.\" -*- nroff -*-
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright (c) 2010-2020 Cisco Systems, Inc.  All rights reserved.
.\" Copyright (c) 2020      Google, LLC. All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Init_thread 3 "May 26, 2022" "4.1.4" "Open MPI"
.
.SH NAME
\fBMPI_Init_thread\fP \- Initializes the MPI execution environment
.
.SH SYNTAX
.ft R
.
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Init_thread(int *\fIargc\fP, char ***\fIargv\fP,
	int \fIrequired\fP, int *\fIprovided\fP)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_INIT_THREAD(\fIREQUIRED, PROVIDED, IERROR\fP)
	INTEGER	\fIREQUIRED, PROVIDED, IERROR\fP

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Init_thread(\fIrequired\fP, \fIprovided\fP, \fIierror\fP)
	INTEGER, INTENT(IN) :: \fIrequired\fP
	INTEGER, INTENT(OUT) :: \fIprovided\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH C++ Syntax
.nf
#include <mpi.h>
int MPI::Init_thread(int& \fIargc\fP, char**& \fIargv\fP, int \fIrequired\fP)
int MPI::Init_thread(int \fIrequired\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
argc
C/C++ only: Pointer to the number of arguments.
.TP 1i
argv
C/C++ only: Argument vector.
.TP 1i
required
Desired level of thread support (integer).
.
.
.SH OUTPUT PARAMETERS
.ft R
.TP 1i
provided
Available level of thread support (integer).
.TP 1i
IERROR
Fortran only: Error status (integer).
.
.
.SH DESCRIPTION
.ft R
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.
.sp
MPI_Init_thread, as compared to MPI_Init, has a provision to request a
certain level of thread support in \fIrequired\fP:
.TP 2.4i
MPI_THREAD_SINGLE
Only one thread will execute.
.TP 2.4i
MPI_THREAD_FUNNELED
If the process is multithreaded, only the thread that called
MPI_Init_thread will make MPI calls.
.TP 2.4i
MPI_THREAD_SERIALIZED
If the process is multithreaded, only one thread will make MPI library
calls at one time.
.TP 2.4i
MPI_THREAD_MULTIPLE
If the process is multithreaded, multiple threads may call MPI at once
with no restrictions.
.
.PP
The level of thread support available to the program is set in
\fIprovided\fP, 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
\fIprovided\fP will be greater than or equal to \fIrequired\fP.
.sp
Also note that calling MPI_Init_thread with a
.I required
value of
.I MPI_THREAD_SINGLE
is equivalent to calling MPI_Init.
.sp
All MPI programs must contain a call to MPI_Init or
MPI_Init_thread. Open MPI accepts the C/C++ \fIargc\fP and \fIargv\fP
arguments to main, but neither modifies, interprets, nor distributes
them:
.sp
.nf
	{
		/* declare variables */
		MPI_Init_thread(&argc, &argv, req, &prov);
		/* parse arguments */
		/* main program */
		MPI_Finalize();
	}
.fi
.
.SH NOTES
.ft R
The Fortran version does not have provisions for \fIargc\fP and
\fIargv\fP and takes only IERROR.
.sp
It is the caller's responsibility to check the value of \fIprovided\fP,
as it may be less than what was requested in \fIrequired\fP.
.sp
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.
.
.
.SH 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
.BR ompi_info (1)
to see if Open MPI has MPI_THREAD_MULTIPLE support:
.
.PP
.nf
shell$ ompi_info | grep "Thread support"
          Thread support: posix (MPI_THREAD_MULTIPLE: yes, OPAL support: yes, OMPI progress: no, Event lib: yes)
shell$
.fi
.
.PP
The "MPI_THREAD_MULTIPLE: yes" portion of the above output indicates
that Open MPI was compiled with MPI_THREAD_MULTIPLE support.
.
.PP
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.
.
.
.SH ERRORS
.ft R
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.
.sp
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.
.
.SH SEE ALSO
.ft R
.nf
MPI_Init
MPI_Initialized
MPI_Finalize
MPI_Finalized