MPI_ALLGATHERV(3)				     Open MPI					 MPI_ALLGATHERV(3)

MPI_Allgatherv, MPI_Iallgatherv <#mpi-iallgatherv>, MPI_Allgatherv_init <#mpi-allgatherv-init> - Gathers data from
all processes and delivers it to all. Each process may contribute a different amount of data.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Allgatherv(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
	       const int displs[], MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Iallgatherv(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
	       const int displs[], MPI_Datatype recvtype, MPI_Comm comm,
		  MPI_Request *request)

	  int MPI_Allgatherv_init(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
	       const int displs[], MPI_Datatype recvtype, MPI_Comm comm,
		  MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF,
		       RECVCOUNT, DISPLS, RECVTYPE, COMM, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*)
	       INTEGER DISPLS(*), RECVTYPE, COMM, IERROR

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

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

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Allgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
		       recvtype, comm, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcounts(*), displs(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Iallgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
		       recvtype, comm, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*)
	       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_Allgatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
			       recvtype, comm, info, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*)
	       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 in send buffer (integer).

       • sendtype: Datatype of send buffer elements (handle).

       • recvcount:  Integer array (of length group size) containing the number of elements that are received from
	 each process.

       • displs: Integer array (of length group size). Entry i specifies the displacement (relative to recvbuf) at
	 which to place the incoming data from process i.

       • recvtype: Datatype of receive buffer elements (handle).

       • comm: Communicator (handle).

       • info: Info (handle, persistent only).

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

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

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

DESCRIPTION
       MPI_Allgatherv is similar to MPI_Allgather <#mpi-allgather> in that all	processes  gather  data	 from  all
       other  processes, except that each process can send a different amount of data. The block of data sent from
       the jth process is received by every process and placed in the jth block of the buffer recvbuf.

       The type signature associated with sendcount, sendtype, at process j must be equal to  the  type	 signature
       associated with recvcounts[j], recvtype at any other process.

       The outcome is as if all processes executed calls to

	  MPI_Allgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcount,
			displs,recvtype,root,comm);

	  // for root = 0 , ..., n-1.

       The rules for correct usage of MPI_Allgatherv are easily found from the corresponding rules for MPI_Gatherv
       <#mpi-gatherv>.

USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform an all-gather 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. Specifically, the outcome of a call to
       MPI_Allgatherv that used the in-place option is identical to the case in which  all  processes  executed	 n
       calls to

	  MPI_Allgatherv ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
			   recvcounts, displs, recvtype, root, comm );

	  // for root =0, ... , n-1.

       Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

       Because	the  in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding
       that includes INTENT must mark these as INOUT, not OUT.

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, concatenated, and received by all the members of the
       second group. Then the data is gathered from all	 the  members  of  the	second	group,	concatenated,  and
       received	 by  all  the  members of the first. The send buffer arguments in the one group must be consistent
       with the receive buffer arguments in the other group, and vice versa. The operation must exhibit symmetric,
       full-duplex behavior.

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_Gatherv <#mpi-gatherv>

	  • MPI_Allgather <#mpi-allgather>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					 MPI_ALLGATHERV(3)
