# 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
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/sys/utils/mpits.c.html#PetscCommBuildTwoSidedFReq">src/sys/utils/mpits.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/sys/utils/mpits.c)


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