Ubuntu Manpage: MPI_Init_thread - Initializes the MPI execution environment

Provided by: openmpi-doc_4.1.6-7ubuntu2_all

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

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

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.