oracular (3) MPI_Init_thread.openmpi.3.gz

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.

SEE ALSO

       MPI_Init
       MPI_Initialized
       MPI_Finalize
       MPI_Finalized