PROLOG  NAME  SYNOPSIS  DESCRIPTION  RETURN VALUE  ERRORS  EXAMPLES  APPLICATION USAGE  RATIONALE  FUTURE DIRECTIONS  SEE ALSO  COPYRIGHT 

ILOGB(3P) POSIX Programmer's Manual ILOGB(3P)
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. delim $$
ilogb, ilogbf, ilogbl — return an unbiased exponent
#include <math.h> int ilogb(double x); int ilogbf(float x); int ilogbl(long double x);
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 $log sub{r}x$ 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.
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 XSI conformant 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 XSI conformant 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.
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.
None.
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.
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.
None.
feclearexcept(3p), fetestexcept(3p), logb(3p), scalbln(3p) The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, float.h(0p), math.h(0p)
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 .
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https://www.kernel.org/doc/manpages/reporting_bugs.html .
IEEE/The Open Group 2013 ILOGB(3P)