--- name: "MPI_Scatter_init" section: "3" platform: "linux" package: "openmpi" version: "5.0.11-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/MPI_Scatter_init" --- # MPI\_Scatter\_init(3) *MPI\_Scatter*, *MPI\_Iscatter*, *MPI\_Scatter\_init* - Sends data from one task to all tasks in a group. ## SYNTAX ### C Syntax ``` #include int MPI_Scatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm) int MPI_Iscatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Request *request) int MPI_Scatter_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, int recvcount, 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_SCATTER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT INTEGER COMM, IERROR MPI_ISCATTER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT INTEGER COMM, REQUEST, IERROR MPI_SCATTER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT INTEGER COMM, INFO, REQUEST, IERROR ``` ### Fortran 2008 Syntax ``` USE mpi_f08 MPI_Scatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm, ierror) TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf TYPE(*), DIMENSION(..) :: recvbuf INTEGER, INTENT(IN) :: sendcount, recvcount, root TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, OPTIONAL, INTENT(OUT) :: ierror MPI_Iscatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount, recvcount, root 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_Scatter_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm, info, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount, recvcount, root 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**: Address of send buffer (choice, significant only at root). - • **sendcount**: Number of elements sent to each process (integer, significant only at root). - • **sendtype**: Datatype of send buffer elements (handle, significant only at root). - • **recvcount**: Number of elements in receive buffer (integer). - • **recvtype**: Datatype of receive buffer elements (handle). - • **root**: Rank of sending process (integer). - • **comm**: Communicator (handle). - • **info**: Info (handle, persistent). ## OUTPUT PARAMETERS - • **recvbuf**: Address of receive buffer (choice). - • **request**: Request (handle, non-blocking only). - • **ierror**: Fortran only: Error status (integer). ## DESCRIPTION *MPI\_Scatter* is the inverse operation to *MPI\_Gather*. The outcome is as if the root executed n send operations, ``` MPI_Send(sendbuf + i * sendcount * extent(sendtype), sendcount, sendtype, i, ...); ``` and each process executed a receive, ``` MPI_Recv(recvbuf, recvcount, recvtype, i, ...).; ``` An alternative description is that the root sends a message with **MPI\_Send(sendbuf, sendcount \* n, sendtype, ...)**. This message is split into *n* equal segments, the ith segment is sent to the ith process in the group, and each process receives this message as above. The send buffer is ignored for all nonroot processes. The type signature associated with *sendcount*, *sendtype* at the root must be equal to the type signature associated with *recvcount*, *recvtype* at all processes (however, the type maps may be different). 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 *recvbuf*, *recvcount*, *recvtype*, *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 read more than once. **Rationale:** Though not needed, the last restriction is imposed so as to achieve symmetry with *MPI\_Gather*, where the corresponding restriction (a multiple-write restriction) is necessary. **Example:** The reverse of Example 1 in the *MPI\_Gather* manpage. Scatter sets of 100 ints from the root to each process in the group. ``` MPI_Comm comm; int gsize,*sendbuf; int root, rbuf[100]; ... MPI_Comm_size(comm, &gsize); sendbuf = (int *)malloc(gsize*100*sizeof(int)); ... MPI_Scatter(sendbuf, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm); ``` ## USE OF IN-PLACE OPTION When the communicator is an intracommunicator, you can perform a scatter 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 *recvbuf*. In this case, *recvcount* and *recvtype* are ignored, and the root process sends no data to itself. 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 sends data to all 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 receive buffer argument of the root process in the first group must be consistent with the receive buffer argument of the processes 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\_Scatterv* - • *MPI\_Gather* - • *MPI\_Gatherv* ## COPYRIGHT 2003-2026, The Open MPI Community