MPI_IGATHERV(3)					     Open MPI					   MPI_IGATHERV(3)

       MPI_Gatherv <#mpi-gatherv>, MPI_Igatherv, MPI_Gatherv_init <#mpi-gatherv-init> - Gathers varying amounts of
       data from all processes to the root process

SYNTAX
   C Syntax
	  #include <mpi.h>

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

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

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

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'

	  MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
		  DISPLS, RECVTYPE, ROOT, COMM, IERROR)
	      <type>  SENDBUF(*), RECVBUF(*)
	      INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
	      INTEGER RECVTYPE, ROOT, COMM, IERROR

	  MPI_IGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
		  DISPLS, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
	      <type>  SENDBUF(*), RECVBUF(*)
	      INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
	      INTEGER RECVTYPE, ROOT, COMM, REQUEST, IERROR

	  MPI_GATHERV_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
		  DISPLS, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
	      <type>  SENDBUF(*), RECVBUF(*)
	      INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
	      INTEGER RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08

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

	  MPI_Igatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
		  recvtype, root, comm, request, ierror)
	      TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	      TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	      INTEGER, INTENT(IN) :: sendcount, root
	      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_Gatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
		  recvtype, root, comm, info, request, ierror)
	      TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	      TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	      INTEGER, INTENT(IN) :: sendcount, root
	      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).

       •

	 recvcounts (Integer array (of length group size) containing the)
		number of elements that are received from each process (significant only at root).

       •

	 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 (significant
		only at root).

       •

	 recvtype (Datatype of recv buffer elements (significant only at)
		root) (handle).

       • root : Rank of receiving process (integer).

       • comm : Communicator (handle).

       • info : Info (handle, persistent only).

OUTPUT PARAMETERS
       •

	 recvbuf (Address of receive buffer (choice, significant only at)
		root).

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

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

DESCRIPTION
       MPI_Gatherv <#mpi-gatherv> extends the functionality of MPI_Gather  <#mpi-gather>  by  allowing	a  varying
       count  of  data	from each process, since recvcounts is now an array. It also allows more flexibility as to
       where the data is placed on the root, by providing the new argument, displs.

       The outcome is as if each process, including the root process, sends a message to the root,

	  MPI_Send(sendbuf, sendcount, sendtype, root, ...);

       and the root executes n receives,

	  MPI_Recv(recvbuf + disp[i] * extent(recvtype), recvcounts[i],
		   recvtype, i, ...);

       Messages are placed in the receive buffer of the root process in rank order, that is, the  data	sent  from
       process	j  is  placed in the jth portion of the receive buffer recvbuf on process root. The jth portion of
       recvbuf begins at offset displs[j] elements (in terms of recvtype) into recvbuf.

       The receive buffer is ignored for all nonroot processes.

       The type signature implied by sendcount, sendtype on process i must be equal to the type signature  implied
       by  recvcounts[i],  recvtype  at	 the  root. This implies that the amount of data sent must be equal to the
       amount of data received, pairwise between each process and the root.  Distinct type maps between sender and
       receiver are still allowed, as illustrated in Example 2, below.

       All arguments to the function are significant on process root, while on	other  processes,  only	 arguments
       sendbuf,	 sendcount,  sendtype, root, comm are significant. The arguments root and comm must have identical
       values on all processes.

       The specification of counts, types, and displacements should not cause any  location  on	 the  root  to	be
       written more than once. Such a call is erroneous.

       Example	1:  Now	 have each process send 100 ints to root, but place each set (of 100) stride ints apart at
       receiving end. Use MPI_Gatherv <#mpi-gatherv> and the displs argument to achieve this effect. Assume stride
       >= 100.

	  MPI_Comm comm;
	  int gsize, sendarray[100];
	  int root, *rbuf, stride;
	  int *displs, i, rcounts;
	  ...

	  MPI_Comm_size(comm, &gsize);
	  rbuf = (int)malloc(gsize * stride * sizeof(int));
	  displs = (int)malloc(gsize * sizeof(int));
	  rcounts = (int )malloc(gsize * sizeof(int));

	  for (i=0; i<gsize; ++i) {
	    displs[i] = i * stride;
	    rcounts[i] = 100;
	  }
	  MPI_Gatherv(sendarray, 100, MPI_INT, rbuf, rcounts, displs, MPI_INT,
		      root, comm);

       Note that the program is erroneous if stride < 100.

       Example 2: Same as Example 1 on the receiving side, but send the 100 ints from the 0th column of a 100  150
       int array, in C.

	  MPI_Comm comm;
	  int gsize, sendarray[100][150];
	  int root, *rbuf, stride;
	  MPI_Datatype stype;
	  int displs,i, rcounts;
	  ...

	  MPI_Comm_size(comm, &gsize);
	  rbuf = (int )malloc(gsize * stride * sizeof(int));
	  displs = (int)malloc(gsize * sizeof(int));
	  rcounts = (int )malloc(gsize * sizeof(int));

	  for (i=0; i<gsize; ++i) {
	    displs[i] = i * stride;
	    rcounts[i] = 100;
	  }

	  // Create datatype for 1 column of array
	  MPI_Type_vector(100, 1, 150, MPI_INT, &stype);
	  MPI_Type_commit( &stype );
	  MPI_Gatherv(sendarray, 1, stype, rbuf, rcounts, displs, MPI_INT,
		      root, comm);

       Example	3: Process i sends (100-i) ints from the ith column of a 100 x 150 int array, in C. It is received
       into a buffer with stride, as in the previous two examples.

	  MPI_Comm comm;
	  int gsize, sendarray[100][150], *sptr;
	  int root, *rbuf, stride, myrank;
	  MPI_Datatype stype;
	  int displs, i, rcounts;
	  ...

	  MPI_Comm_size(comm, &gsize);
	  MPI_Comm_rank( comm, &myrank );
	  rbuf = (int)malloc(gsize * stride * sizeof(int));
	  displs = (int)malloc(gsize * sizeof(int));
	  rcounts = (int )malloc(gsize * sizeof(int));

	  for (i=0; i<gsize; ++i) {
	    displs[i] = i * stride;
	    rcounts[i] = 100-i; // note change from previous example
	  }

	  // Create datatype for the column we are sending
	  MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
	  MPI_Type_commit( &stype );
	  // sptr is the address of start of "myrank" column
	  sptr = &sendarray[0][myrank];
	  MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
		      root, comm);

       Note that a different amount of data is received from each process.

       Example 4: Same as Example 3, but done in a different way at the sending end. We	 create	 a  datatype  that
       causes the correct striding at the sending end so that we read a column of a C array.

	  MPI_Comm comm;
	  int gsize, sendarray[100][150], *sptr;
	  int root, *rbuf, stride, myrank, disp[2], blocklen[2];
	  MPI_Datatype stype, type[2];
	  int displs, i, rcounts;
	  ...

	  MPI_Comm_size(comm, &gsize);
	  MPI_Comm_rank(comm, &myrank );
	  rbuf = (int )alloc(gsize * stride * sizeof(int));
	  displs = (int )malloc(gsize * sizeof(int));
	  rcounts = (int)malloc(gsize * sizeof(int));

	  for (i=0; i<gsize; ++i) {
	    displs[i] = i* stride;
	    rcounts[i] = 100-i;
	  }
	  // Create datatype for one int, with extent of entire row
	  disp[0] = 0;
	  disp[1] = 150 * sizeof(int);
	  type[0] = MPI_INT;
	  type[1] = MPI_UB;
	  blocklen[0] = 1;
	  blocklen[1] = 1;

	  MPI_Type_struct( 2, blocklen, disp, type, &stype );
	  MPI_Type_commit(&stype );
	  sptr = &sendarray[0][myrank];
	  MPI_Gatherv(sptr, 100-myrank, stype, rbuf, rcounts, displs, MPI_INT,
		      root, comm);

       Example	5:  Same  as  Example 3 at sending side, but at receiving side we make the stride between received
       blocks vary from block to block.

	  MPI_Comm comm;
	  int gsize, sendarray[100][150], *sptr;
	  int root, *rbuf, *stride, myrank, bufsize;
	  MPI_Datatype stype;
	  int *displs, i, *rcounts, offset;
	  ...

	  MPI_Comm_size( comm, &gsize);
	  MPI_Comm_rank( comm, &myrank );
	  de = (int )malloc(gsize * sizeof(int));
	  ...
	  // stride[i] for i = 0 to gsize-1 is set somehow

	  // set up displs and rcounts vectors first
	  displs = (int)malloc(gsize * sizeof(int));
	  rcounts = (int )malloc(gsize * sizeof(int));
	  offset = 0;

	  for (i=0; i<gsize; ++i) {
	    displs[i] = offset;
	    offset += stride[i];
	    rcounts[i] = 100-i;
	  }

	  // the required buffer size for rbuf is now easily obtained
	  bufsize = displs[gsize-1]+rcounts[gsize-1];
	  rbuf = (int )malloc(bufsize * sizeof(int));
	  // Create datatype for the column we are sending
	  MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
	  MPI_Type_commit( &stype );
	  sptr = &sendarray[0][myrank];
	  MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
		      root, comm);

       Example 6: Process i sends num ints from the ith column of a 100 x 150 int array, in  C.	 The  complicating
       factor  is  that the various values of num are not known to root, so a separate gather must first be run to
       find these out. The data is placed contiguously at the receiving end.

	  MPI_Comm comm;
	  int gsize, sendarray[100][150], *sptr;
	  int root, *rbuf, stride, myrank, disp[2], blocklen[2];
	  MPI_Datatype stype,types[2];
	  int *displs, i, *rcounts, num;
	  ...

	  MPI_Comm_size( comm, &gsize);
	  MPI_Comm_rank( comm, &myrank );

	  // First, gather nums to root
	  rcounts = (int )malloc(gsize * sizeof(int));
	  MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, root, comm);
	  // root now has correct rcounts, using these we set
	  // displs[] so that data is placed contiguously (or concatenated) at receive end

	  displs = (int)malloc(gsize * sizeof(int));
	  displs[0] = 0;
	  for (i=1; i<gsize; ++i) {
	    displs[i] = displs[i-1]+rcounts[i-1];
	  }

	  // And, create receive buffer
	  rbuf = (int *)malloc(gsize * (displs[gsize-1]+rcounts[gsize-1]) * sizeof(int));
	  // Create datatype for one int, with extent of entire row
	  disp[0] = 0;
	  disp[1] = 150 * sizeof(int);
	  type[0] = MPI_INT;
	  type[1] = MPI_UB;
	  blocklen[0] = 1;
	  blocklen[1] = 1;
	  MPI_Type_struct(2, blocklen, disp, type, &stype );
	  MPI_Type_commit( &stype );
	  sptr = &sendarray[0][myrank];
	  MPI_Gatherv(sptr, num, stype, rbuf, rcounts, displs, MPI_INT, root, comm);

USE OF IN-PLACE OPTION
       The in-place option operates  in	 the  same  way	 as  it	 does  for  MPI_Gather	<#mpi-gather>.	 When  the
       communicator  is	 an  intracommunicator,	 you can perform a gather operation in-place (the output buffer is
       used as the input buffer). Use the variable MPI_IN_PLACE as the value of the root process sendbuf. In  this
       case,  sendcount	 and sendtype are ignored, and the contribution of the root process to the gathered vector
       is assumed to already be in the correct place in the receive buffer.

       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 root process in the first group gathers data from all
       the processes in the second group. The first group defines the root process. That process uses MPI_ROOT	as
       the  value  of  its  root  argument.  The  remaining processes use MPI_PROC_NULL as the value of their root
       argument. All processes in the second group use the rank of that root process in the  first  group  as  the
       value  of  their	 root  argument.  The  send  buffer  argument  of the processes in the first group must be
       consistent with the receive buffer argument of the root process in the second group.

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_Gather <#mpi-gather>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					   MPI_IGATHERV(3)
