#  rpnexpr.pod - Torrus RPN expressions guide
#  Copyright (C) 2002  Stanislav Sinyagin
#
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#  You should have received a copy of the GNU General Public License
#  along with this program; if not, write to the Free Software
#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.

# $Id: rpnexpr.pod.in,v 1.1 2010-12-27 00:04:31 ivan Exp $
# Stanislav Sinyagin <ssinyagin@yahoo.com>
#
#

=head1 RPN expressions in Torrus

In Torrus framework, RPN expressions are the superset of those
in RRDtool version 1.0. See the C<rrdtool graph> manual at
E<lt>http://oss.oetiker.ch/rrdtool/doc/rrdgraph_rpn.en.htmlE<gt>.

=head2 New functions added

=over 4

=item * NE

Pops two arguments from stack, and pushes 0 if the arguments are equal,
and 1 otherwise.

=item * AND, OR

These functions pop two arguments from stack, and push back the result of
logical operation. Unlike C operators,

=item * NOT

Pops one value from stack and pushes 0 if the argument is nonzero,
otherwise 1.

=item * ABS

Pops one value from stack and pushes the absolute value of it.

=item * NOW

Pushes the current time, in seconds since Epoch.

=item * MOD

Equivalent of C<%>, the modulo operator. In Torrus parameter value,
percent sign is reserved for parameter substitution.

=item * NUM

Returns zero if the argument is undefined, and the argument's numeric value
otherwise

=item * INF, NEGINF

Returns the positive or negative infinity.

=back

=head2 Data access

In certain context, the values of the datasources can be evaluated
into RPN expression.

The general format for data access is following:

  {FUNC@PATH(-OFFSET)}

C<FUNC@> specifies a special function to be performed on the
data being accessed.

For monitor expressions, C<T@> returns the timestamp of the data source.

For C<rrd-cdef> leaf types and for C<rrd-multigraph> datasource types,
the following functions affect the graph shape: C<AVERAGE@>, C<MIN@>,
C<MAX@>, and C<LAST@>. They cause the corresponding Consolidation Function
being used when creating a graph.

C<PATH> specifies the relative name for the data source.
If omitted, the current leaf value is taken. If starts with C</>,
the path is considered as absolute.
Path starting with letter denotes the child of the parent subtree.
Double dot (C<../>) in the beginning of the path is interpreted as
current parent's parent subtree.

C<(OFFSET)> determines the time reference, as described in C<rrdtool fetch>
manual. In addition, the word C<LAST> refers to the latest data timestamp
available.

C<(OFFSET)> is currently supported in Monitor expressions only.

=head1 Author

Copyright (c) 2002-2004 Stanislav Sinyagin E<lt>ssinyagin@yahoo.comE<gt>