MPI_IALLGATHERV(3)				     Open MPI					MPI_IALLGATHERV(3)

       MPI_Allgatherv  <#mpi-allgatherv>,  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  <#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 <#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  <#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_IALLGATHERV(3)
