:ref:`MPI_Iprobe` |mdash| Nonblocking test for a message.
#include <mpi.h>
int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag,
MPI_Status *status)
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
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
source
: Source rank orMPI_ANY_SOURCE
(integer).tag
: Tag value orMPI_ANY_TAG
(integer).comm
: Communicator (handle).
flag
: Message-waiting flag (logical).status
: Status object (status).ierror
: Fortran only: Error status (integer).
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.
Multi-threaded application developers should remember that two threads calling :ref:`MPI_Iprobe` may return true for the same message in both threads.
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`