17.2.232. MPI_Init_thread — Open MPI 5.0.10 documentation

# 17.2.232. MPI_Init_thread

MPI_Init_thread — Initializes the MPI world model

## 17.2.232.1. SYNTAX

### 17.2.232.1.1. C Syntax

    #include <mpi.h>

    int MPI_Init_thread(int *argc, char ***argv,
         int required, int *provided)

### 17.2.232.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR)
         INTEGER REQUIRED, PROVIDED, IERROR

### 17.2.232.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Init_thread(required, provided, ierror)
         INTEGER, INTENT(IN) :: required
         INTEGER, INTENT(OUT) :: provided
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.232.2. INPUT PARAMETERS

- `argc`: C only: Pointer to the number of arguments.

- `argv`: C only: Argument vector.

- `required`: Desired level of thread support (integer).

## 17.2.232.3. OUTPUT PARAMETERS

- `provided`: Available level of thread support (integer).

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

## 17.2.232.4. DESCRIPTION

This routine, or MPI_Init, initializes the MPI world model. Either of these routines must be called before MPI communication routines are called within the MPI world model. The MPI world model can be initialized at most exactly once in the lifetime of an MPI process. This is different than the MPI session model, which can be initialized and finalized multiple times in an MPI process. See MPI_Session_init and MPI_Session_finalize.

See MPI-5.0:11.4.1 for a list of MPI functionality that is available (e.g., even when the MPI world model has not yet initialized or has already been finalized).

The MPI world model can be initialized at most once; subsequent calls to MPI_Init or MPI_Init_thread are erroneous.

Alternatively, instead of the MPI world model, MPI applications can use the sessions model; see MPI_Session_init.

Upon return, the level of thread support available to the program is set in provided. In Open MPI, the value is dependent on how the library was configured and built. Note that there is no guarantee that provided will be greater than or equal to required.

Open MPI accepts the C argc and argv arguments to main, but neither modifies, interprets, nor distributes them:

    #include <mpi.h>

    int main(int argv, char *argv[]) {
        int provided;
        MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided);
        /* ...body of main MPI pogram... */
        MPI_Finalize();
        return 0;
    }

MPI_Init_thread has both a direct and an indirect mechanism to request a specific level of thread support. MPI_Init only has an indirect mechanism to request a specific level of thread support.

### 17.2.232.4.1. Direct request of thread level

MPI_Init_thread has the required parameter, which can be set to any one of the following constants (from `mpi.h`):

- `MPI_THREAD_SINGLE`: Indicating that only one thread will execute.

- `MPI_THREAD_FUNNELED`: Indicating that if the process is multithreaded, only the thread that called MPI_Init_thread will make MPI calls.

- `MPI_THREAD_SERIALIZED`: Indicating that if the process is multithreaded, only one thread will make MPI library calls at one time.

- `MPI_THREAD_MULTIPLE`: Indicating that if the process is multithreaded, multiple threads may call MPI at once with no restrictions.

The values of these constants adhere to the following relationships:

\begin{eqnarray} MPI\_THREAD\_SINGLE & < & MPI\_THREAD\_FUNNELED \\ MPI\_THREAD\_FUNNELED & < & MPI\_THREAD\_SERIALIZED \\ MPI\_THREAD\_SERIALIZED & < & MPI\_THREAD\_MULTIPLE \\ \end{eqnarray}

### 17.2.232.4.2. Indirect request of thread level

Both MPI_Init_thread and MPI_Init support an indirect method of indicating the required thread level: setting the `OMPI_MPI_THREAD_LEVEL` environment variable:

- If the `OMPI_MPI_THREAD_LEVEL` environment variable is set at the time MPI_Init is invoked, it behaves as if MPI_Init_thread was invoked with the corresponding `MPI_THREAD_*` constant value passed via the required parameter.

- If the `OMPI_MPI_THREAD_LEVEL` environment variable is set at the time MPI_Init_thread is invoked, the `MPI_THREAD_*` constant value corresponding to the environment variable value overrides the value passed via the required parameter.

The `OMPI_MPI_THREAD_LEVEL` environment variable can be set to any of the values listed below.

Value that Open MPI uses | Allowable values (case-insensitive)
--- | ---
`MPI_THREAD_SINGLE` | `MPI_THREAD_SINGLE`, `SINGLE`, 0
`MPI_THREAD_FUNNELED` | `MPI_THREAD_FUNNELED`, `FUNNELED`, 1
`MPI_THREAD_SERIALIZED` | `MPI_THREAD_SERIALIZED`, `SERIALIZED`, 2
`MPI_THREAD_MULTIPLE` | `MPI_THREAD_MULTIPLE`, `MULTIPLE`, 3

Note

In Open MPI v5.0.8 and prior, only the integer values 0 through 3 were acceptable values for the `OMPI_MPI_THREAD_LEVEL` environment variable.

Starting with Open MPI v5.0.9, the Open MPI community recomends using one of the string name variants.

## 17.2.232.5. NOTES

The Fortran version does not have provisions for `argc` and `argv` and takes only `REQUIRED`, `PROVIDED`, and `IERROR`.

It is the caller’s responsibility to check the value of `provided`, as it may be less than what was requested in `required`.

The MPI Standard does not specify what a program using the MPI world model can do before invoking MPI_Init or MPI_Init_thread or after invoking MPI_Finalize. In the Open MPI implementation, it should do as little as possible. In particular, avoid anything that changes the external state of the program, such as opening files, reading standard input, or writing to standard output.

### 17.2.232.5.1. MPI_THREAD_MULTIPLE Support

`MPI_THREAD_MULTIPLE` support is included if the environment in which Open MPI was built supports threading. You can check the output of ompi_info(1) to see if Open MPI has `MPI_THREAD_MULTIPLE` support:

    shell$ ompi_info | grep "Thread support"
              Thread support: posix (MPI_THREAD_MULTIPLE: yes, OPAL support: yes, OMPI progress: no, Event lib: yes)
    shell$

The `MPI_THREAD_MULTIPLE: yes` portion of the above output indicates that Open MPI was compiled with `MPI_THREAD_MULTIPLE` support.

Note that there is a small performance penalty for using `MPI_THREAD_MULTIPLE` support; latencies for short messages will be higher as compared to when using `MPI_THREAD_SINGLE`, for example.

## 17.2.232.6. 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.

See also

- MPI_Init

- MPI_Initialized

- MPI_Finalize

- MPI_Finalized

- MPI_Session_finalize

- MPI_Session_init