260 lines
9.2 KiB
Groff
260 lines
9.2 KiB
Groff
.\" Man page generated from reStructuredText.
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.TH "MPI_UNPACK_EXTERNAL" "3" "Feb 14, 2025" "" "Open MPI"
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.sp
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\fI\%MPI_Unpack_external\fP — Reads data from a portable format
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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_Unpack_external(const char datarep[], const void *inbuf,
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MPI_Aint insize, MPI_Aint *position,
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void *outbuf, int outcount,
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MPI_Datatype datatype)
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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_UNPACK_EXTERNAL(DATAREP, INBUF, INSIZE, POSITION,
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OUTBUF, OUTCOUNT, DATATYPE, IERROR)
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INTEGER OUTCOUNT, DATATYPE, IERROR
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INTEGER(KIND=MPI_ADDRESS_KIND) INSIZE, POSITION
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CHARACTER*(*) DATAREP
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<type> INBUF(*), OUTBUF(*)
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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_Unpack_external(datarep, inbuf, insize, position, outbuf, outcount,
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datatype, ierror)
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CHARACTER(LEN=*), INTENT(IN) :: datarep
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TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
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TYPE(*), DIMENSION(..) :: outbuf
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INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: insize
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INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(INOUT) :: position
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INTEGER, INTENT(IN) :: outcount
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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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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\fBdatarep\fP: Data Representation (string).
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.IP \(bu 2
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\fBinbuf\fP: Input buffer start (choice).
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.IP \(bu 2
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\fBinsize\fP: Size of input buffer, in bytes (integer).
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.IP \(bu 2
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\fBoutcount\fP: Number of items to be unpacked (integer).
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.IP \(bu 2
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\fBdatatype\fP: Datatype of each output data item (handle).
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.UNINDENT
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.SH INPUT/OUTPUT PARAMETER
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.INDENT 0.0
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.IP \(bu 2
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\fBposition\fP: Current position in buffer, in bytes (integer).
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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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\fBoutbuf\fP: Output buffer start (choice).
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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_Unpack_external\fP unpacks data from the external32 format, a universal
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data representation defined by the MPI Forum. This format is useful for
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exchanging data between MPI implementations, or when writing data to a
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file.
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.sp
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The input buffer is a contiguous storage area pointed to by \fIinbuf\fP
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containing \fIinsize\fP bytes. The output buffer can be any communication
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buffer allowed in \fI\%MPI_Recv\fP, and is specified by \fIoutbuf\fP, \fIoutcount\fP,
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and \fIdatatype\fP\&.
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.sp
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The input value of \fIposition\fP is the first position in \fIinbuf\fP to be
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read for unpacking (measured in bytes, not elements, relative to the
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start of the buffer). When the function returns, \fIposition\fP is
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incremented by the size of the packed message, so that it points to the
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first location in \fIinbuf\fP following the message that was unpacked. This
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way it may be used as input to a subsequent call to \fI\%MPI_Unpack_external\fP\&.
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.SH NOTES
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.sp
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Note the difference between \fI\%MPI_Recv\fP and \fI\%MPI_Unpack_external\fP: In
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\fI\%MPI_Recv\fP, the \fIcount\fP argument specifies the maximum number of items
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that can be received. In \fI\%MPI_Unpack_external\fP, the \fIoutcount\fP argument
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specifies the actual number of items that are to be unpacked. With a
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regular receive operation, the incoming message size determines the
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number of components that will be received. With \fI\%MPI_Unpack_external\fP, it
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is up to the user to specify how many components to unpack, since the
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user may wish to unpack the received message multiple times into various
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buffers.
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.sp
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To understand the behavior of pack and unpack, it is convenient to think
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of the data part of a message as being the sequence obtained by
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concatenating the successive values sent in that message. The pack
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operation stores this sequence in the buffer space, as if sending the
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message to that buffer. The unpack operation retrieves this sequence
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from buffer space, as if receiving a message from that buffer. (It is
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helpful to think of internal Fortran files or sscanf in C for a similar
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function.)
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.sp
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Several messages can be successively packed into one packing unit. This
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is effected by several successive related calls to \fI\%MPI_Pack_external\fP,
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where the first call provides \fIposition\fP=0, and each successive call
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inputs the value of \fIposition\fP that was output by the previous call,
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along with the same values for \fIoutbuf\fP and \fIoutcount\fP\&. This packing
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unit now contains the equivalent information that would have been stored
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in a message by one send call with a send buffer that is the
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“concatenation” of the individual send buffers.
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.sp
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A packing unit can be sent using type MPI_BYTE. Any point\-to\-point or
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collective communication function can be used to move the sequence of
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bytes that forms the packing unit from one process to another. This
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packing unit can now be received using any receive operation, with any
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datatype: The type\-matching rules are relaxed for messages sent with
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type MPI_BYTE.
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.sp
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A packing unit can be unpacked into several successive messages. This is
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effected by several successive related calls to \fI\%MPI_Unpack_external\fP,
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where the first call provides \fIposition\fP=0, and each successive call
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inputs the value of position that was output by the previous call, and
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the same values for \fIinbuf\fP and \fIinsize\fP\&.
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.sp
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The concatenation of two packing units is not necessarily a packing
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unit; nor is a substring of a packing unit necessarily a packing unit.
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Thus, one cannot concatenate two packing units and then unpack the
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result as one packing unit; nor can one unpack a substring of a packing
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unit as a separate packing unit. Each packing unit that was created by a
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related sequence of pack calls must be unpacked as a unit by a sequence
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of related unpack calls.
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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_Pack_external\fP
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.IP \(bu 2
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\fI\%MPI_Pack_external_size\fP
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.IP \(bu 2
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\fI\%MPI_Recv\fP
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.IP \(bu 2
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sscanf(3C)
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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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