# 18.2.302. MPI_Request_free

MPI_Request_free — Frees a communication request object.

## 18.2.302.1. SYNTAX

### 18.2.302.1.1. C Syntax

    #include <mpi.h>

    int MPI_Request_free(MPI_Request *request)

### 18.2.302.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_REQUEST_FREE(REQUEST, IERROR)
         INTEGER REQUEST, IERROR

### 18.2.302.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Request_free(request, ierror)
         TYPE(MPI_Request), INTENT(INOUT) :: request
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.302.2. INPUT/OUTPUT PARAMETER

- `request`: Communication request (handle).

## 18.2.302.3. DESCRIPTION

This operation allows a request object to be deallocated without waiting for the associated communication to complete.

MPI_Request_free marks the request object for deallocation and sets request to MPI_REQUEST_NULL. Any ongoing communication that is associated with the request will be allowed to complete. The request will be deallocated only after its completion.

## 18.2.302.4. NOTES

Once a request is freed by a call to MPI_Request_free, it is not possible to check for the successful completion of the associated communication with calls to MPI_Wait or MPI_Test. Also, if an error occurs subsequently during the communication, an error code cannot be returned to the user — such an error must be treated as fatal. Questions arise as to how one knows when the operations have completed when using MPI_Request_free. Depending on the program logic, there may be other ways in which the program knows that certain operations have completed and this makes usage of MPI_Request_free practical. For example, an active send request could be freed when the logic of the program is such that the receiver sends a reply to the message sent — the arrival of the reply informs the sender that the send has completed and the send buffer can be reused. An active receive request should never be freed, as the receiver will have no way to verify that the receive has completed and the receive buffer can be reused.

Example:

    CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
    IF(rank == 0) THEN
        DO i=1, n
          CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
          CALL MPI_REQUEST_FREE(req, ierr)
          CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
          CALL MPI_WAIT(req, status, ierr)
        END DO
    ELSE IF (rank == 1) THEN
        CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
        CALL MPI_WAIT(req, status)
        DO I=1, n-1
           CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
           CALL MPI_REQUEST_FREE(req, ierr)
           CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
           CALL MPI_WAIT(req, status, ierr)
        END DO
        CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
        CALL MPI_WAIT(req, status)
    END IF

This routine is normally used to free persistent requests created with either MPI_Recv_init or MPI_Send_init and friends. However, it can be used to free a request created with MPI_Irecv or MPI_Isend and friends; in that case the use can not use the test/wait routines on the request.

It is permitted to free an active request. However, once freed, you can not use the request in a wait or test routine (e.g., MPI_Wait ).

## 18.2.302.5. 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.

See also

- MPI_Isend

- MPI_Irecv

- MPI_Issend

- MPI_Ibsend

- MPI_Irsend

- MPI_Recv_init

- MPI_Send_init

- MPI_Ssend_init

- MPI_Rsend_init

- MPI_Test

- MPI_Wait

- MPI_Waitall

- MPI_Waitany

- MPI_Waitsome

- MPI_Testall

- MPI_Testany

- MPI_Testsome