# 17.2.284. MPI_Probe

MPI_Probe — Blocking test for a message.

## 17.2.284.1. SYNTAX

### 17.2.284.1.1. C Syntax

    #include <mpi.h>

    int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)

### 17.2.284.1.2. 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

### 17.2.284.1.3. 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

## 17.2.284.2. INPUT PARAMETERS

- `source`: Source rank or `MPI_ANY_SOURCE` (integer).

- `tag`: Tag value or `MPI_ANY_TAG` (integer).

- `comm`: Communicator (handle).

## 17.2.284.3. OUTPUT PARAMETERS

- `status`: Status object (status).

- `ierror`: Fortran only: Error status (integer).

## 17.2.284.4. 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 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 == 0) THEN
       CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
    ELSE IF(rank == 1) THEN
       CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
    ELSE   ! rank == 2
       DO i=1, 2
          CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
                         comm, status, ierr)
          IF (status(MPI_SOURCE) = 0) THEN
             CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
          ELSE
             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 == 0) THEN
       CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
    ELSE IF(rank == 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
             CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE, &
                           0, status, ierr)
          ELSE
             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.

## 17.2.284.5. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard, MPI errors on messages queried by MPI_Probe 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 MPI_Probe if the back-end error handler returns it. The pre-defined MPI error handler `MPI_ERRORS_RETURN` exhibits this behavior, for example.

See also

- MPI_Iprobe

- MPI_Cancel