271 lines
9.2 KiB
Groff
271 lines
9.2 KiB
Groff
.\" Man page generated from reStructuredText.
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.TH "MPI_EXSCAN_INIT" "3" "Feb 14, 2025" "" "Open MPI"
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.\" new: \\n[rst2man-indent\\n[rst2man-indent-level]]
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.in \\n[rst2man-indent\\n[rst2man-indent-level]]u
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..
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.INDENT 0.0
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.INDENT 3.5
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.UNINDENT
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.UNINDENT
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.sp
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\fI\%MPI_Exscan\fP, \fI\%MPI_Iexscan\fP \- Computes an exclusive scan (partial
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reduction)
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.SH SYNTAX
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.SS C Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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#include <mpi.h>
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int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
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int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
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MPI_Request *request)
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int MPI_Exscan_init(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
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MPI_Info info, MPI_Request *request)
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SS Fortran Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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USE MPI
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! or the older form: INCLUDE \(aqmpif.h\(aq
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MPI_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, IERROR
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MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR
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MPI_EXSCAN_INIT(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SS Fortran 2008 Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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USE mpi_f08
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MPI_Exscan(sendbuf, recvbuf, count, datatype, op, comm, ierror)
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TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
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TYPE(*), DIMENSION(..) :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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MPI_Iexscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror)
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TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
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TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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TYPE(MPI_Request), INTENT(OUT) :: request
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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MPI_Exscan_init(sendbuf, recvbuf, count, datatype, op, comm, info, request, ierror)
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TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
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TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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TYPE(MPI_Info), INTENT(IN) :: info
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TYPE(MPI_Request), INTENT(OUT) :: request
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SH INPUT PARAMETERS
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.INDENT 0.0
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.IP \(bu 2
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\fBsendbuf\fP: Send buffer (choice).
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.IP \(bu 2
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\fBcount\fP: Number of elements in input buffer (integer).
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.IP \(bu 2
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\fBdatatype\fP: Data type of elements of input buffer (handle).
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.IP \(bu 2
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\fBop\fP: Operation (handle).
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.IP \(bu 2
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\fBcomm\fP: Communicator (handle).
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.UNINDENT
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.SH OUTPUT PARAMETERS
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.INDENT 0.0
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.IP \(bu 2
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\fBrecvbuf\fP: Receive buffer (choice).
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.IP \(bu 2
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\fBrequest\fP: Request (handle, non\-blocking only).
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.IP \(bu 2
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\fBierror\fP: Fortran only: Error status (integer).
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.UNINDENT
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.SH DESCRIPTION
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.sp
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\fI\%MPI_Exscan\fP is used to perform an exclusive prefix reduction on data
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distributed across the calling processes. The operation returns, in the
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\fIrecvbuf\fP of the process with rank i, the reduction (calculated
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according to the function \fIop\fP) of the values in the \fIsendbuf\fPs of
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processes with ranks 0, …, i\-1. Compare this with the functionality of
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\fI\%MPI_Scan\fP, which calculates over the range 0, …, i (inclusive). The
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type of operations supported, their semantics, and the constraints on
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send and receive buffers are as for \fI\%MPI_Reduce\fP\&.
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.sp
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The value in \fIrecvbuf\fP on process 0 is undefined and unreliable as
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\fIrecvbuf\fP is not significant for process 0. The value of \fIrecvbuf\fP on
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process 1 is always the value in \fIsendbuf\fP on process 0.
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.SH USE OF IN-PLACE OPTION
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.sp
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The \(gain place’ option for intracommunicators is specified by passing
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MPI_IN_PLACE in the \fIsendbuf\fP argument. In this case, the input data is
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taken from the receive buffer, and replaced by the output data.
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.sp
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Note that MPI_IN_PLACE is a special kind of value; it has the same
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restrictions on its use as MPI_BOTTOM.
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.sp
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Because the in\-place option converts the receive buffer into a
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send\-and\-receive buffer, a Fortran binding that includes INTENT must
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mark these as INOUT, not OUT.
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.SH NOTES
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.sp
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MPI does not specify which process computes which operation. In
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particular, both processes 0 and 1 may participate in the computation
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even though the results for both processes’ \fIrecvbuf\fP are degenerate.
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Therefore, all processes, including 0 and 1, must provide the same \fIop\fP\&.
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.sp
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It can be argued, from a mathematical perspective, that the definition
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of \fI\%MPI_Exscan\fP is unsatisfactory because the output at process 0 is
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undefined. The “mathematically correct” output for process 0 would be
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the unit element of the reduction operation. However, such a definition
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of an exclusive scan would not work with user\-defined \fIop\fP functions as
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there is no way for MPI to “know” the unit value for these custom
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operations.
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.SH NOTES ON COLLECTIVE OPERATIONS
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.sp
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The reduction functions of type MPI_Op do not return an error value. As
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a result, if the functions detect an error, all they can do is either
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call \fI\%MPI_Abort\fP or silently skip the problem. Thus, if the error handler
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is changed from MPI_ERRORS_ARE_FATAL to something else (e.g.,
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MPI_ERRORS_RETURN), then no error may be indicated.
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.sp
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The reason for this is the performance problems in ensuring that all
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collective routines return the same error value.
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.SH ERRORS
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.sp
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Almost all MPI routines return an error value; C routines as the return result
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of the function and Fortran routines in the last argument.
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.sp
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Before the error value is returned, the current MPI error handler associated
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with the communication object (e.g., communicator, window, file) is called.
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If no communication object is associated with the MPI call, then the call is
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considered attached to MPI_COMM_SELF and will call the associated MPI error
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handler. When MPI_COMM_SELF is not initialized (i.e., before
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\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
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Model exclusively) the error raises the initial error handler. The initial
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error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
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MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
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argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
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error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error
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handler is called for all other MPI functions.
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.sp
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Open MPI includes three predefined error handlers that can be used:
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.INDENT 0.0
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.IP \(bu 2
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\fBMPI_ERRORS_ARE_FATAL\fP
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Causes the program to abort all connected MPI processes.
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.IP \(bu 2
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\fBMPI_ERRORS_ABORT\fP
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An error handler that can be invoked on a communicator,
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window, file, or session. When called on a communicator, it
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acts as if \fI\%MPI_Abort\fP was called on that communicator. If
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called on a window or file, acts as if \fI\%MPI_Abort\fP was called
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on a communicator containing the group of processes in the
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corresponding window or file. If called on a session,
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aborts only the local process.
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.IP \(bu 2
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\fBMPI_ERRORS_RETURN\fP
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Returns an error code to the application.
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.UNINDENT
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.sp
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MPI applications can also implement their own error handlers by calling:
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.INDENT 0.0
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.IP \(bu 2
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\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
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.IP \(bu 2
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\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
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.IP \(bu 2
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\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
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.IP \(bu 2
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\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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.UNINDENT
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.sp
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Note that MPI does not guarantee that an MPI program can continue past
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an error.
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.sp
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See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
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.sp
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See the Error Handling section of the MPI\-3.1 standard for
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more information.
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.sp
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\fBSEE ALSO:\fP
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.INDENT 0.0
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.INDENT 3.5
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.INDENT 0.0
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.IP \(bu 2
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\fI\%MPI_Op_create\fP
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.IP \(bu 2
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\fI\%MPI_Reduce\fP
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.IP \(bu 2
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\fI\%MPI_Scan\fP
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.UNINDENT
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.UNINDENT
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.UNINDENT
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.SH COPYRIGHT
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2003-2025, The Open MPI Community
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.\" Generated by docutils manpage writer.
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.
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