# MPI_Alltoallw

Generalized all-to-all communication allowing different datatypes, counts, and displacements for each partner

## Synopsis

    int MPI_Alltoallw(const void *sendbuf, const int sendcounts[],
                      const int sdispls[], const MPI_Datatype sendtypes[],
                      void *recvbuf, const int recvcounts[], const int rdispls[],
                      const MPI_Datatype recvtypes[], MPI_Comm comm)

    int MPI_Alltoallw_c(const void *sendbuf, const MPI_Count sendcounts[],
                        const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
                        void *recvbuf, const MPI_Count recvcounts[],
                        const MPI_Aint rdispls[], const MPI_Datatype recvtypes[],
                        MPI_Comm comm)

## Input Parameters

sendbuf
  starting address of send buffer (choice)
sendcounts
  non-negative integer array (of length group size) specifying the number of elements to send to each rank (non-negative integer)
sdispls
  integer array (of length group size). Entry j specifies the displacement in bytes (relative to sendbuf) from which to take the outgoing data destined for process j (integer)
sendtypes
  array of datatypes (of length group size). Entry j specifies the type of data to send to process j (handle)
recvcounts
  non-negative integer array (of length group size) specifying the number of elements that can be received from each rank (non-negative integer)
rdispls
  integer array (of length group size). Entry i specifies the displacement in bytes (relative to recvbuf) at which to place the incoming data from process i (integer)
recvtypes
  array of datatypes (of length group size). Entry i specifies the type of data received from process i (handle)
comm
  communicator (handle)

## Output Parameters

recvbuf
  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.