PetscGatherMessageLengths

Computes info about messages that a MPI-node will receive, including (from-id,length) pairs for each message.

#include "petsc.h"  
int PetscGatherMessageLengths(MPI_Comm comm,int nsends,int nrecvs,int *ilengths,int **onodes,int **olengths)

Collective on MPI_Comm

	comm	- Communicator
	nsends	- number of messages that are to be sent.
	nrecvs	- number of messages being received
	ilengths	- an array of integers of length sizeof(comm) a non zero ilengths[i] represent a message to i of length ilengths[i]

	onodes	- list of node-ids from which messages are expected
	olengths	- corresponding message lengths

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.

The calling function deallocates the memory in onodes and olengths

To determine nrecevs, one can use PetscGatherNumberOfMessages()