MPI_SESSION_GET_NUM_PSETS(3)			     Open MPI			      MPI_SESSION_GET_NUM_PSETS(3)

MPI_Session_get_num_psets — Query runtime for number of available process sets

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Session_get_num_psets(MPI_Session session, MPI_Info info, int *npset_names)

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

	  MPI_SESSION_GET_NUM_PSETS(SESSION, INFO, NPSET_NAMES, IERROR)
	      INTEGER SESSION, INFO, SESSION, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Session_get_num_psets(session, info, npset_names, ierror)
	      TYPE(MPI_Session), INTENT(IN) :: session
	      TYPE(MPI_Info), INTENT(IN) :: info
	      INTEGER, INTENT(OUT) :: npset_names
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • session : session (handle)

       • info : info object (handle)

OUTPUT PARAMETERS
       • npset_names : number of available process sets (non-negtive integer)

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

DESCRIPTION
       MPI_Session_get_num_psets  is  used  to query the runtime for the number of available process sets in which
       the calling MPI process is a member. An MPI implementation is allowed to increase the number  of	 available
       process	sets  during  the execution of an MPI application when new process sets become available. However,
       MPI implementations are not allowed to change the index of a particular process set name, or to change  the
       name of the process set at a particular index, or to delete a process set name once it has been added.

NOTES
       When  a process set becomes invalid, for example, when some processes become unreachable due to failures in
       the communication system, subsequent usage of the process  set  name  may  raise	 an  error.  For  example,
       creating	 an  MPI_Group from such a process set might succeed because it is a local operation, but creating
       an MPI_Comm from that group and attempting collective communication may raise 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-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_Session_init <#mpi-session-init>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026			      MPI_SESSION_GET_NUM_PSETS(3)
