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SYNTAX
C Syntax
#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)
Fortran Syntax
USE MPI
USE MPI_EXT
! or the older form: INCLUDE 'mpif.h'
MPIX_COMM_AGREE(COMM, FLAG, IERROR)
INTEGER COMM, FLAG, IERROR
MPIX_COMM_IAGREE(COMM, FLAG, REQUEST, IERROR)
INTEGER COMM, FLAG, REQUEST, IERROR
Fortran 2008 Syntax
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
INPUT PARAMETERS
• comm: Communicator (handle).
• flag: Binary flags (integer).
OUTPUT PARAMETERS
• flag: Reduced binary flags (integer).
• request: Request (handle, non-blocking only).
• ierror: Fortran only: Error status (integer).
DESCRIPTION
This collective communication agrees on the integer value flag and (implicitly) on the group of failed
processes in comm.
On completion, all non-failed MPI processes have agreed to set the output integer value of flag to the
result of a bitwise AND operation over the contributed input values of flag.
MPIX_Comm_iagree is the non-blocking variant of MPIX_Comm_agree.
PROCESS FAILURES
When an MPI process fails before contributing to the agree operation, the flag is computed ignoring its
contribution, and the operation raises an error of class MPIX_ERR_PROC_FAILED.
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently raised at all MPI processes in
the group(s) of comm.
After MPIX_Comm_agree raised an error of class MPIX_ERR_PROC_FAILED, the group produced by a subsequent
call to MPIX_Comm_get_failed on comm contains every MPI process that didn’t contribute to the computation
of flag.
WHEN THE COMMUNICATOR CONTAINS ACKNOWLEDGED FAILURES
If all MPI processes in the group of comm have acknowledged the failure of an MPI process (using
MPIX_Comm_ack_failed) prior to the call to MPIX_Comm_agree (or MPIX_Comm_iagree), the
MPIX_ERR_PROC_FAILED error is not raised when the output value of flag 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 comm, all processes raise an error of class MPIX_ERR_PROC_FAILED.
Example 1: Using a combination of MPIX_Comm_ack_failed and MPIX_Comm_agree users can propagate and
synchronize the knowledge of failures across all MPI processes in comm.
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's 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);
}
}
WHEN THE COMMUNICATOR IS REVOKED
This function never raises an error of class MPIX_ERR_REVOKED. The defined semantics of MPIX_Comm_agree
are maintained when comm is revoked, or when the group of comm contains failed MPI processes. In
particular, MPIX_Comm_agree is a collective operation, even when comm is revoked.
WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
When the communicator is an inter-communicator, the value of flag is a bitwise AND operation over the
values contributed by the remote group.
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently raised at all MPI processes in
the group(s) of comm, that is, both the local and remote groups of the inter-communicator.
ERRORS
Almost all MPI routines return an error value; C routines as the return result of the function and
Fortran routines in the last argument.
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 MPI_Init/MPI_Init_thread, after
MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler.
The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/‐
MPI_Comm_spawn_multiple. 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.
Open MPI includes three predefined error handlers that can be used:
• MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.
• MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a
window or file, acts as if MPI_Abort 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.
• MPI_ERRORS_RETURN Returns an error code to the application.
MPI applications can also implement their own error handlers by calling:
• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
SEE ALSO:
• MPIX_Comm_is_revoked
• MPIX_Comm_ack_failed
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPIX_COMM_AGREE(3)