18.2.283. MPI_Probe

MPI_Probe - Blocking test for a message.

18.2.283.1. SYNTAX

18.2.283.1.1. C Syntax

#include <mpi.h>

int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)

18.2.283.1.2. Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
     INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

18.2.283.1.3. Fortran 2008 Syntax

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

18.2.283.2. INPUT PARAMETERS

  • source: Source rank or MPI_ANY_SOURCE (integer).

  • tag: Tag value or MPI_ANY_TAG (integer).

  • comm: Communicator (handle).

18.2.283.3. OUTPUT PARAMETERS

  • status: Status object (status).

  • IERROR: Fortran only: Error status (integer).

18.2.283.4. DESCRIPTION

The MPI_Probe and MPI_Iprobe 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.

MPI_Probe behaves like MPI_Iprobe except that it is a blocking call that returns only after a matching message has been found.

If your application does not need to examine the status field, you can save resources by using the predefined constant MPI_STATUS_IGNORE as a special value for the status argument.

The semantics of MPI_Probe and MPI_Iprobe guarantee progress: If a call to MPI_Probe has been issued by a process, and a send that matches the probe has been initiated by some process, then the call to MPI_Probe 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 MPI_Iprobe and a matching message has been issued, then the call to MPI_Iprobe will eventually return flag = true unless the message is received by another concurrent receive operation.

Example 1: Use blocking probe to wait for an incoming message.

CALL MPI_COMM_RANK(comm, rank, ierr)
       IF (rank.EQ.0) THEN
            CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
       ELSE IF(rank.EQ.1) THEN
            CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
       ELSE   ! rank.EQ.2
           DO i=1, 2
              CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
                              comm, status, ierr)
              IF (status(MPI_SOURCE) = 0) THEN
100                CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
              ELSE
200                CALL MPI_RECV(x, 1, MPI_REAL, 1, 0, status, ierr)
              END IF
           END DO
       END IF

Each message is received with the right type.

Example 2: A program similar to the previous example, but with a problem.

CALL MPI_COMM_RANK(comm, rank, ierr)
       IF (rank.EQ.0) THEN
            CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
       ELSE IF(rank.EQ.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
100                CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE,
                                 0, status, ierr)
              ELSE
200                CALL MPI_RECV(x, 1, MPI_REAL, MPI_ANY_SOURCE,
                                 0, status, ierr)
              END IF
           END DO
       END IF

We slightly modified Example 2, using MPI_ANY_SOURCE 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 MPI_Probe.

18.2.283.5. 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[_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.

In the sessions model, the error handler can be set during MPI_Session_init.

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.

Custom MPI error handlers can be created by calling: MPI_Comm_create_errhandler(3) MPI_File_create_errhandler(3) MPI_Session_create_errhandler(3) MPI_Win_create_errhandler(3)

Predefined and custom error handlers can be set by calling: MPI_Comm_set_errhandler(3) MPI_File_set_errhandler(3) MPI_Session_set_errhandler(3) MPI_Win_set_errhandler(3)

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 standard for more information.