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.TH "MPI_ALLGATHER_INIT" "3" "May 30, 2025" "" "Open MPI"
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.INDENT 3.5
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.sp
\fI\%MPI_Allgather\fP, \fI\%MPI_Iallgather\fP, \fI\%MPI_Allgather_init\fP \- Gathers data
from all processes and distributes it to all processes
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Allgather(const void *sendbuf, int  sendcount,
      MPI_Datatype sendtype, void *recvbuf, int recvcount,
      MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Iallgather(const void *sendbuf, int  sendcount,
      MPI_Datatype sendtype, void *recvbuf, int recvcount,
      MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)

int MPI_Allgather_init(const void *sendbuf, int  sendcount,
      MPI_Datatype sendtype, void *recvbuf, int recvcount,
      MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, COMM, IERROR)
     <type>  SENDBUF (*), RECVBUF (*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
     INTEGER IERROR

MPI_IALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF (*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
     INTEGER REQUEST, IERROR

MPI_ALLGATHER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                     RECVTYPE, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF (*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
     INTEGER INFO, REQUEST, IERROR
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE mpi_f08
MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     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_Allgather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                     comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     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
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBsendbuf\fP: Starting address of send buffer (choice).
.IP \(bu 2
\fBsendcount\fP: Number of elements in send buffer (integer).
.IP \(bu 2
\fBsendtype\fP: Datatype of send buffer elements (handle).
.IP \(bu 2
\fBrecvbuf\fP: Starting address of recv buffer (choice).
.IP \(bu 2
\fBrecvcount\fP: Number of elements received from any process (integer).
.IP \(bu 2
\fBrecvtype\fP: Datatype of receive buffer elements (handle).
.IP \(bu 2
\fBcomm\fP: Communicator (handle).
.IP \(bu 2
\fBinfo\fP: Info (handle, persistent only).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBrecvbuf\fP: Address of receive buffer (choice).
.IP \(bu 2
\fBrequest\fP: Request (handle, non\-blocking only).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
\fI\%MPI_Allgather\fP is similar to \fI\%MPI_Gather\fP, except that all processes
receive the result, instead of just the root. In other words, all
processes contribute to the result, and all processes receive the
result.
.sp
The type signature associated with sendcount, sendtype at a process must
be equal to the type signature associated with recvcount, recvtype at
any other process.
.sp
The outcome of a call to \fI\%MPI_Allgather\fP is as if all processes
executed n calls to
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
MPI_Gather(sendbuf, sendcount, sendtype, recvbuf, recvcount,
           recvtype, root, comm);

// for root = 0 , ..., n\-1.
.ft P
.fi
.UNINDENT
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.sp
The rules for correct usage of \fI\%MPI_Allgather\fP
are easily found from the corresponding rules for \fI\%MPI_Gather\fP\&.
.sp
\fBExample:\fP The all\-gather version of Example 1 in \fI\%MPI_Gather\fP\&. Using
\fI\%MPI_Allgather\fP, we will gather 100 ints from every process in the group
to every process.
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.INDENT 3.5
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.nf
.ft C
MPI_Comm comm;
int gsize,sendarray[100];
int *rbuf;
\&...
MPI_Comm_size( comm, &gsize);
rbuf = (int *)malloc(gsize*100*sizeof(int));
MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);
.ft P
.fi
.UNINDENT
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.sp
After the call, every process has the group\-wide concatenation of the
sets of data.
.SH USE OF IN-PLACE OPTION
.sp
When the communicator is an intracommunicator, you can perform an
all\-gather operation in\-place (the output buffer is used as the input
buffer). Use the variable MPI_IN_PLACE as the value of \fIsendbuf\fP\&. In
this case, \fIsendcount\fP and \fIsendtype\fP are ignored. The input data of
each process is assumed to be in the area where that process would
receive its own contribution to the receive buffer. Specifically, the
outcome of a call to \fI\%MPI_Allgather\fP that used the in\-place option is
identical to the case in which all processes executed \fIn\fP calls to
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.INDENT 3.5
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.nf
.ft C
MPI_Allgather( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
               recvcount, recvtype, root, comm )

// for root =0, ... , n\-1.
.ft P
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.sp
Note that MPI_IN_PLACE is a special kind of value; it has the same
restrictions on its use as MPI_BOTTOM.
.sp
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.
.SH WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
.sp
When the communicator is an inter\-communicator, the gather operation
occurs in two phases. The data is gathered from all the members of the
first group and received by all the members of the second group. Then
the data is gathered from all the members of the second group and
received by all the members of the first. The operation, however, need
not be symmetric. The number of items sent by the processes in first
group need not be equal to the number of items sent by the the processes
in the second group. You can move data in only one direction by giving
\fIsendcount\fP a value of 0 for communication in the reverse direction.
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
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.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
\fBSEE ALSO:\fP
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.IP \(bu 2
\fI\%MPI_Allgatherv\fP
.IP \(bu 2
\fI\%MPI_Gather\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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