| 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 | datatype 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 | datatype of recv buffer elements (handle, significant only at root) |
| IN root | rank of receiving MPI process (integer) |
| IN comm | communicator (handle) |
If comm is an intra-communicator, each MPI process (the root included) sends the contents of its send buffer to the root. The root receives the messages and stores them in rank order. The outcome is as if each of the n MPI processes in the group (including the root) 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_get_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 nonroot MPI processes.
General, derived datatypes are allowed for both sendtype and recvtype. The type signature of sendcount, sendtype on each MPI 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 MPI process and the root. Distinct type maps between sender and receiver MPI processes are still allowed.
All arguments to the function are significant on the root, while on other MPI processes, only the arguments sendbuf, sendcount, sendtype, root, and comm are significant. The arguments root and comm must have identical values on all MPI 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 MPI process, not the total number of items it receives.
The ``in place'' option for intra-communicators 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 inter-communicator, then the call involves all MPI processes in the inter-communicator, but with one group (group A) defining the root. All MPI 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 MPI processes in group A pass the value MPI_PROC_NULL in root. Data is gathered from all MPI processes in group B to the root. The send buffer arguments of the MPI 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 | datatype of send buffer elements (handle) |
| OUT recvbuf | address of receive buffer (choice, significant only at root) |
| IN recvcounts | nonnegative integer array (of length group size) containing the number of elements that are received from each MPI 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 MPI process i (significant only at root) |
| IN recvtype | datatype of recv buffer elements (handle, significant only at root) |
| IN root | rank of receiving MPI process (integer) |
| IN comm | communicator (handle) |
MPI_GATHERV extends the functionality of MPI_GATHER by allowing a varying count of data from each MPI 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 intra-communicator, the outcome is as if each MPI process, including the root, sends a message to the root,
and the root executes n receives,
The data received from MPI process j is placed into recvbuf of the root beginning at offset displs[j] elements (in terms of the recvtype).
The receive buffer is ignored for all nonroot MPI processes.
The type signature implied by sendcount, sendtype on MPI 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 MPI 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 the root, while on other MPI processes, only arguments sendbuf, sendcount, sendtype, root, and comm are significant. The arguments root and comm must have identical values on all MPI 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 intra-communicators 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 inter-communicator, then the call involves all MPI processes in the inter-communicator, but with one group (group A) defining the root. All MPI 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 MPI processes in group A pass the value MPI_PROC_NULL in root. Data is gathered from all MPI processes in group B to the root. The send buffer arguments of the MPI processes in group B must be consistent with the receive buffer argument of the root.