MPI_INIT(3)

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version 5.0.10 · 7 versions
, in ubuntu stonking

💖 MPI_INIT — NAME 💖

okay bestie, welcome to the MPI_INIT page from the Open MPI manual (that's the manual name, section 3, library calls ✨). Here's the whole vibe: MPI_Init initializes the MPI world model. That's it, that's the girl. 🌈

✨ 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

🌸 INPUT PARAMETERS 🌸

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

🍓 OUTPUT PARAMETER 🍓

  • ierror — Fortran only: error status (integer). That's the whole list, she's tiny and she knows it 💜

👑 DESCRIPTION 👑

So here's the tea. This routine — or its twin MPI_Init_thread — initializes the MPI world model. One of these two must be called before you call any MPI communication routines inside the MPI world model. Periodt.

And the way that the MPI world model works? It can be initialized at most exactly once in the lifetime of an MPI process. Once. That's her one shot, no do-overs 💀. This is different from the MPI session model, which can be initialized and finalized multiple times in an MPI process. Go look at MPI_Session_init and MPI_Session_finalize if that's what you're after.

Also, check out MPI-5.0:11.4.1 at <https://www.mpi-forum.org/> for a list of MPI functionality that's available even when the MPI world model hasn't been initialized yet, or has already been finalized. iykyk ✨

Now, Open MPI's MPI_Init and MPI_Init_thread both accept the C argc and argv arguments to main, but neither one modifies, interprets, nor distributes them. Not one single thing. She just holds them 💅:

#include <mpi.h>

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

By default, MPI_Init is effectively equivalent to invoking MPI_Init_thread with a required value of MPI_THREAD_SINGLE. However — and this is the plot twist 🫶 — if the OMPI_MPI_THREAD_LEVEL environment variable is set to a valid value at the moment MPI_Init is invoked, it's equivalent to invoking MPI_Init_thread with required set to the corresponding value of that OMPI_MPI_THREAD_LEVEL environment variable. See MPI_Init_thread for more details, bestie.

📝 NOTES 📝

  • The Fortran version doesn't have provisions for argc and argv at all — it takes only IERROR. That's it. That's the whole signature. 🎀
  • The MPI Standard does not specify what a program using the MPI world model is allowed to 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. Specifically: avoid anything that changes the external state of the program, such as opening files, reading standard input, or writing to standard output. Seriously, keep it lowkey. 😭

💀 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) gets called. If no communication object is associated with the MPI call, then the call counts as attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF isn't initialized — i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when you're using the Sessions Model exclusively — the error raises the initial error handler. That initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when you're using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or via the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. And 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 queen 💅
  • 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, it 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, it aborts only the local process.
  • MPI_ERRORS_RETURN — returns an error code to the application.

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

  • MPI_Comm_create_errhandler and then MPI_Comm_set_errhandler
  • MPI_File_create_errhandler and then MPI_File_set_errhandler
  • MPI_Session_create_errhandler and then MPI_Session_set_errhandler, or at MPI_Session_init
  • MPI_Win_create_errhandler and then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error. No promises, no safety net 🌧️

See the MPI man page for a full list of MPI error codes, and see the Error Handling section of the MPI-3.1 standard for more information.

See also:

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

Copyright: 2003-2026, The Open MPI Community.

okay that's the whole page bestie, you ate 🫶 and one last crumb for the sign-off: this was the Open MPI manual, MPI_INIT(3), dated Sep 10, 2026. slay, go initialize something 💅✨🌸

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