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<H2><A NAME="SECTION000810000000000000000">
psb_sum -- Global sum</A>
</H2>

<P>
<PRE>
call psb_sum(icontxt, dat, root)
</PRE>

<P>
This subroutine implements a sum reduction  operation based on the
underlying communication library. 
<DL>
<DT><STRONG>Type:</STRONG></DT>
<DD>Synchronous.
</DD>
<DT><STRONG><B>On Entry </B></STRONG></DT>
<DD>
</DD>
<DT><STRONG>icontxt</STRONG></DT>
<DD>the communication context identifying the virtual
  parallel machine.
<BR>
Scope: <B>global</B>.
<BR>
Type: <B>required</B>.
<BR>
Intent: <B>in</B>.
<BR>
Specified as: an integer variable.
</DD>
<DT><STRONG>dat</STRONG></DT>
<DD>The local contribution to the global sum.
<BR>
Scope: <B>global</B>.
<BR>
Type: <B>required</B>.
<BR>
Intent: <B>inout</B>.
<BR>
Specified as: an integer, real or complex variable, which may be a
scalar, or a rank 1 or 2 array.  Type, kind, rank and size must agree on all processes.
</DD>
<DT><STRONG>root</STRONG></DT>
<DD>Process to hold the final sum, or <IMG
 WIDTH="24" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
 SRC="img78.png"
 ALT="$-1$"> to make it available
  on all processes.
<BR>
Scope: <B>global</B>.
<BR>
Type: <B>optional</B>.
<BR>
Intent: <B>in</B>.
<BR>
Specified as: an integer value <!-- MATH
 $-1<= root <= np-1$
 -->
<IMG
 WIDTH="165" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
 SRC="img132.png"
 ALT="$-1&lt;= root &lt;= np-1$">, default -1.  </DD>
</DL>

<P>
<DL>
<DT><STRONG><B>On Return</B></STRONG></DT>
<DD>
</DD>
<DT><STRONG>dat</STRONG></DT>
<DD>On destination process(es), the result of the sum operation.
<BR>
Scope: <B>global</B>.
<BR>
Type: <B>required</B>.
<BR>
Intent: <B>inout</B>.
<BR>
Specified as: an integer, real or complex variable, which may be a
scalar, or a rank 1 or 2 array. 
<BR>
Type, kind, rank and size must agree on all processes.
</DD>
</DL>

<P>
<FONT SIZE="+1"><B>Notes</B></FONT>

<OL>
<LI>The <code>dat</code> argument is both input and output, and its
  value may be changed even on processes different from the final
  result destination.
</LI>
<LI>The <code>dat</code> argument may also be a long integer scalar.
</LI>
</OL>

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