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.TH "MPIX_COMM_AGREE" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPIX_Comm_agree\fP, \fI\%MPIX_Comm_iagree\fP \- Agree on a flag value
from all live processes and distributes the result back to all live
processes, even after process failures.
.sp
This is part of the User Level Fault Mitigation \fI\%ULFM extension\fP\&.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>
#include <mpi\-ext.h>

int MPIX_Comm_agree(MPI_Comm comm, int *flag)

int MPIX_Comm_iagree(MPI_Comm comm, int *flag, MPI_Request *request)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE MPI
USE MPI_EXT
! or the older form: INCLUDE \(aqmpif.h\(aq

MPIX_COMM_AGREE(COMM, FLAG, IERROR)
     INTEGER COMM, FLAG, IERROR

MPIX_COMM_IAGREE(COMM, FLAG, REQUEST, IERROR)
     INTEGER COMM, FLAG, REQUEST, IERROR
.ft P
.fi
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.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE mpi_f08
USE mpi_ext_f08

MPIX_Comm_agree(comm, flag, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(INOUT) :: flag
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPIX_COMM_IAGREE(COMM, FLAG, REQUEST, IERROR)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(INOUT), ASYNCHRONOUS :: flag
     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
\fBcomm\fP: Communicator (handle).
.IP \(bu 2
\fBflag\fP: Binary flags (integer).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBflag\fP: Reduced binary flags (integer).
.IP \(bu 2
\fBrequest\fP: Request (handle, non\-blocking only).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
This collective communication agrees on the integer value \fIflag\fP and
(implicitly) on the group of failed processes in \fIcomm\fP\&.
.sp
On completion, all non\-failed MPI processes have agreed to set the
output integer value of \fIflag\fP to the result of a \fIbitwise AND\fP
operation over the contributed input values of \fIflag\fP\&.
.sp
\fI\%MPIX_Comm_iagree\fP is the non\-blocking variant of \fI\%MPIX_Comm_agree\fP\&.
.SH PROCESS FAILURES
.sp
When an MPI process fails before contributing to the agree operation,
the \fIflag\fP is computed ignoring its contribution, and the operation
raises an error of class MPIX_ERR_PROC_FAILED.
.sp
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently
raised at all MPI processes in the group(s) of \fIcomm\fP\&.
.sp
After \fI\%MPIX_Comm_agree\fP raised an error of class MPIX_ERR_PROC_FAILED,
the group produced by a subsequent call to \fI\%MPIX_Comm_get_failed\fP on
\fIcomm\fP contains every MPI process that didn’t contribute to the
computation of \fIflag\fP\&.
.SH WHEN THE COMMUNICATOR CONTAINS ACKNOWLEDGED FAILURES
.sp
If \fBall\fP MPI processes in the group of \fIcomm\fP have acknowledged the failure
of an MPI process (using \fI\%MPIX_Comm_ack_failed\fP) prior to the call to
\fI\%MPIX_Comm_agree\fP (or \fI\%MPIX_Comm_iagree\fP), the MPIX_ERR_PROC_FAILED
error is not raised when the output value of \fIflag\fP ignores the
contribution of that failed process. Note that this is an uniform property:
if a non\-contributing process is found to be not\-acknowledged at any live
process in \fIcomm\fP, all processes raise an error of class MPIX_ERR_PROC_FAILED.
.sp
\fBExample 1:\fP Using a combination of \fI\%MPIX_Comm_ack_failed\fP and
\fI\%MPIX_Comm_agree\fP users can propagate and synchronize the knowledge
of failures across all MPI processes in \fIcomm\fP\&.
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
Comm_get_failed_consistent(MPI_Comm c, MPI_Group * g) {
    int rc; int T=1;
    int size; int num_acked;
    MPI_Group gf;
    int ranges[3] = {0, 0, 1};

    MPI_Comm_size(c, &size);

    do {
        /* this routine is not pure: calling MPI_Comm_ack_failed
         * affects the state of the communicator c */
        MPIX_Comm_ack_failed(c, size, &num_acked);
        /* we simply ignore the T value in this example */
        rc = MPIX_Comm_agree(c, &T);
    } while( rc != MPI_SUCCESS );
    /* after this loop, MPIX_Comm_agree has returned MPI_SUCCESS at
     * all processes, so all processes have Acknowledged the same set of
     * failures. Let\(aqs get that set of failures in the g group. */
    if( 0 == num_acked ) {
        *g = MPI_GROUP_EMPTY;
    }
    else {
        MPIX_Comm_get_failed(c, &gf);
        ranges[1] = num_acked \- 1;
        MPI_Group_range_incl(gf, 1, ranges, g);
        MPI_Group_free(&gf);
    }
}
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.fi
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.SH WHEN THE COMMUNICATOR IS REVOKED
.sp
This function never raises an error of class MPIX_ERR_REVOKED.
The defined semantics of \fI\%MPIX_Comm_agree\fP are maintained when \fIcomm\fP
is revoked, or when the group of \fIcomm\fP contains failed MPI processes.
In particular, \fI\%MPIX_Comm_agree\fP is a collective operation, even
when \fIcomm\fP is revoked.
.SH WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
.sp
When the communicator is an inter\-communicator, the value of \fIflag\fP is
a \fIbitwise AND\fP operation over the values contributed by the remote
group.
.sp
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently
raised at all MPI processes in the group(s) of \fIcomm\fP, that is, both
the local and remote groups of the inter\-communicator.
.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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.sp
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\%MPIX_Comm_is_revoked\fP
.IP \(bu 2
\fI\%MPIX_Comm_ack_failed\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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