# 4.5. Gather

Up: Collective Communication Next: Examples using MPI_GATHER, MPI_GATHERV Previous: Example using MPI_BCAST

MPI_GATHER( sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm)
[ IN sendbuf] starting address of send buffer (choice)
[ IN sendcount] number of elements in send buffer (integer)
[ IN sendtype] data type of send buffer elements (handle)
[ OUT recvbuf] address of receive buffer (choice, significant only at root)
[ IN recvcount] number of elements for any single receive (integer, significant only at root)
[ IN recvtype] data type of recv buffer elements (significant only at root) (handle)
[ IN root] rank of receiving process (integer)
[ IN comm] communicator (handle)

`int MPI_Gather(void* sendbuf, int sendcount, MPI_Datatype sendtype, void* recvbuf, int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm)
`

`MPI_GATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR
`

Each process (root process included) sends the contents of its send buffer to the root process. The root process receives the messages and stores them in rank order. The outcome is as if each of the `n` processes in the group (including the root process) had executed a call to

and the root had executed `n` calls to

where `extent(recvtype)` is the type extent obtained from a call to `MPI_Type_extent()`.

An alternative description is that the `n` messages sent by the processes in the group are concatenated in rank order, and the resulting message is received by the root as if by a call to MPI_RECV(recvbuf, recvcount n, recvtype, ...).

The receive buffer is ignored for all non-root processes.

General, derived datatypes are allowed for both sendtype and recvtype. The type signature of sendcount, sendtype on process `i` must be equal to the type signature of recvcount, 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.

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 and types should not cause any location on the root to be written more than once. Such a call is erroneous.

Note that the recvcount argument at the root indicates the number of items it receives from each process, not the total number of items it receives.

MPI_GATHERV( sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, root, comm)
[ IN sendbuf] starting address of send buffer (choice)
[ IN sendcount] number of elements in send buffer (integer)
[ IN sendtype] data type of send buffer elements (handle)
[ OUT recvbuf] address of receive buffer (choice, significant only at root)
[ IN recvcounts] integer array (of length group size) containing the number of elements that are received from each process (significant only at root)
[ IN 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)
[ IN recvtype] data type of recv buffer elements (significant only at root) (handle)
[ IN root] rank of receiving process (integer)
[ IN comm] communicator (handle)

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

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

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,

and the root executes `n` receives,

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 `j`th portion of the receive buffer recvbuf on process root. The `j`th portion of recvbuf begins at offset displs[j] elements (in terms of recvtype) into recvbuf.

The receive buffer is ignored for all non-root 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 Examples using MPI_GATHER, MPI_GATHERV .

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.

Up: Collective Communication Next: Examples using MPI_GATHER, MPI_GATHERV Previous: Example using MPI_BCAST

Return to MPI 1.1 Standard Index
Return to MPI-2 Standard Index
Return to MPI Forum Home Page

MPI-1.1 of June 12, 1995
HTML Generated on August 6, 1997