IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group A newer edition of this document exists here
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.1-2017 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]
otherwise, a [CX]error may occur.
If x is ±Inf, the value {INT_MAX} shall be returned. [XSI]
otherwise, a [CX]If x is a NaN, the value FP_ILOGBNAN shall be returned. [XSI]
otherwise, a [CX]If the correct value is greater than {INT_MAX}, [MX]
If the correct value is less than {INT_MIN}, [MX]
ERRORS
These functions shall fail if:
- Domain Error
- [XSI|MX]
[XSI]
[XSI|MX]
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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