# PetscGatherMessageLengths

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

### Synopsis

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

Collective on MPI_Comm

### Input Parameters

| 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]

### Output Parameters

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

### Notes

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()

### See Also

PetscGatherNumberOfMessages()

Level:developer
Location: src/sys/src/utils/mpimesg.c
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