MPI_ALLTOALLW_INIT(3)				     Open MPI				     MPI_ALLTOALLW_INIT(3)

       MPI_Alltoallw  <#mpi-alltoallw>,	 MPI_Ialltoallw <#mpi-ialltoallw>, MPI_Alltoallw_init - All processes send
       data of different types to, and receive data of different types from, all processes

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  <#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  <#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 <#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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Alltoall <#mpi-alltoall>

	  • MPI_Alltoallv <#mpi-alltoallv>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				     MPI_ALLTOALLW_INIT(3)
