--- name: "MPI_Exscan_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_Exscan_init" --- # MPI\_Exscan\_init(3) *MPI\_Exscan*, *MPI\_Iexscan* - Computes an exclusive scan (partial reduction) ## SYNTAX ### C Syntax ``` #include int MPI_Exscan(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm) int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request) int MPI_Exscan_init(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Info info, MPI_Request *request) ``` ### Fortran Syntax ``` USE MPI ! or the older form: INCLUDE 'mpif.h' MPI_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR) SENDBUF(*), RECVBUF(*) INTEGER COUNT, DATATYPE, OP, COMM, IERROR MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR MPI_EXSCAN_INIT(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR ``` ### Fortran 2008 Syntax ``` USE mpi_f08 MPI_Exscan(sendbuf, recvbuf, count, datatype, op, comm, ierror) TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf TYPE(*), DIMENSION(..) :: recvbuf INTEGER, INTENT(IN) :: count TYPE(MPI_Datatype), INTENT(IN) :: datatype TYPE(MPI_Op), INTENT(IN) :: op TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, OPTIONAL, INTENT(OUT) :: ierror MPI_Iexscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: count TYPE(MPI_Datatype), INTENT(IN) :: datatype TYPE(MPI_Op), INTENT(IN) :: op TYPE(MPI_Comm), INTENT(IN) :: comm TYPE(MPI_Request), INTENT(OUT) :: request INTEGER, OPTIONAL, INTENT(OUT) :: ierror MPI_Exscan_init(sendbuf, recvbuf, count, datatype, op, comm, info, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: count TYPE(MPI_Datatype), INTENT(IN) :: datatype TYPE(MPI_Op), INTENT(IN) :: op 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**: Send buffer (choice). - • **count**: Number of elements in input buffer (integer). - • **datatype**: Data type of elements of input buffer (handle). - • **op**: Operation (handle). - • **comm**: Communicator (handle). ## OUTPUT PARAMETERS - • **recvbuf**: Receive buffer (choice). - • **request**: Request (handle, non-blocking only). - • **ierror**: Fortran only: Error status (integer). ## DESCRIPTION *MPI\_Exscan* is used to perform an exclusive prefix reduction on data distributed across the calling processes. The operation returns, in the *recvbuf* of the process with rank i, the reduction (calculated according to the function *op*) of the values in the *sendbuf*s of processes with ranks 0, …, i-1. Compare this with the functionality of *MPI\_Scan*, which calculates over the range 0, …, i (inclusive). The type of operations supported, their semantics, and the constraints on send and receive buffers are as for *MPI\_Reduce*. The value in *recvbuf* on process 0 is undefined and unreliable as *recvbuf* is not significant for process 0. The value of *recvbuf* on process 1 is always the value in *sendbuf* on process 0. ## USE OF IN-PLACE OPTION The \`in place’ option for intracommunicators is specified by passing MPI\_IN\_PLACE in the *sendbuf* argument. In this case, the input data is taken from the receive buffer, and replaced by the output data. 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. ## NOTES MPI does not specify which process computes which operation. In particular, both processes 0 and 1 may participate in the computation even though the results for both processes’ *recvbuf* are degenerate. Therefore, all processes, including 0 and 1, must provide the same *op*. It can be argued, from a mathematical perspective, that the definition of *MPI\_Exscan* is unsatisfactory because the output at process 0 is undefined. The “mathematically correct” output for process 0 would be the unit element of the reduction operation. However, such a definition of an exclusive scan would not work with user-defined *op* functions as there is no way for MPI to “know” the unit value for these custom operations. ## NOTES ON COLLECTIVE OPERATIONS The reduction functions of type MPI\_Op do not return an error value. As a result, if the functions detect an error, all they can do is either call *MPI\_Abort* or silently skip the problem. Thus, if the error handler is changed from MPI\_ERRORS\_ARE\_FATAL to something else (e.g., MPI\_ERRORS\_RETURN), then no error may be indicated. The reason for this is the performance problems in ensuring that all collective routines return the same error value. ## 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\_Op\_create* - • *MPI\_Reduce* - • *MPI\_Scan* ## COPYRIGHT 2003-2026, The Open MPI Community