SYNTAX

C Syntax

#include <mpi.h>
int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag,
        MPI_Status *status)

Fortran Syntax

INCLUDE 'mpif.h'
MPI_IPROBE(SOURCE, TAG, COMM, FLAG, STATUS, IERROR)
        LOGICAL FLAG
        INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR 

C++ Syntax

#include <mpi.h>
bool Comm::Iprobe(int source, int tag, Status& status) const
bool Comm::Iprobe(int source, int tag) const

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 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.

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.