# 18.2.237. MPI_Iprobe

MPI_Iprobe — Nonblocking test for a message.

## 18.2.237.1. SYNTAX

### 18.2.237.1.1. C Syntax

    #include <mpi.h>

    int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag,
         MPI_Status *status)

### 18.2.237.1.2. Fortran Syntax

    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

### 18.2.237.1.3. Fortran 2008 Syntax

    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

## 18.2.237.2. INPUT PARAMETERS

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

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

- `comm`: Communicator (handle).

## 18.2.237.3. OUTPUT PARAMETERS

- `flag`: Message-waiting flag (logical).

- `status`: Status object (status).

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

## 18.2.237.4. 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 (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 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 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.

## 18.2.237.5. NOTE

Multi-threaded application developers should remember that two threads calling MPI_Iprobe may return true for the same message in both threads.

## 18.2.237.6. 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_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 MPI_Iprobe 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_Probe

- MPI_Cancel