.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "LOG2" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" log2 
.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
log2, log2f, log2l \- compute base 2 logarithm functions
.SH SYNOPSIS
.LP
\fB#include <math.h>
.br
.sp
double log2(double\fP \fIx\fP\fB);
.br
float log2f(float\fP \fIx\fP\fB);
.br
long double log2l(long double\fP \fIx\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
These functions shall compute the base 2 logarithm of their argument
\fIx\fP, log_2(\fIx\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 base
2 logarithm of \fIx\fP.
.LP
If \fIx\fP is \(+-0, a pole error shall occur and \fIlog2\fP(), \fIlog2f\fP(),
and \fIlog2l\fP() shall return -HUGE_VAL,
-HUGE_VALF, and -HUGE_VALL, respectively.
.LP
For finite values of \fIx\fP that are less than 0,  or if \fIx\fP
is -Inf,  a domain error shall occur, and  either a NaN (if supported),
or an implementation-defined value shall be returned.
.LP
If
\fIx\fP is NaN, a NaN shall be returned.
.LP
If \fIx\fP is 1, +0 shall be returned.
.LP
If \fIx\fP is +Inf, \fIx\fP shall be returned. 
.SH ERRORS
.LP
These functions shall fail if:
.TP 7
Domain\ Error
The finite value of \fIx\fP is less than zero,  or \fIx\fP is -Inf.
.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.
.TP 7
Pole\ Error
The value of \fIx\fP is zero. 
.LP
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then \fIerrno\fP shall be set to [ERANGE]. If the
integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the divide-by-zero 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
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfeclearexcept\fP(), \fIfetestexcept\fP(), \fIlog\fP(), the Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Section 4.18, Treatment
of Error Conditions for Mathematical Functions, \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 .