MPI_ALLTOALLV(3)				     Open MPI					  MPI_ALLTOALLV(3)

MPI_Alltoallv,	MPI_Ialltoallv	<#mpi-ialltoallv>,  MPI_Alltoallv_init	<#mpi-alltoallv-init> - All processes send
different amount of data to, and receive different amount of data from, all processes

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Alltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Ialltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Request *request)

	  int MPI_Alltoallv_init(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, IERROR

	  MPI_IALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, REQUEST, IERROR

	  MPI_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
		       rdispls, recvtype, comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
	       rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ialltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
		       rdispls, recvtype, 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) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
			       rdispls, recvtype, comm, info, 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) :: 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.

       • 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  (offset  from  sendbuf,  in	 units	of
	 sendtype) from which to send data to rank i.

       • sendtype: Datatype of send buffer elements.

       • 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 (offset from recvbuf, in units of
	 recvtype) to which data from rank j should be written.

       • recvtype: Datatype of receive buffer elements.

       • 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_Alltoallv is a generalized collective operation in which all processes send data to	and  receive  data
       from  all  other	 processes.  It	 adds  flexibility to MPI_Alltoall <#mpi-alltoall> by allowing the user to
       specify data to send and receive vector-style (via a displacement and element  count).	The  operation	of
       this  routine can be thought of 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 + sdispls[i] * extent(sendtype),
		  sendcounts[i], sendtype, i, ..., comm);
	  for (i = 0, i < n; i++)
	      MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
		  recvcounts[i], recvtype, 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 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 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
       The specification of counts 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 units of sendtype and	 recvtype,  respectively.  Compare
       this to MPI_Alltoallw <#mpi-alltoallw>, where these offsets are measured in bytes.

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_Alltoallw <#mpi-alltoallw>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_ALLTOALLV(3)
