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MPI_Iprobe.3.rst

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MPI_Iprobe

:ref:`MPI_Iprobe` |mdash| Nonblocking test for a message.

SYNTAX

C Syntax

#include <mpi.h>

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

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

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

INPUT PARAMETERS

  • source: Source rank or MPI_ANY_SOURCE (integer).
  • tag: Tag value or MPI_ANY_TAG (integer).
  • comm: Communicator (handle).

OUTPUT PARAMETERS

  • flag: Message-waiting flag (logical).
  • status: Status object (status).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

The :ref:`MPI_Probe` and :ref:`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 :ref:`MPI_Recv`. Otherwise, the call returns flag = false, and leaves status undefined.

If :ref:`MPI_Iprobe` returns flag = true, then the content of the status object can be subsequently accessed as described in the "Return Status" subsection of the "Point-to-Point Communication" chapter in the MPI Standard 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 :ref:`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 :ref:`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.

NOTE

Multi-threaded application developers should remember that two threads calling :ref:`MPI_Iprobe` may return true for the same message in both threads.

ERRORS

Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, MPI errors on messages queried by :ref:`MPI_Iprobe` do not set the status.MPI_ERROR field in the returned status. 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 :ref:`MPI_Iprobe` if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example.

.. seealso::
   * :ref:`MPI_Probe`
   * :ref:`MPI_Cancel`