Gather

  gather

  MPI_GATHER(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm)

    IN       sendbuf                 starting address of send buffer
    IN       sendcount               number of elements in send buffer
    IN       sendtype                data type of send buffer elements
    OUT      recvbuf                 address of receive buffer
    IN       recvcount               number of elements for any single
                                     receive
    IN       recvtype                data type of recv buffer elements
    IN       root                    rank of reciving process
    IN       comm                    communicator

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 MPI_Send(sendbuf, sendcount, sendtype, root, ...), and the root had executed n calls to MPI_Recv(recvbuf+i-recvcount-extent(recvtype), recvcount, recvtype, i ,...), 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 and sendtype on process i must be equal to the type signature of recvcount and 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, and comm are significant. The argument root must have identical values on all processes and comm must represent the same intragroup communication domain.

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.

• Examples Using MPI_GATHER
• Gather, Vector Variant
• Examples Using MPI_GATHERV