ilogb man page
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.
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
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 ${\text{log}}_{r}x$ as a signed integral value, for nonzero x, where r is the radix of the machine's floatingpoint 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 nonzero or fetestexcept(FE_INVALID  FE_DIVBYZERO  FE_OVERFLOW  FE_UNDERFLOW) is nonzero, 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 XSIconformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If x is ±Inf, the value {INT_MAX} shall be returned. On XSIconformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If x is a NaN, the value FP_ILOGBNAN shall be returned. On XSIconformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If the correct value is greater than {INT_MAX}, a domain error shall occur and an unspecified value shall be returned. On XSIconformant systems, a domain error shall occur and {INT_MAX} shall be returned.
If the correct value is less than {INT_MIN}, a domain error shall occur and an unspecified value shall be returned. On XSIconformant systems, a domain error shall occur and {INT_MIN} shall be returned.
Errors
These functions shall fail if:
 Domain Error

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 nonzero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is nonzero, then the invalid floatingpoint 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 nonzero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is nonzero, then the invalid floatingpoint 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 nonzero.
Rationale
The errors come from taking the expected floatingpoint 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_EXPLDBL_MANT_DIG)*log2(FLT_RADIX) if subnormals are supported.
Future Directions
None.
See Also
feclearexcept(), fetestexcept(), logb(), scalbln()
The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>
Copyright
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology  Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.12008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/manpages/reporting_bugs.html .