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.TH "MPI_INIT" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPI_Init\fP — Initializes the MPI execution environment
.SH SYNTAX
.SS C Syntax
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.INDENT 3.5
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.nf
.ft C
#include <mpi.h>

int MPI_Init(int *argc, char ***argv)
.ft P
.fi
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.SS Fortran Syntax
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.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_INIT(IERROR)
     INTEGER IERROR
.ft P
.fi
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.SS Fortran 2008 Syntax
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.nf
.ft C
USE mpi_f08
MPI_Init(ierror)
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
.ft P
.fi
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBargc\fP: C only: Pointer to the number of arguments.
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\fBargv\fP: C only: Argument vector.
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.SH OUTPUT PARAMETER
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.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
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.SH DESCRIPTION
.sp
This routine, or \fI\%MPI_Init_thread\fP, must be called before most other MPI
routines are called. There are a small number of errors, such as
\fI\%MPI_Initialized\fP and \fI\%MPI_Finalized\fP\&. MPI can be initialized at most once;
subsequent calls to \fI\%MPI_Init\fP or \fI\%MPI_Init_thread\fP are erroneous.
.sp
All MPI programs must contain a call to \fI\%MPI_Init\fP or \fI\%MPI_Init_thread\fP\&.
Open MPI accepts the C \fIargc\fP and \fIargv\fP arguments to main, but neither
modifies, interprets, nor distributes them:
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.ft C
/* declare variables */
MPI_Init(&argc, &argv);
/* parse arguments */
/* main program */
MPI_Finalize();
.ft P
.fi
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.SH NOTES
.sp
The Fortran version does not have provisions for \fIargc\fP and \fIargv\fP and
takes only IERROR.
.sp
The MPI Standard does not say what a program can do before an \fI\%MPI_Init\fP
or after an \fI\%MPI_Finalize\fP\&. 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 ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler associated
with the communication object (e.g., communicator, window, file) is called.
If no communication object is associated with the MPI call, then the call is
considered attached to MPI_COMM_SELF and will call the associated MPI error
handler. When MPI_COMM_SELF is not initialized (i.e., before
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error
handler is called for all other MPI functions.
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Open MPI includes three predefined error handlers that can be used:
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\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP was called
on a communicator containing the group of processes in the
corresponding window or file. If called on a session,
aborts only the local process.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
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See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
\fBSEE ALSO:\fP
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.IP \(bu 2
\fI\%MPI_Init_thread\fP
.IP \(bu 2
\fI\%MPI_Initialized\fP
.IP \(bu 2
\fI\%MPI_Finalize\fP
.IP \(bu 2
\fI\%MPI_Finalized\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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