
ILOGB(P) POSIX Programmer's Manual ILOGB(P)
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
These functions shall return the exponent part of their argument x. Formally, the return
value is the integral part of log_rx 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 non
zero 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, a domain error shall occur, and the value FP_ILOGB0 shall be returned.
If x is +Inf, a domain error shall occur, and the value {INT_MAX} shall be returned.
If x is a NaN, a domain error shall occur, and the value FP_ILOGBNAN shall be returned.
If the correct value is greater than {INT_MAX}, {INT_MAX} shall be returned and a domain
error shall occur.
If the correct value is less than {INT_MIN}, {INT_MIN} shall be returned and a domain
error shall occur.
ERRORS
These functions shall fail if:
Domain Error
The x argument is zero, NaN, or +Inf, or the correct value is not representable as
an integer.
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 7541985 (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() , scalb() , the Base Definitions volume of
IEEE Std 1003.12001, Section 4.18, Treatment of Error Conditions for Mathematical Func
tions, <float.h>, <math.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology  Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 20012003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 orig
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 ILOGB(P) 
