# 17.2.373. MPI_Test

MPI_Test — Tests for the completion of a specific send or receive.

## 17.2.373.1. SYNTAX

### 17.2.373.1.1. C Syntax

    #include <mpi.h>

    int MPI_Test(MPI_Request *request, int *flag, MPI_Status *status)

### 17.2.373.1.2. Fortran Syntax

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

### 17.2.373.1.3. Fortran 2008 Syntax

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

## 17.2.373.2. INPUT PARAMETER

- `request`: Communication request (handle).

## 17.2.373.3. OUTPUT PARAMETERS

- `flag`: True if operation completed (logical).

- `status`: Status object (status).

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

## 17.2.373.4. DESCRIPTION

A call to MPI_Test returns flag = true if the operation identified by request is complete. In such a case, the status object is set to contain information on the completed operation; if the communication object was created by a nonblocking send or receive, then it is deallocated and the request handle is set to `MPI_REQUEST_NULL`. The call returns flag = false, otherwise. In this case, the value of the status object is undefined. MPI_Test is a local operation.

The return status object for a receive operation carries information that 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 carries information that can be accessed by a call to MPI_Test_cancelled (see the “Probe and Cancel” section of 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_Test with a null or inactive request argument. In such a case the operation returns with flag = true and empty status.

The functions MPI_Wait and MPI_Test can be used to complete both sends and receives.

## 17.2.373.5. NOTES

The use of the nonblocking MPI_Test call allows the user to schedule alternative activities within a single thread of execution. An event-driven thread scheduler can be emulated with periodic calls to MPI_Test.

## 17.2.373.6. 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_Test 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_Test 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_Testall

- MPI_Testany

- MPI_Testsome

- MPI_Wait

- MPI_Waitall

- MPI_Waitany

- MPI_Waitsome

- MPI_Win_set_errhandler