17.2.236. MPI_Iprobe

MPI_Iprobe - Nonblocking test for a message.

17.2.236.1. SYNTAX

17.2.236.1.1. C Syntax

#include <mpi.h>

int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag,
     MPI_Status *status)

17.2.236.1.2. Fortran Syntax

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

17.2.236.1.3. Fortran 2008 Syntax

USE mpi_f08
MPI_Iprobe(source, tag, comm, flag, status, ierror)
     INTEGER, INTENT(IN) :: source, tag
     TYPE(MPI_Comm), INTENT(IN) :: comm
     LOGICAL, INTENT(OUT) :: flag
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

17.2.236.2. INPUT PARAMETERS

  • source: Source rank or MPI_ANY_SOURCE (integer).

  • tag: Tag value or MPI_ANY_TAG (integer).

  • comm: Communicator (handle).

17.2.236.3. OUTPUT PARAMETERS

  • flag: Message-waiting flag (logical).

  • status: Status object (status).

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

17.2.236.4. 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 (basically, the information returned by status). In particular, the user may allocate memory for the receive buffer, according to the length of the probed message.

MPI_Iprobe(source, tag, comm, flag, status) returns flag = true if there is a message that can be received and that matches the pattern specified by the arguments source, tag, and comm. The call matches the same message that would have been received by a call to MPI_Recv(…, source, tag, comm, status) executed at the same point in the program, and returns in status the same value that would have been returned by MPI_Recv(). Otherwise, the call returns flag = false, and leaves status undefined.

If MPI_Iprobe returns flag = true, then the content of the status object can be subsequently accessed as described in Section 3.2.5 of the MPI-1 Standard, “Return Status,” to find the source, tag, and length of the probed message.

A subsequent receive executed with the same context, and the source and tag returned in status by MPI_Iprobe will receive the message that was matched by the probe if no other intervening receive occurs after the probe. If the receiving process is multithreaded, it is the user’s responsibility to ensure that the last condition holds.

The source argument of MPI_Probe can be MPI_ANY_SOURCE, and the tag argument can be MPI_ANY_TAG, so that one can probe for messages from an arbitrary source and/or with an arbitrary tag. However, a specific communication context must be provided with the comm argument.

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.

It is not necessary to receive a message immediately after it has been probed for, and the same message may be probed for several times before it is received.

17.2.236.5. NOTE

Users of libmpi-mt should remember that two threads may do an MPI_Iprobe that actually returns true for the same message for both threads.

17.2.236.6. 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.