MPI_Probe(3) Open MPI MPI_Probe(3)
NAME
MPI_Probe - Blocking test for a message.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Probe(source, tag, comm, status, ierror)
INTEGER, INTENT(IN) :: source, tag
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Status) :: status
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
C++ Syntax
#include <mpi.h>
void Comm::Probe(int source, int tag, Status& status) const
void Comm::Probe(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
status Status object (status).
IERROR Fortran only: Error status (integer).
DESCRIPTION
The MPI_Probe and MPI_Iprobe operations allow checking of incoming mes-
sages, 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 re-
ceive 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 re-
ceive 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 prob-
lem.
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 ar-
gument 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.
ERRORS
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++ func-
tions 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.
SEE ALSO
MPI_Iprobe
MPI_Cancel
4.1.2 Nov 24, 2021 MPI_Probe(3)
Generated by dwww version 1.14 on Fri Jan 24 09:42:03 CET 2025.