plucky (3) MPI_Igatherv.openmpi.3.gz
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 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. 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_thread, after
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 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_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 was called on that communicator. If called on a
window or file, acts as if 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 then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
SEE ALSO:
MPI_Gather
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_IGATHERV(3)