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.TH "MPI_COMM_TEST_INTER" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPI_Comm_test_inter\fP — Tests to see if a comm is an
intercommunicator.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Comm_test_inter(MPI_Comm comm, int *flag)
.ft P
.fi
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.UNINDENT
.SS Fortran Syntax
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.INDENT 3.5
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.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_COMM_TEST_INTER(COMM, FLAG, IERROR)
     INTEGER COMM, IERROR
     LOGICAL FLAG
.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
MPI_Comm_test_inter(comm, flag, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     LOGICAL, INTENT(OUT) :: flag
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETER
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.IP \(bu 2
\fBcomm\fP: Communicator (handle).
.UNINDENT
.SH OUTPUT PARAMETERS
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.IP \(bu 2
\fBflag (Logical.)\fP:
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
This local routine allows the calling process to determine the type of a
communicator. It returns true for an intercommunicator, false for an
intracommunicator.
.sp
The type of communicator also affects the value returned by three other
functions. When dealing with an intracommunicator (enables communication
within a single group), the functions listed below return the expected
values, group size, group, and rank. When dealing with an
inter\-communicator, however, they return the following values:
.INDENT 0.0
.TP
.B \fI\%MPI_Comm_size\fP
Returns the size of the local group.
.TP
.B \fI\%MPI_Comm_group\fP
Returns the local group.
.TP
.B \fI\%MPI_Comm_rank\fP
Returns the rank in the local group.
.UNINDENT
.sp
To return the remote group and remote group size of an
inter\-communicator, use the \fI\%MPI_Comm_remote_group\fP and
\fI\%MPI_Comm_remote_size\fP functions.
.sp
The operation \fI\%MPI_Comm_compare\fP is valid for intercommunicators. Both
communicators must be either intra\- or intercommunicators, or else
MPI_UNEQUAL results. Both corresponding local and remote groups must
compare correctly to get the results MPI_CONGRUENT and MPI_SIMILAR. In
particular, it is possible for MPI_SIMILAR to result because either the
local or remote groups were similar but not identical.
.sp
The following accessors provide consistent access to the remote group of
an intercommunicator: \fI\%MPI_Comm_remote_size\fP, \fI\%MPI_Comm_remote_group\fP\&.
.sp
The intercommunicator accessors (\fI\%MPI_Comm_test_inter\fP,
\fI\%MPI_Comm_remote_size\fP, MPI_Comm_remote_group) are all local operations.
.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.
.UNINDENT
.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
.UNINDENT
.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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.INDENT 3.5
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.IP \(bu 2
\fI\%MPI_Comm_remote_group\fP
.IP \(bu 2
\fI\%MPI_Comm_remote_size\fP
.IP \(bu 2
\fI\%MPI_Intercomm_create\fP
.IP \(bu 2
\fI\%MPI_Intercomm_merge\fP
.UNINDENT
.UNINDENT
.UNINDENT
.SH COPYRIGHT
2003-2025, The Open MPI Community
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