Ubuntu Manpage: MPI_Init_thread - Initializes the MPI execution environment

Provided by: openmpi-doc_4.1.4-3ubuntu2_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.