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MPI_Probe - Blocking test for a message.
#include <mpi.h>
int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)
INCLUDE ’mpif.h’
MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR
#include <mpi.h>
void Comm::Probe(int source, int tag, Status& status) const
void Comm::Probe(int source, int tag) const
- source
- Source rank or MPI_ANY_SOURCE (integer).
- tag
- Tag
value or MPI_ANY_TAG (integer).
- comm
- Communicator (handle).
- status
- Status object (status).
- IERROR
- Fortran only: Error status (integer).
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.
Almost
all MPI routines return an error value; C routines as the value of the
function and Fortran routines in the last argument. C++ functions do not
return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS,
then on error the C++ exception mechanism will be used to throw an MPI:Exception
object.
Before the error value is returned, the current MPI error handler
is called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler;
the predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarantee that an MPI program
can continue past an error.
MPI_Iprobe
MPI_Cancel
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