MPI_Init_thread(3) Open MPI 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
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 rou-
tines 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 restric-
tions.
The level of thread support available to the program is set in pro-
vided, 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 implementa-
tion, 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++ func-
tions 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
4.1.2 Nov 24, 2021 MPI_Init_thread(3)
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