| 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 (non-negative 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 (non-negative 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
void MPI::Comm::Gather(const void* sendbuf, int sendcount, const MPI::Datatype& sendtype, void* recvbuf, int recvcount, const MPI::Datatype& recvtype, int root) const = 0
If comm is an intracommunicator, 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, sendtype on each process 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, and 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. The ``in place'' option for intracommunicators is specified by passing MPI_IN_PLACE as the value of sendbuf at the root. In such a case, sendcount and sendtype are ignored, and the contribution of the root to the gathered vector is assumed to be already in the correct place in the receive buffer.
If comm is an intercommunicator, then the call involves all processes in the intercommunicator, but with one group (group A) defining the root process. All processes in the other group (group B) pass the same value in argument root, which is the rank of the root in group A. The root passes the value MPI_ROOT in root. All other processes in group A pass the value MPI_PROC_NULL in root. Data is gathered from all processes in group B to the root. The send buffer arguments of the processes in group B must be consistent with the receive buffer argument of the root.
| 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 (non-negative integer) |
| IN sendtype | data type of send buffer elements (handle) |
| OUT recvbuf | address of receive buffer (choice, significant only at root) |
| IN recvcounts | non-negative 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
void MPI::Comm::Gatherv(const void* sendbuf, int sendcount, const MPI::Datatype& sendtype, void* recvbuf, const int recvcounts[], const int displs[], const MPI::Datatype& recvtype, int root) const = 0
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.
If comm is an intracommunicator, 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,
The data received from process j is placed into recvbuf of the root process beginning at offset displs[j] elements (in terms of the recvtype). 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, and 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. The ``in place'' option for intracommunicators is specified by passing MPI_IN_PLACE as the value of sendbuf at the root. In such a case, sendcount and sendtype are ignored, and the contribution of the root to the gathered vector is assumed to be already in the correct place in the receive buffer
If comm is an intercommunicator, then the call involves all processes in the intercommunicator, but with one group (group A) defining the root process. All processes in the other group (group B) pass the same value in argument root, which is the rank of the root in group A. The root passes the value MPI_ROOT in root. All other processes in group A pass the value MPI_PROC_NULL in root. Data is gathered from all processes in group B to the root. The send buffer arguments of the processes in group B must be consistent with the receive buffer argument of the root.