MPI_IALLTOALL(3)				     Open MPI					  MPI_IALLTOALL(3)

       MPI_Alltoall  <#mpi-alltoall>,  MPI_Ialltoall,  MPI_Alltoall_init <#mpi-alltoall-init> - All processes send
       data to all processes

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Alltoall(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Ialltoall(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)

	  int MPI_Alltoall_init(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, IERROR)

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

	  MPI_IALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, REQUEST, IERROR)

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

	  MPI_ALLTOALL_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, INFO, REQUEST, IERROR)

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

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
		       comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ialltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
		       comm, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Alltoall_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
			       comm, info, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       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 (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).

       • info: Info (handle, persistent only)

OUTPUT PARAMETERS
       • recvbuf: Starting address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Alltoall <#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.

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,
       sendcount  and sendtype 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
       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-alltoall>. MPI_Alltoallv
       <#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	 <#mpi-alltoallw>  expands
       MPI_Alltoallv <#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 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_Alltoallv <#mpi-alltoallv>

	  • MPI_Alltoallw <#mpi-alltoallw>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_IALLTOALL(3)
