MPI_INIT(3)					     Open MPI					       MPI_INIT(3)

MPI_Init — Initializes the MPI world model

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: Pointer to the number of arguments.

       • argv: C only: Argument vector.

OUTPUT PARAMETER
       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       This  routine,  or  MPI_Init_thread  <#mpi-init-thread>,	 initializes the MPI world model.  Either of these
       routines must be called before MPI communication routines are called within the MPI world model.	  The  MPI
       world  model  can be initialized at most exactly once in the lifetime of an MPI process.	 This is different
       than the MPI session model, which can be initialized and finalized multiple times in an MPI  process.   See
       MPI_Session_init <#mpi-session-init> and MPI_Session_finalize <#mpi-session-finalize>.

       See  MPI-5.0:11.4.1  <https://www.mpi-forum.org/>  for a list of MPI functionality that is available (e.g.,
       even when the MPI world model has not yet initialized or has already been finalized).

       Open MPI’s MPI_Init and MPI_Init_thread <#mpi-init-thread> both accept the C argc  and  argv  arguments	to
       main, but neither modifies, interprets, nor distributes 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 <#mpi-init-thread> with a
       required value of MPI_THREAD_SINGLE.  However, if the OMPI_MPI_THREAD_LEVEL environment variable is set	to
       a  valid	 value	when  MPI_Init is invoked, it is equivalent to invoking MPI_Init_thread <#mpi-init-thread>
       with required set to the corresponding  value  of  the  OMPI_MPI_THREAD_LEVEL  environment  variable.   See
       MPI_Init_thread <#mpi-init-thread> for more details.

NOTES
       The Fortran version does not have provisions for argc and argv and takes only IERROR.

       The  MPI Standard does not specify what a program using the MPI world model can do before invoking MPI_Init
       or MPI_Init_thread <#mpi-init-thread> or after invoking	MPI_Finalize  <#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.

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	<#mpi-init-thread>,  after
       MPI_Finalize <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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.

       • 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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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.

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

       • MPI_Comm_create_errhandler	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Init_thread <#mpi-init-thread>

	  • MPI_Initialized <#mpi-initialized>

	  • MPI_Finalize <#mpi-finalize>

	  • MPI_Finalized <#mpi-finalized>

	  • MPI_Session_finalize <#mpi-session-finalize>

	  • MPI_Session_init <#mpi-session-init>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					       MPI_INIT(3)
