released in Issue 4, Version 2.

#### Issue 5

> Moved from X/OPEN UNIX extension to BASE.

#### Issue 6

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

> The ilogbf() and ilogbl() functions are added for alignment with the ISO/IEC 9899:1999 standard.

> The RETURN VALUE section is revised for alignment with the ISO/IEC 9899:1999 standard.

> Functionality relating to the XSI option is marked.

#### Issue 7

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

End of informative text.

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