The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 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] [Option Start] 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. [Option End]

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] [Option Start]  On XSI-conformant systems, a domain error shall occur; [Option End]
otherwise, a [CX] [Option Start]  domain [Option End]  error may occur.

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

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

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

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

ERRORS

These functions shall fail if:

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

[XSI] [Option Start] The x argument is zero, NaN, or ±Inf. [Option End]

[XSI|MX] [Option Start] 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. [Option End]

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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