The Great and Terrible implementation of MPI-2

function index


Sends data from all to all processes; each process may send a different amount of data and provide displacements for the input and output data.
int MPI_Alltoallv(
  void *sendbuf,
  int *sendcnts,
  int *sdispls,
  MPI_Datatype sendtype,
  void *recvbuf,
  int *recvcnts,
  int *rdispls,
  MPI_Datatype recvtype,
  MPI_Comm comm


[in] starting address of send buffer (choice)
[in] integer array equal to the group size specifying the number of elements to send to each processor
[in] integer array (of length group size). Entry j specifies the displacement (relative to sendbuf from which to take the outgoing data destined for process j
[in] data type of send buffer elements (handle)
[out] address of receive buffer (choice)
[in] integer array equal to the group size specifying the maximum number of elements that can be received from each processor
[in] integer array (of length group size). Entry i specifies the displacement (relative to recvbuf at which to place the incoming data from process i
[in] data type of receive buffer elements (handle)
[in] communicator (handle)


MPI_ALLTOALLV adds flexibility to MPI_ALLTOALL in that the location of data for the send is specified by sdispls and the location of the placement of the data on the receive side is specified by rdispls.

The jth block sent from process i is received by process j and is placed in the ith block of recvbuf. These blocks need not all have the same size.

The type signature associated with sendcount[j], sendtype at process i must be equal to the type signature associated with recvcount[i], recvtype at process j. This implies that the amount of data sent must be equal to the amount of data received, pairwise between every pair of processes. Distinct type maps between sender and receiver are still allowed.

The outcome is as if each process sent a message to every other process with,

and received a message from every other process with a call to

All arguments on all processes are significant. The argument comm must have identical values on all processes.

No "in place" option is supported.

If comm is an intercommunicator, then the outcome is as if each process in group A sends a message to each process in group B, and vice versa. The j-th send buffer of process i in group A should be consistent with the i-th receive buffer of process j in group B, and vice versa.


The definitions of MPI_ALLTOALL and MPI_ALLTOALLV give as much flexibility as one would achieve by specifying n independent, point-to-point communications, with two exceptions: all messages use the same datatype, and messages are scattered from (or gathered to) sequential storage.

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.


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 guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid buffer pointer. Usually a null buffer where one is not valid.

Example Code

The following sample code illustrates MPI_Alltoallv.

#include "mpi.h"
#include <stdlib.h>
#include <stdio.h>
This program tests MPI_Alltoallv by having processor i send different
amounts of data to each processor.
The first test sends i items to processor i from all processors.
int main( int argc, char **argv )
    MPI_Comm comm;
int *sbuf, *rbuf;
int rank, size;
int *sendcounts, *recvcounts, *rdispls, *sdispls;
int i, j, *p, err;

    MPI_Init( &argc, &argv );
    err = 0;
    comm = MPI_COMM_WORLD;
/* Create the buffer */
MPI_Comm_size( comm, &size );
    MPI_Comm_rank( comm, &rank );
    sbuf = (
int *)malloc( size * size * sizeof(int) );
    rbuf = (
int *)malloc( size * size * sizeof(int) );
if (!sbuf || !rbuf) {
        fprintf( stderr, "Could not allocated buffers!\n" );
        MPI_Abort( comm, 1 );
/* Load up the buffers */
for (i=0; i<size*size; i++) {
        sbuf[i] = i + 100*rank;
        rbuf[i] = -i;
/* Create and load the arguments to alltoallv */
sendcounts = (int *)malloc( size * sizeof(int) );
    recvcounts = (
int *)malloc( size * sizeof(int) );
    rdispls = (
int *)malloc( size * sizeof(int) );
    sdispls = (
int *)malloc( size * sizeof(int) );
if (!sendcounts || !recvcounts || !rdispls || !sdispls) {
        fprintf( stderr, "Could not allocate arg items!\n" );fflush(stderr);
        MPI_Abort( comm, 1 );
for (i=0; i<size; i++) {
        sendcounts[i] = i;
        recvcounts[i] = rank;
        rdispls[i] = i * rank;
        sdispls[i] = (i * (i+1))/2;
    MPI_Alltoallv( sbuf, sendcounts, sdispls, MPI_INT,
                       rbuf, recvcounts, rdispls, MPI_INT, comm );
/* Check rbuf */
for (i=0; i<size; i++) {
        p = rbuf + rdispls[i];
for (j=0; j<rank; j++) {
if (p[j] != i * 100 + (rank*(rank+1))/2 + j) {
                fprintf( stderr, "[%d] got %d expected %d for %dth\n",
                                    rank, p[j],(i*(i+1))/2 + j, j );
    free( sdispls );
    free( rdispls );
    free( recvcounts );
    free( sendcounts );
    free( rbuf );
    free( sbuf );
return 0;