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_r|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, 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 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() , scalb() , the Base
Definitions volume of IEEE Std 1003.1-2001, Section 4.18, 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, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .