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SYNTAX
C Syntax
#include <mpi.h>
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_Ialltoallw(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,
MPI_Request *request)
int MPI_Alltoallw_init(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, MPI_Info info,
MPI_Request *request)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, IERROR
MPI_IALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, REQUEST, IERROR
MPI_ALLTOALLW_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, INFO, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, INFO, REQUEST, IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf, recvcounts,
rdispls, recvtypes, comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
rdispls(*)
TYPE(MPI_Datatype), INTENT(IN) :: sendtypes(*)
TYPE(MPI_Datatype), INTENT(IN) :: recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ialltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
recvcounts, rdispls, recvtypes, comm, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
recvcounts(*), rdispls(*)
TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Alltoallw_init(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
recvcounts, rdispls, recvtypes, comm, fIinfo, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
recvcounts(*), rdispls(*)
TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Info), INTENT(IN) :: info
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
INPUT PARAMETERS
• sendbuf: Starting address of send buffer.
• sendcounts: Integer array, where entry i specifies the number of elements to send to rank i.
• sdispls: Integer array, where entry i specifies the displacement (in bytes, offset from sendbuf) from
which to send data to rank i.
• sendtypes: Datatype array, where entry i specifies the datatype to use when sending data to rank i.
• recvcounts: Integer array, where entry j specifies the number of elements to receive from rank j.
• rdispls: Integer array, where entry j specifies the displacement (in bytes, offset from recvbuf) to
which data from rank j should be written.
• recvtypes: Datatype array, where entry j specifies the datatype to use when receiving data from rank j.
• comm: Communicator over which data is to be exchanged.
• info: Info (handle, persistent only)
OUTPUT PARAMETERS
• recvbuf: Address of receive buffer.
• request: Request (handle, non-blocking only).
• ierror: Fortran only: Error status.
DESCRIPTION
MPI_Alltoallw is a generalized collective operation in which all processes send data to and receive data
from all other processes. It adds flexibility to MPI_Alltoallv by allowing the user to specify the
datatype of individual data blocks (in addition to displacement and element count). Its operation can be
thought of in the following way, 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 + sdispls[i], sendcounts[i],
sendtypes[i], i, ..., comm);
for (i = 0, i < n; i++)
MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
recvtypes[i], i, ..., comm);
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.
When a pair of processes exchanges data, each may pass different element count and datatype arguments so
long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to
receive.
Note that process i may send a different amount of data to process j than it receives from process j.
Also, a process may send entirely different amounts and types of data to different processes in the
communicator.
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.
USE OF IN-PLACE OPTION
When the communicator is an intracommunicator, you can perform an all-to-all operation in-place (the
output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf. In
this case, sendcounts, sdispls, and sendtypes are ignored. The input data of each process is assumed to
be in the area where that process would receive its own contribution to the receive buffer.
NOTES
The specification of counts, types, and displacements should not cause any location to be written more
than once.
All arguments on all processes are significant. The comm argument, in particular, must describe the same
communicator on all processes.
The offsets of sdispls and rdispls are measured in bytes. Compare this to MPI_Alltoallv, where these
offsets are measured in units of sendtype and recvtype, respectively.
ERRORS
Almost all MPI routines return an error value; C routines as the return result of the function and
Fortran routines in the last argument.
Before the error value is returned, the current MPI error handler associated with the communication
object (e.g., communicator, window, file) is called. If no communication object is associated with the
MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error
handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after
MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler.
The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/‐
MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all
other MPI functions.
Open MPI includes three predefined error handlers that can be used:
• MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.
• MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a
window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in
the corresponding window or file. If called on a session, aborts only the local process.
• MPI_ERRORS_RETURN Returns an error code to the application.
MPI applications can also implement their own error handlers by calling:
• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
SEE ALSO:
• MPI_Alltoall
• MPI_Alltoallv
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_IALLTOALLW(3)