# MPI_Neighbor_alltoallv

The vector variant of MPI_Neighbor_alltoall allows sending/receiving different numbers of elements to and from each neighbor.

## Synopsis

    int MPI_Neighbor_alltoallv(const void *sendbuf, const int sendcounts[],
                               const int sdispls[], MPI_Datatype sendtype,
                               void *recvbuf, const int recvcounts[],
                               const int rdispls[], MPI_Datatype recvtype,
                               MPI_Comm comm)

    int MPI_Neighbor_alltoallv_c(const void *sendbuf, const MPI_Count sendcounts[],
                                 const MPI_Aint sdispls[], MPI_Datatype sendtype,
                                 void *recvbuf, const MPI_Count recvcounts[],
                                 const MPI_Aint rdispls[], MPI_Datatype recvtype,
                                 MPI_Comm comm)

## Input Parameters

sendbuf
  starting address of send buffer (choice)
sendcounts
  non-negative integer array (of length outdegree) specifying the number of elements to send to each neighbor (non-negative integer)
sdispls
  integer array (of length outdegree). Entry j specifies the displacement (relative to sendbuf) from which to send the outgoing data to neighbor j (integer)
sendtype
  data type of send buffer elements (handle)
recvcounts
  non-negative integer array (of length indegree) specifying the number of elements that are received from each neighbor (non-negative integer)
rdispls
  integer array (of length indegree). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from neighbor i (integer)
recvtype
  data type of receive buffer elements (handle)
comm
  communicator with topology structure (handle)

## Output Parameters

recvbuf
  starting address of receive buffer (choice)

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. The predefined error handler `MPI_ERRORS_RETURN` may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_BUFFER
  Invalid buffer pointer. Usually a null buffer where one is not valid.

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_COUNT
  Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.