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<A NAME="PetscGatherMessageLengths"><H1>PetscGatherMessageLengths</H1></A>
Computes info about messages that a MPI-node will receive, including (from-id,length) pairs for each message.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petsc.h"
int PetscGatherMessageLengths(MPI_Comm comm,int nsends,int nrecvs,int *ilengths,int **onodes,int **olengths)
</PRE>
Collective on MPI_Comm
<P>
<H3><FONT COLOR="#CC3333">Input Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B><A HREF="../Sys/comm.html#comm">comm</A> </B></TD><TD>- Communicator
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nsends </B></TD><TD>- number of messages that are to be sent.
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nrecvs </B></TD><TD>- number of messages being received
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>ilengths </B></TD><TD>- an array of integers of length sizeof(<A HREF="../Sys/comm.html#comm">comm</A>)
a non zero ilengths[i] represent a message to i of length ilengths[i]
</TD></TR></TABLE>
<P>
<P>
<H3><FONT COLOR="#CC3333">Output Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>onodes </B></TD><TD>- list of node-ids from which messages are expected
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>olengths </B></TD><TD>- corresponding message lengths
</TD></TR></TABLE>
<P>
<P>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
With this info, the correct MPI_Irecv() can be posted with the correct
from-id, with a buffer with the right amount of memory required.
<P>
The calling function deallocates the memory in onodes and olengths
<P>
To determine nrecevs, one can use <A HREF="../Sys/PetscGatherNumberOfMessages.html#PetscGatherNumberOfMessages">PetscGatherNumberOfMessages</A>()
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../Sys/PetscGatherNumberOfMessages.html#PetscGatherNumberOfMessages">PetscGatherNumberOfMessages</A>()
<BR><P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>developer
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/sys/src/utils/mpimesg.c.html#PetscGatherMessageLengths">src/sys/src/utils/mpimesg.c</A>
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