PETSc version 3.17.0
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# PetscCommBuildTwoSidedFReq

discovers communicating ranks given one-sided information, calling user-defined functions during rendezvous, returns requests

### Synopsis

    #include "petscsys.h"
    PetscErrorCode PetscCommBuildTwoSidedFReq(MPI_Comm comm,PetscMPIInt count,MPI_Datatype dtype,PetscMPIInt nto,const PetscMPIInt *toranks,const void *todata,
                                              PetscMPIInt *nfrom,PetscMPIInt **fromranks,void *fromdata,PetscMPIInt ntags,MPI_Request **toreqs,MPI_Request **fromreqs,
                                              PetscErrorCode (*send)(MPI_Comm,const PetscMPIInt[],PetscMPIInt,PetscMPIInt,void*,MPI_Request[],void*),
                                              PetscErrorCode (*recv)(MPI_Comm,const PetscMPIInt[],PetscMPIInt,void*,MPI_Request[],void*),void *ctx)

Collective

### Input Parameters

| comm | - communicator
| count | - number of entries to send/receive in initial rendezvous (must match on all ranks)
| dtype | - datatype to send/receive from each rank (must match on all ranks)
| nto | - number of ranks to send data to
| toranks | - ranks to send to (array of length nto)
| todata | - data to send to each rank (packed)
| ntags | - number of tags needed by send/recv callbacks
| send | - callback invoked on sending process when ready to send primary payload
| recv | - callback invoked on receiving process after delivery of rendezvous message
| ctx | - context for callbacks

### Output Parameters

| nfrom | - number of ranks receiving messages from
| fromranks | - ranks receiving messages from (length nfrom; caller should PetscFree())
| fromdata | - packed data from each rank, each with count entries of type dtype (length nfrom, caller responsible for PetscFree())
| toreqs | - array of nto*ntags sender requests (caller must wait on these, then PetscFree())
| fromreqs | - array of nfrom*ntags receiver requests (caller must wait on these, then PetscFree())

### Notes

This memory-scalable interface is an alternative to calling PetscGatherNumberOfMessages() and PetscGatherMessageLengths(), possibly with a subsequent round of communication to send other data.

Basic data types as well as contiguous types are supported, but non-contiguous (e.g., strided) types are not.

### References

| * | - Hoefler, Siebert and Lumsdaine, The MPI_Ibarrier implementation uses the algorithm in Scalable communication protocols for dynamic sparse data exchange, 2010.

### See Also

PetscCommBuildTwoSided(), PetscCommBuildTwoSidedF(), PetscGatherNumberOfMessages(), PetscGatherMessageLengths()

### Level

developer

### Location

src/sys/utils/mpits.c
Index of all Sys routines
Table of Contents for all manual pages
Index of all manual pages