# 18.2.423. MPI_Wait

MPI_Wait — Waits for an MPI send or receive to complete.

## 18.2.423.1. SYNTAX

### 18.2.423.1.1. C Syntax

    #include <mpi.h>

    int MPI_Wait(MPI_Request *request, MPI_Status *status)

### 18.2.423.1.2. Fortran Syntax

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

### 18.2.423.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Wait(request, status, ierror)
         TYPE(MPI_Request), INTENT(INOUT) :: request
         TYPE(MPI_Status) :: status
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.423.2. INPUT PARAMETER

- `request`: Request (handle).

## 18.2.423.3. OUTPUT PARAMETERS

- `status`: Status object (status).

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

## 18.2.423.4. DESCRIPTION

A call to MPI_Wait returns when the operation identified by request is complete. If the communication object associated with this request was created by a nonblocking send or receive call, then the object is deallocated by the call to MPI_Wait and the request handle is set to `MPI_REQUEST_NULL`.

The call returns, in status, information on the completed operation. The content of the status object for a receive operation can be accessed as described in the “Return Status” subsection of the “Point-to-Point Communication” chapter in the MPI Standard. The status object for a send operation may be queried by a call to MPI_Test_cancelled (see the “Probe and Cancel” section in the MPI Standard).

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.

One is allowed to call MPI_Wait with a null or inactive request argument. In this case the operation returns immediately with empty status.

## 18.2.423.5. NOTES

Successful return of MPI_Wait after an MPI_Ibsend implies that the user send buffer can be reused i.e., data has been sent out or copied into a buffer attached with MPI_Buffer_attach. Note that, at this point, we can no longer cancel the send (for more information, see the “Probe and Cancel” section in the MPI Standard). If a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of MPI_Cancel (always being able to free program space that was committed to the communication subsystem).

Example: Simple usage of nonblocking operations and MPI_Wait.

    CALL MPI_COMM_RANK(comm, rank, ierr)
    IF(rank == 0) THEN
        CALL MPI_ISEND(a(1), 10, MPI_REAL, 1, tag, comm, request, ierr)
        **** do some computation ****
        CALL MPI_WAIT(request, status, ierr)
    ELSE IF (rank == 1) THEN
        CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr)
        **** do some computation ****
        CALL MPI_WAIT(request, status, ierr)
    END IF

## 18.2.423.6. 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 requests passed to MPI_Wait 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_Wait 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_Comm_set_errhandler

- MPI_File_set_errhandler

- MPI_Test

- MPI_Testall

- MPI_Testany

- MPI_Testsome

- MPI_Waitall

- MPI_Waitany

- MPI_Waitsome

- MPI_Win_set_errhandler