MPI_SESSION_INIT(3)				     Open MPI				       MPI_SESSION_INIT(3)

MPI_Session_init — Creates a new session handle

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Session_init(MPI_Info info, MPI_Errhandler errhandler, MPI_Session *session)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'

	  MPI_SESSION_INIT(INFO, ERRHANDLER, SESSION, IERROR)
	      INTEGER INFO, ERRHANDLER, SESSION, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Session_init(info, errhandler, session, ierror)
	      TYPE(MPI_Info), INTENT(IN) :: info
	      TYPE(MPI_Errhandler), INTENT(IN) :: errhandler
	      TYPE(MPI_Session), INTENT(OUT) :: session
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • info : info object (handle)

       •

	 errhandler (error handler to be attached to the returned session)
		(handle)

OUTPUT PARAMETERS
       • session : New session (handle).

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

DESCRIPTION
       MPI_Session_init	 is  used  to instantiate an MPI Session. The returned session handle can be used to query
       the runtime system about characteristics of the job within which the process is running, as well	 as  other
       system  resources.  Other MPI communications can also be initiated in the context of an MPI session handle.
       All sessions must be finalized via MPI_Session_finalize	<#mpi-session-finalize>	 before	 the  MPI  process
       terminates.

       Multiple	 sessions  can	be created and destroyed during the lifetime of an MPI process.	 This is different
       than MPI world model, which can be initialized at most  exactly	once  (and  then  subsequently	finalized)
       during the lifetime of an MPI process.

NOTES
       The  info  argument  is	used  to  request  MPI	functionality requirements and possible MPI implementation
       specific capabilities.

       The errhandler argument specifies an error handler to invoke in the event that  the  Session  instantiation
       call encounters an error.

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>/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_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 <#mpi-init>

	  • MPI_Initialized <#mpi-initialized>

	  • MPI_Init_thread <#mpi-init-thread>

	  • MPI_Finalize <#mpi-finalize>

	  • MPI_Finalized <#mpi-finalized>

	  • MPI_Group_from_session_pset <#mpi-group-from-session-pset>

	  • MPI_Session_c2f <#mpi-session-c2f>

	  • MPI_Session_call_errhandler <#mpi-session-call-errhandler>

	  • MPI_Session_create_errhandler <#mpi-session-create-errhandler>

	  • MPI_Session_f2c <#mpi-session-f2c>

	  • MPI_Session_finalize <#mpi-session-finalize>

	  • MPI_Session_get_errhandler <#mpi-session-get-errhandler>

	  • MPI_Session_get_info <#mpi-session-get-info>

	  • MPI_Session_get_nth_pset <#mpi-session-get-nth-pset>

	  • MPI_Session_get_num_psets <#mpi-session-get-num-psets>

	  • MPI_Session_get_pset_info <#mpi-session-get-pset-info>

	  • MPI_Session_init

	  • MPI_Session_set_errhandler <#mpi-session-set-errhandler>

	  • MPI_T_pvar_session_create <#mpi-t-pvar-session-create>

	  • MPI_T_pvar_session_free <#mpi-t-pvar-session-free>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				       MPI_SESSION_INIT(3)
