MPI_Init_thread(3)

NAME
MPI_Init_thread - Initializes the MPI execution environment

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Init_thread(int *argc, char ***argv,
	int required, int *provided)

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

C++ Syntax
#include <mpi.h>
int MPI::Init_thread(int& argc, char**& argv, int required)
int MPI::Init_thread(int required)

INPUT PARAMETERS

argc
    C/C++ only: Pointer to the number of arguments.

argv
    C/C++ only: Argument vector.

required
    Desired level of thread support (integer).

OUTPUT PARAMETERS

provided
    Available level of thread support (integer).

IERROR
    Fortran only: Error status (integer).

DESCRIPTION
This routine, or MPI_Init, must be called before most other MPI routines are called. There are a small number of exceptions, such as MPI_Initialized and MPI_Finalized. MPI can be initialized at most once; subsequent calls to MPI_Init or MPI_Init_thread are erroneous.

MPI_Init_thread, as compared to MPI_Init, has a provision to request a certain level of thread support in required:

MPI_THREAD_SINGLE
    Only one thread will execute.

MPI_THREAD_FUNNELED
    If the process is multithreaded, only the thread that called MPI_Init_thread will make MPI calls.

MPI_THREAD_SERIALIZED
    If the process is multithreaded, only one thread will make MPI library calls at one time.

MPI_THREAD_MULTIPLE
    If the process is multithreaded, multiple threads may call MPI at once with no restrictions.

The level of thread support available to the program is set in provided, except in the C++ binding, where it is the return value of the function. 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.

Also note that calling MPI_Init_thread with a required value of MPI_THREAD_SINGLE is equivalent to calling MPI_Init.

All MPI programs must contain a call to MPI_Init or MPI_Init_thread. Open MPI accepts the C/C++ argc and argv arguments to main, but neither modifies, interprets, nor distributes them:

	{
		/* declare variables */
		MPI_Init_thread(&argc, &argv, req, &prov);
		/* parse arguments */
		/* main program */
		MPI_Finalize();
	}

NOTES
The Fortran version does not have provisions for argc and argv and takes only 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 say what a program can do before an MPI_Init_thread or after an 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.

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.

ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO
MPI_Init
MPI_Initialized
MPI_Finalize
MPI_Finalized