# MPI_Neighbor_allgather

Gathers data from all neighboring processes and distribute the combined data to all neighboring processes

## Synopsis

    int MPI_Neighbor_allgather(const void *sendbuf, int sendcount,
                               MPI_Datatype sendtype, void *recvbuf, int recvcount,
                               MPI_Datatype recvtype, MPI_Comm comm)

    int MPI_Neighbor_allgather_c(const void *sendbuf, MPI_Count sendcount,
                                 MPI_Datatype sendtype, void *recvbuf,
                                 MPI_Count recvcount, MPI_Datatype recvtype,
                                 MPI_Comm comm)

## Input Parameters

sendbuf
  starting address of send buffer (choice)
sendcount
  number of elements sent to each neighbor (non-negative integer)
sendtype
  data type of send buffer elements (handle)
recvcount
  number of elements received from each neighbor (non-negative integer)
recvtype
  data type of receive buffer elements (handle)
comm
  communicator with topology structure (handle)

## Output Parameters

recvbuf
  starting address of receive buffer (choice)

## Notes

In this function, each process i gathers data items from each process j if an edge (j,i) exists in the topology graph, and each process i sends the same data items to all processes j where an edge (i,j) exists. The send buffer is sent to each neighboring process and the l-th block in the receive buffer is received from the l-th neighbor.

## 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.