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<!Converted with LaTeX2HTML 95.1 (Fri Jan 20 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>6.7 Argument Passing</TITLE>
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<H1><A NAME=SECTION03370000000000000000>6.7 Argument Passing</A></H1>
<P>
<A NAME=secarg>&#160;</A>
<P>
FM extends the basic task/channel model by
<A NAME=9351>&#160;</A>
allowing ordinary variables, as well as ports, to be passed as actual
arguments in a process call.  These values can be both read and
modified inside the process, and updated values are available to the
parent process when the process returns.
<P>
This capability is not essential: a value can always be passed to a
process by defining a channel, sending the value on the outport, and
passing the inport to the process as an argument.  A similar strategy
<A NAME=9352>&#160;</A>
can be used to return a value from a process.  However, normal
argument passing often produces more concise and easy-to-understand
programs.  See Program <A HREF="node74.html#progfmarg" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node74.html#progfmarg">6.8</A> for a comparison of the two
approaches.
<P>
<P><A NAME=progfmarg>&#160;</A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img894.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img894.gif"><P><H2><A NAME=SECTION03371000000000000000>6.7.1 Copying and Determinism</A></H2>
<P>
If unrestricted, argument passing could compromise determinism. The
result computed by a program that passed the same variable to two
<A NAME=9389>&#160;</A>
processes could depend on the order in which those processes executed.
For example, the following code fragments could be nondeterministic if
a call to <tt> proc</tt> modifies the argument <tt> x</tt> and this variable
is used in subsequent computation.
<P>

<PRE>  PROCESSDO i = 1,2             PROCESSES
    PROCESSCALL proc(i,x)         PROCESSCALL proc(1,x)
  ENDPROCESSDO                    PROCESSCALL proc(2,x)
    ...                         ENDPROCESSES
</PRE>

<P>
FM semantics ensure deterministic execution in these situations.
Variables named as process arguments in a process block or do-loop are
passed by value; that is, they are copied.  In the case of arrays, the
number of values copied is determined by the dummy argument
declaration in the called process.  Values are also copied back upon
termination of the process block or do-loop, in textual and do-loop
order.  These copy operations ensure deterministic execution even
when concurrent processes update the same value.  Hence, these code
fragments are deterministic even if <tt> proc</tt> does modify its
argument.  In both cases, the value computed by the process <tt>
proc(2,x)</tt> is used to update <tt> x</tt>.
<P>
<H2><A NAME=SECTION03372000000000000000>6.7.2 Avoiding Copying</A></H2>
<P>
<A NAME=secfmintent>&#160;</A>
<P>
Copying variables on process call and return can be expensive.
Sometimes this copying is not necessary; for example, if a variable is
only read by a process, not modified, then there is no need to copy it
on return.  We can provide <tt> INTENT</tt> declarations for dummy
arguments to specify when copying is not to be performed.  Three <tt>
INTENT</tt> declarations are supported, as follows.
<P>

<PRE><TT>     &#175;<tt> INTENT(in)</tt>    <em> var-list</em>       &#175; : Copy on call only.
<P>
		 <tt> INTENT(out)</tt>   <em> var-list</em> 		 : 		 Copy on return only.
<P>
		 <tt> INTENT(inout)</tt> <em> var-list</em> 		 : 		 (Default) Copy on call and return.
<P>
</TT></PRE>

<P>
We note that FM and
<A NAME=9408>&#160;</A>
Fortran 90 intent declarations have slightly different semantics.  In
a Fortran 90 subroutine, these declarations are assertions: <tt>
intent(in)</tt> asserts that an argument is not written by the subroutine,
and <tt> intent(out)</tt> asserts that the argument is not read.  These
assertions have no semantic content: a program with incorrect
declarations is invalid.  In contrast, <tt> INTENT</tt> declarations in an
FM process have semantic content: they specify whether copying is to
be performed, and they cannot be invalid.
<P>
<BR><HR>
<b> Example <IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img899.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img899.gif">.<IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img895.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img895.gif">    Ring Pipeline</b>:<A NAME=exrpx>&#160;</A>
<P>
We use a modified version of Program <A HREF="node70.html#progfmring" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node70.html#progfmring">6.2</A> to illustrate
the use of <tt> INTENT</tt> declarations.  Program <A HREF="node74.html#progfmring3" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node74.html#progfmring3">6.9</A>
extends Program <A HREF="node70.html#progfmring" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node70.html#progfmring">6.2</A> by incorporating statements that
read input data and write output data.
<A NAME=9418>&#160;</A>
The inport and outport declarations and the <tt> CHANNEL</tt> statements
are as in Program <A HREF="node70.html#progfmring" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node70.html#progfmring">6.2</A> and are not shown here.  The main
program uses routines <tt> ring_read</tt> and <tt> ring_write</tt> to read
and write two-dimensional arrays <tt> input</tt> and <tt> output</tt>,
respectively.  These arrays contain initial particle positions and
final particle positions, respectively.
<P>
<P><A NAME=10361>&#160;</A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img898.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img898.gif">
<BR><STRONG>Figure 6.6:</STRONG> <em> Argument passing and <tt> INTENT</tt> declarations in the ring
pipeline program.  The parent process reads the <tt> input</tt> array, which is of
size <IMG BORDER=0 ALIGN=MIDDLE ALT="" SRC="img897.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img897.gif">, from a file and passes one column to each of four 
<tt> ringnode</tt> processes.  The ring processes communicate among
themselves and compute a column of the <tt> output</tt> array; upon
termination, this array is copied back to the parent, which writes it
to a file.</em><A NAME=figfmintent>&#160;</A><BR>
<P>
<P>
Argument passing is used to pass the appropriate components of these
arrays to the subdomain processes, with <tt> INTENT</tt> declarations
ensuring that <tt> input</tt> is copied only on call and <tt> output</tt>
only on return (Figure <A HREF="node74.html#figfmintent" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node74.html#figfmintent">6.6</A>).  This centralized I/O
strategy has the advantage of simplicity.  A disadvantage on many
computer systems is that it limits scalability, as the size of problem
that can be solved is constrained by the need to fit the <tt> input</tt>
and <tt> output</tt> arrays into a single processor's memory.
<P>
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<I>&#169 Copyright 1995 by <A href="msgs0.htm#6" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/tppmsgs/msgs0.htm#6">Ian Foster</a></I>
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