MPI_SESSION_GET_NUM_PSETS(3)

💖 MPI_SESSION_GET_NUM_PSETS 💖

okay bestie, welcome to the Open MPI manual, and specifically to page MPI_SESSION_GET_NUM_PSETS(3) — the lil function that counts your process sets for you. let's get into it 💅

---

✨ NAME ✨

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). it's giving "the session you're working in", slay.
• 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.

and now the part that's actually kinda iconic: an MPI implementation is allowed to increase the number of available process sets during the execution of an MPI application, whenever new process sets become available. but — and this is the ✨ rule ✨ — MPI implementations are not allowed to:

• change the index of a particular process set name,
• change the name of the process set at a particular index,
• or delete a process set name once it has been added.

periodt. once it's in, it stays in. 💅

---

🙈 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. so like, it's not the group's fault, it's the collective's 💀

---

💀 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.
• 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.

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

lowkey important: note that MPI does not guarantee that an MPI program can continue past an error. fr fr.

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.

---

🔥 SEE ALSO 🔥

• MPI_Session_init

---

👑 Copyright 👑

2003-2026, The Open MPI Community

---

and that's the whole page bestie, no cap 🫶 this was Open MPI, manual page MPI_SESSION_GET_NUM_PSETS(3), dated Sep 10, 2026 — go count those process sets, you got this 💖✨