# MPI_Alltoall

## NAME

MPI_Alltoall − All processes send data to all processes

## SYNTAX

## C Syntax

#include <mpi.h>
int MPI_Alltoall(void *sendbuf, int sendcount,

|  | MPI_Datatype sendtype, void *recvbuf, int recvcount,
|  | MPI_Datatype recvtype, MPI_Comm comm)

## Fortran Syntax

INCLUDE ’mpif.h’
MPI_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,

| RECVTYPE, COMM, IERROR) |  |  |
| <type> |  |  | SENDBUF(*), RECVBUF(*)
| INTEGER |  |  | SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
| INTEGER |  |  | COMM, IERROR

## C++ Syntax

#include <mpi.h>
void MPI::Comm::Alltoall(const void* sendbuf, int sendcount,

| const MPI::Datatype& sendtype, void* recvbuf,
| int recvcount, const MPI::Datatype& recvtype)

## INPUT PARAMETERS

| sendbuf |  | Starting address of send buffer (choice).
| sendcount |  | Number of elements to send to each process (integer).
| sendtype |  | Datatype of send buffer elements (handle).
| recvcount |  | Number of elements to receive from each process (integer).
| recvtype |  | Datatype of receive buffer elements (handle).
| comm |  | Communicator over which data is to be exchanged (handle).

## OUTPUT PARAMETERS

| recvbuf |  | Starting address of receive buffer (choice). |
| IERROR |  | Fortran only: Error status (integer). |

## DESCRIPTION

MPI_Alltoall is a collective operation in which all processes send the same amount of data to each other, and receive the same amount of data from each other. The operation of this routine can be represented as follows, where each process performs 2n (n being the number of processes in communicator comm) independent point-to-point communications (including communication with itself).

| MPI_Comm_size(comm, &n);
| for (i = 0, i < n; i++)
| MPI_Send(sendbuf + i * sendcount * extent(sendtype),
| sendcount, sendtype, i, ..., comm);
| for (i = 0, i < n; i++)
| MPI_Recv(recvbuf + i * recvcount * extent(recvtype),
| recvcount, recvtype, i, ..., comm);

Each process breaks up its local sendbuf into n blocks − each containing sendcount elements of type sendtype − and divides its recvbuf similarly according to recvcount and recvtype. Process j sends the k-th block of its local sendbuf to process k, which places the data in the j-th block of its local recvbuf. The amount of data sent must be equal to the amount of data received, pairwise, between every pair of processes.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

When the communicator is an inter-communicator, the gather operation occurs in two phases. The data is gathered from all the members of the first group and received by all the members of the second group. Then the data is gathered from all the members of the second group and received by all the members of the first. The operation exhibits a symmetric, full-duplex behavior.

The first group defines the root process. The root process uses MPI_ROOT as the value of root. All other processes in the first group use MPI_PROC_NULL as the value of root. All processes in the second group use the rank of the root process in the first group as the value of root.

When the communicator is an intra-communicator, these groups are the same, and the operation occurs in a single phase.

## NOTES

The MPI_IN_PLACE option is not available for this function.

All arguments on all processes are significant. The comm argument, in particular, must describe the same communicator on all processes.

There are two MPI library functions that are more general than MPI_Alltoall. MPI_Alltoallv allows all-to-all communication to and from buffers that need not be contiguous; different processes may send and receive different amounts of data. MPI_Alltoallw expands MPI_Alltoallv’s functionality to allow the exchange of data with different datatypes.

## ERRORS

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI:Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; 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.

## SEE ALSO

MPI_Alltoallv
MPI_Alltoallw

MPI API Index