386 lines
12 KiB
Groff
386 lines
12 KiB
Groff
.\" Man page generated from reStructuredText.
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.TH "MPI_GATHER_INIT" "3" "Feb 14, 2025" "" "Open MPI"
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.nr rst2man-indent-level 0
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..
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.de1 INDENT
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.de UNINDENT
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. RE
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.\" indent \\n[an-margin]
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.\" old: \\n[rst2man-indent\\n[rst2man-indent-level]]
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.nr rst2man-indent-level -1
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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_Gather\fP, \fI\%MPI_Igather\fP, \fI\%MPI_Gather_init\fP \- Gathers values from a group
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of processes.
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.SH SYNOPSIS
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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_Gather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
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void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
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MPI_Comm comm)
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int MPI_Igather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
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void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
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MPI_Comm comm, MPI_Request *request)
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int MPI_Gather_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
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void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
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MPI_Comm comm, 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_GATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
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RECVTYPE, ROOT, COMM, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
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INTEGER COMM, IERROR
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MPI_IGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
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RECVTYPE, ROOT, COMM, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
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INTEGER COMM, REQUEST, IERROR
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MPI_GATHER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
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RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
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INTEGER 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_Gather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
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root, 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) :: sendcount, recvcount, root
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TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
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TYPE(MPI_Comm), INTENT(IN) :: comm
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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MPI_Igather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
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root, 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) :: sendcount, recvcount, root
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TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
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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_Gather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
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root, 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) :: sendcount, recvcount, root
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TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
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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 : Starting address of send buffer (choice).
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.IP \(bu 2
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\fBsendcount\fP : Number of elements in send buffer (integer).
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.IP \(bu 2
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\fBsendtype\fP : Datatype of send buffer elements (handle).
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.IP \(bu 2
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\fBrecvcount\fP : Number of elements for any single receive (integer,
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significant only at root).
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.IP \(bu 2
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\fBrecvtype\fP : Datatype of recvbuffer elements (handle, significant only
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at root).
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.IP \(bu 2
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\fBroot\fP : Rank of receiving process (integer).
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.IP \(bu 2
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\fBcomm\fP : Communicator (handle).
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.IP \(bu 2
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\fBinfo\fP : Info (handle, persistent only).
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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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.INDENT 2.0
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.TP
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.B \fBrecvbuf\fP
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Address of receive buffer (choice, significant only at
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root).
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.UNINDENT
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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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Each process (root process included) sends the contents of its send
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buffer to the root process. The root process receives the messages and
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stores them in rank order. The outcome is as if each of the n processes
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in the group (including the root process) had executed a call to
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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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MPI_Send(sendbuf, sendcount, sendtype, root, ...)
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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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.sp
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and the root had executed n calls to
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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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MPI_Recv(recfbuf + i * recvcount * extent(recvtype), recvcount,
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recvtype, i, ...)
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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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.sp
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where extent(recvtype) is the type extent obtained from a call to
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\fI\%MPI_Type_extent\fP\&.
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.sp
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An alternative description is that the n messages sent by the processes
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in the group are concatenated in rank order, and the resulting message
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is received by the root as if by a call to
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\fBMPI_Recv(recvbuf, recvcount * n, recvtype, ... )\fP\&.
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.sp
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The receive buffer is ignored for all nonroot processes.
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.sp
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General, derived datatypes are allowed for both sendtype and recvtype.
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The type signature of sendcount, sendtype on process i must be equal to
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the type signature of recvcount, recvtype at the root. This implies that
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the amount of data sent must be equal to the amount of data received,
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pairwise between each process and the root. Distinct type maps between
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sender and receiver are still allowed.
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.sp
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All arguments to the function are significant on process root, while on
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other processes, only arguments sendbuf, sendcount, sendtype, root, comm
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are significant. The arguments root and comm must have identical values
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on all processes.
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.sp
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The specification of counts and types should not cause any location on
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the root to be written more than once. Such a call is erroneous. Note
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that the recvcount argument at the root indicates the number of items it
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receives from each process, not the total number of items it receives.
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.sp
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Example 1: Gather 100 ints from every process in group to root.
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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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MPI_Comm comm;
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int gsize, sendarray[100];
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int root, *rbuf;
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\&...
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MPI_Comm_size( comm, &gsize);
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rbuf = (int*)malloc(gsize* 100*sizeof(int));
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MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);
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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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.sp
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Example 2: Previous example modified — only the root allocates memory
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for the receive buffer.
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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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MPI_Comm comm;
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int gsize, sendarray[100];
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int root, myrank, *rbuf;
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\&...
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MPI_Comm_rank( comm, myrank);
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if ( myrank == root) {
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MPI_Comm_size( comm, &gsize);
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rbuf = (int *)malloc(gsize * 100*sizeof(int));
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}
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MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);
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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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.sp
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Example 3: Do the same as the previous example, but use a derived
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datatype. Note that the type cannot be the entire set of \fBgsize * 100\fP
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ints since type matching is defined pairwise between the root and each
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process in the gather.
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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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MPI_Comm comm;
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int gsize, sendarray[100];
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int root, *rbuf;
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MPI_Datatype rtype;
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\&...
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MPI_Comm_size( comm, &gsize);
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MPI_Type_contiguous( 100, MPI_INT, &rtype);
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MPI_Type_commit( &rtype );
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rbuf = (int*)malloc(gsize* 100*sizeof(int));
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MPI_Gather( sendarray, 100, MPI_INT, rbuf, 1, rtype, root, comm);
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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 USE OF IN-PLACE OPTION
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.sp
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When the communicator is an intracommunicator, you can perform a gather
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operation in\-place (the output buffer is used as the input buffer). Use
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the variable MPI_IN_PLACE as the value of the root process sendbuf. In
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this case, sendcount and sendtype are ignored, and the contribution of
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the root process to the gathered vector is assumed to already be in the
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correct place in the receive buffer. Note that MPI_IN_PLACE is a special
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kind of value; it has the same restrictions on its use as MPI_BOTTOM.
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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 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
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.sp
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When the communicator is an inter\-communicator, the root process in the
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first group gathers data from all the processes in the second group. The
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first group defines the root process. That process uses MPI_ROOT as the
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value of its root argument. The remaining processes use \fBMPI_PROC_NULL\fP as
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the value of their root argument. All processes in the second group use
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the rank of that root process in the first group as the value of their
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root argument. The send buffer argument of the processes in the first
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group must be consistent with the receive buffer argument of the root
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process in the second group.
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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_Gatherv\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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