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.TH "MPI_PROBE" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPI_Probe\fP — Blocking test for a message.
.SH SYNTAX
.SS C Syntax
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.INDENT 3.5
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.nf
.ft C
#include <mpi.h>

int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)
.ft P
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.SS Fortran Syntax
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.INDENT 3.5
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.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
     INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR
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.SS Fortran 2008 Syntax
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.INDENT 3.5
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.ft C
USE mpi_f08
MPI_Probe(source, tag, comm, status, ierror)
     INTEGER, INTENT(IN) :: source, tag
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBsource\fP: Source rank or \fBMPI_ANY_SOURCE\fP (integer).
.IP \(bu 2
\fBtag\fP: Tag value or \fBMPI_ANY_TAG\fP (integer).
.IP \(bu 2
\fBcomm\fP: Communicator (handle).
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.SH OUTPUT PARAMETERS
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.IP \(bu 2
\fBstatus\fP: Status object (status).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
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.SH DESCRIPTION
.sp
The \fI\%MPI_Probe\fP and \fI\%MPI_Iprobe\fP operations allow checking of incoming
messages, without actual receipt of them. The user can then decide how
to receive them, based on the information returned by the probe in the
status variable. For example, the user may allocate memory for the
receive buffer, according to the length of the probed message.
.sp
\fI\%MPI_Probe\fP behaves like \fI\%MPI_Iprobe\fP except that it is a blocking call that
returns only after a matching message has been found.
.sp
If your application does not need to examine the \fIstatus\fP field, you can
save resources by using the predefined constant \fBMPI_STATUS_IGNORE\fP as a
special value for the \fIstatus\fP argument.
.sp
The semantics of \fI\%MPI_Probe\fP and \fI\%MPI_Iprobe\fP guarantee progress: If a call
to \fI\%MPI_Probe\fP has been issued by a process, and a send that matches the
probe has been initiated by some process, then the call to \fI\%MPI_Probe\fP
will return, unless the message is received by another concurrent
receive operation (that is executed by another thread at the probing
process). Similarly, if a process busy waits with \fI\%MPI_Iprobe\fP and a
matching message has been issued, then the call to \fI\%MPI_Iprobe\fP will
eventually return \fIflag\fP = true unless the message is received by another
concurrent receive operation.
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\fBExample 1:\fP Use blocking probe to wait for an incoming message.
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.ft C
CALL MPI_COMM_RANK(comm, rank, ierr)
IF (rank == 0) THEN
   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
ELSE IF(rank == 1) THEN
   CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
ELSE   ! rank == 2
   DO i=1, 2
      CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
                     comm, status, ierr)
      IF (status(MPI_SOURCE) = 0) THEN
         CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
      ELSE
         CALL MPI_RECV(x, 1, MPI_REAL, 1, 0, status, ierr)
      END IF
   END DO
END IF
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Each message is received with the right type.
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\fBExample 2:\fP A program similar to the previous example, but with a
problem.
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.ft C
CALL MPI_COMM_RANK(comm, rank, ierr)
IF (rank == 0) THEN
   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
ELSE IF(rank == 1) THEN
   CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
ELSE
   DO i=1, 2
      CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
                     comm, status, ierr)
      IF (status(MPI_SOURCE) == 0) THEN
         CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE, &
                       0, status, ierr)
      ELSE
         CALL MPI_RECV(x, 1, MPI_REAL, MPI_ANY_SOURCE, &
                       0, status, ierr)
      END IF
   END DO
END IF
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We slightly modified Example 2, using \fBMPI_ANY_SOURCE\fP as the source
argument in the two receive calls in statements labeled 100 and 200. The
program is now incorrect: The receive operation may receive a message
that is distinct from the message probed by the preceding call to
\fI\%MPI_Probe\fP\&.
.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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\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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Note that MPI does not guarantee that an MPI program can continue past
an error.
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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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See the Error Handling section of the MPI\-3.1 standard for
more information.
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Note that per the “Return Status” section in the “Point\-to\-Point
Communication” chapter in the \fI\%MPI Standard\fP, MPI errors on messages queried
by \fI\%MPI_Probe\fP do not set the \fBstatus.MPI_ERROR\fP field in the
returned \fIstatus\fP\&.  The error code is always passed to the back\-end
error handler and may be passed back to the caller through the return
value of \fI\%MPI_Probe\fP if the back\-end error handler returns it.
The pre\-defined MPI error handler \fBMPI_ERRORS_RETURN\fP exhibits this
behavior, for example.
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\fBSEE ALSO:\fP
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.IP \(bu 2
\fI\%MPI_Iprobe\fP
.IP \(bu 2
\fI\%MPI_Cancel\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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