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.1-2024 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 \(\log _r|x|\) 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.

^[MX] When the correct result is representable in the range of the return type, the returned value shall be exact and shall be independent of the current rounding direction mode.

If x is 0, the value FP_ILOGB0 shall be returned. ^[XSI|MX]  On XSI-conformant systems and on systems that support the IEC 60559 Floating-Point option, a domain error shall occur;  otherwise, a
^[CX] domain  error may occur.

If x is ±Inf, the value {INT_MAX} shall be returned. ^[XSI|MX]  On XSI-conformant systems and on systems that support the IEC 60559 Floating-Point option, 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|MX]  On XSI-conformant systems and on systems that support the IEC 60559 Floating-Point option, a domain error shall occur; otherwise, a ^[CX]  domain  error may occur.

If the correct value is greater than {INT_MAX} or less than {INT_MIN}, an unspecified value shall be returned. ^[XSI] On XSI-conformant systems, a domain error shall occur and {INT_MAX} or {INT_MIN}, respectively, shall be returned;
^[MX] if the IEC 60559 Floating-Point option is supported, a domain error shall occur;  otherwise, a domain error or range error may occur.

ERRORS

These functions shall fail if:

Domain Error

^[XSI|MX] 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.

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 4.23 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.

Issue 8

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.