.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "ILOGB" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" ilogb 
.SH 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.
.SH NAME
ilogb, ilogbf, ilogbl \- return an unbiased exponent
.SH SYNOPSIS
.LP
\fB#include <math.h>
.br
.sp
int ilogb(double\fP \fIx\fP\fB);
.br
int ilogbf(float\fP \fIx\fP\fB);
.br
int ilogbl(long double\fP \fIx\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
These functions shall return the exponent part of their argument \fIx\fP.
Formally, the return value is the integral part of
log_r|x| as a signed integral value, for non-zero \fIx\fP, where \fIr\fP
is the radix of the machine's floating-point
arithmetic, which is the value of FLT_RADIX defined in \fI<float.h>\fP.
.LP
An application wishing to check for error situations should set \fIerrno\fP
to zero and call
\fIfeclearexcept\fP(FE_ALL_EXCEPT) before calling these functions.
On return, if \fIerrno\fP is non-zero or
\fIfetestexcept\fP(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an error has occurred.
.SH RETURN VALUE
.LP
Upon successful completion, these functions shall return the exponent
part of \fIx\fP as a signed integer value. They are
equivalent to calling the corresponding \fIlogb\fP() function and
casting the returned value
to type \fBint\fP.
.LP
If \fIx\fP is 0,  a domain error shall occur, and 
the value FP_ILOGB0 shall be returned.
.LP
If \fIx\fP is \(+-Inf,  a domain error shall occur, and 
the value {INT_MAX} shall be returned.
.LP
If \fIx\fP is a NaN,  a domain error shall occur, and 
the value FP_ILOGBNAN shall be returned.
.LP
If the correct value is greater than {INT_MAX}, {INT_MAX} shall be
returned and a domain error shall occur.
.LP
If the correct value is less than {INT_MIN}, {INT_MIN} shall be returned
and a domain error shall occur. 
.SH ERRORS
.LP
These functions shall fail if:
.TP 7
Domain\ Error
The \fIx\fP argument is zero, NaN, or \(+-Inf, or the correct value
is not representable as an integer. 
.LP
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then \fIerrno\fP 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.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
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.
.SH RATIONALE
.LP
The errors come from taking the expected floating-point value and
converting it to \fBint\fP, which is an invalid operation in
IEEE\ Std\ 754-1985 (since overflow, infinity, and NaN are not representable
in a type \fBint\fP), so should be a domain
error.
.LP
There are no known implementations that overflow. For overflow to
happen, {INT_MAX} must be less than
LDBL_MAX_EXP*\fIlog2\fP(FLT_RADIX) or {INT_MIN} must be greater than
LDBL_MIN_EXP*\fIlog2\fP(FLT_RADIX) if subnormals are not
supported, or {INT_MIN} must be greater than (LDBL_MIN_EXP-LDBL_MANT_DIG)*\fIlog2\fP(FLT_RADIX)
if subnormals are supported.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfeclearexcept\fP(), \fIfetestexcept\fP(), \fIlogb\fP(), \fIscalb\fP(),
the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 4.18, Treatment of Error Conditions
for
Mathematical Functions, \fI<float.h>\fP, \fI<math.h>\fP
.SH 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 .