MPI_INIT(3)

✨ MPI_INIT(3) ✨

okay so this is the Open MPI page for MPI_Init and bestie, it's literally the first thing your MPI program ever does — it's giving main character energy for real 💅

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🌸 NAME 🌸

MPI_Init — initializes the MPI world model 🫶

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✨ SYNTAX ✨

💜 C Syntax

#include <mpi.h>

int MPI_Init(int *argc, char ***argv)

🦋 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_INIT(IERROR)
     INTEGER IERROR

🎀 Fortran 2008 Syntax

USE mpi_f08
MPI_Init(ierror)
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🍓 INPUT PARAMETERS 🍓

• argc — C only, babes: pointer to the number of arguments.
• argv — C only: the argument vector.

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⭐ OUTPUT PARAMETER ⭐

• ierror — Fortran only: error status, and yes it's an integer 💖

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👑 DESCRIPTION 👑

So this routine — or its sibling MPI_Init_thread — initializes the MPI world model. Either one of them must be called before you go calling MPI communication routines inside the MPI world model. No skipping, no cap 🙅‍♀️

The MPI world model can be initialized at most exactly once in the lifetime of an MPI process. Like, once. That's it. That's the whole vibe. This is different from the MPI session model, which can be initialized and finalized multiple times in one MPI process — see MPI_Session_init and MPI_Session_finalize iykyk.

Also: see MPI-5.0:11.4.1 for a list of MPI functionality that's available (e.g., even when the MPI world model has not yet been initialized or has already been finalized). Yes, there's stuff that works outside the world model, iconic.

Open MPI's MPI_Init and MPI_Init_thread both accept the C argc and argv arguments to main, but neither modifies, interprets, nor distributes them — they literally just hold them, no thoughts head empty 🧚

#include <mpi.h>

int main(int argv, char *argv[]) {
    MPI_Init(&argc, &argv);
    /* ...body of main MPI program... */
    MPI_Finalize();
    return 0;
}

By default, MPI_Init is effectively equivalent to invoking MPI_Init_thread with a required value of MPI_THREAD_SINGLE. BUT 👀 — if the OMPI_MPI_THREAD_LEVEL environment variable is set to a valid value when MPI_Init is invoked, it's equivalent to invoking MPI_Init_thread with required set to the corresponding value of the OMPI_MPI_THREAD_LEVEL environment variable. See MPI_Init_thread for more details, bestie, there's lore there.

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💀 NOTES 💀

The Fortran version does not have provisions for argc and argv and takes only IERROR. That's it. That's all it gets.

The MPI Standard does not specify what a program using the MPI world model can do before invoking MPI_Init or MPI_Init_thread or after invoking 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. Like fr fr, don't do it 🙈

seriously, keep it minimal before MPI_Init and after MPI_Finalize — no sneaky file opens, no reading stdin, no printing 💅

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😭 ERRORS 😭

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

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 MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. 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.

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. Dramatic? yes. Effective? also yes 💀
• MPI_ERRORS_ABORT — an error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort 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.
• MPI_ERRORS_RETURN — returns an error code to the application. Chill of it honestly.

MPI applications can also implement their own error handlers by calling:

• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error. So like… don't count on it 🫠

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

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🦋 SEE ALSO 🦋

• MPI_Init_thread
• MPI_Initialized
• MPI_Finalize
• MPI_Finalized
• MPI_Session_finalize
• MPI_Session_init

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🌈 COPYRIGHT 🌈

2003-2026, The Open MPI Community

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and that's the whole page bestie — shoutout to the Open MPI girlies, this one dropped Sep 11, 2026, go init your world model and slay 💖✨🎀