MPI_GATHERV(3)					     Open MPI					    MPI_GATHERV(3)

MPI_Gatherv,  MPI_Igatherv <#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  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 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_GATHERV(3)
