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](#SYNTAX)

## [C Syntax](#C_Syntax)

```
#include <mpi.h>
int MPI_Session_get_num_psets(MPI_Session session, MPI_Info info, int *npset_names)
```

## [Fortran Syntax](#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](#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](#INPUT_PARAMETERS)

- **session** : session (handle)
- **info** : info object (handle)

# [OUTPUT PARAMETERS](#OUTPUT_PARAMETERS)

- **npset\_names** : number of available process sets (non-negtive integer)
- **ierror** : Fortran only: Error status (integer).

# [DESCRIPTION](#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](#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](#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 &lt;#mpi-init&gt;/MPI\_Init\_thread &lt;#mpi-init-thread&gt;, after MPI\_Finalize &lt;#mpi-finalize&gt;, 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 &lt;#mpi-comm-set-errhandler&gt; 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 &lt;#mpi-comm-spawn&gt;/MPI\_Comm\_spawn\_multiple &lt;#mpi-comm-spawn-multiple&gt;. 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 &lt;#mpi-abort&gt; was called on that communicator. If called on a window or file, acts as if MPI\_Abort &lt;#mpi-abort&gt; 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 &lt;#mpi-comm-create-errhandler&gt; then MPI\_Comm\_set\_errhandler &lt;#mpi-comm-set-errhandler&gt;
- MPI\_File\_create\_errhandler &lt;#mpi-file-create-errhandler&gt; then MPI\_File\_set\_errhandler &lt;#mpi-file-set-errhandler&gt;
- MPI\_Session\_create\_errhandler &lt;#mpi-session-create-errhandler&gt; then MPI\_Session\_set\_errhandler &lt;#mpi-session-set-errhandler&gt; or at MPI\_Session\_init &lt;#mpi-session-init&gt;
- MPI\_Win\_create\_errhandler &lt;#mpi-win-create-errhandler&gt; then MPI\_Win\_set\_errhandler &lt;#mpi-win-set-errhandler&gt;

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

See the MPI man page &lt;#open-mpi&gt; for a full list of MPI error codes &lt;#open-mpi-errors&gt;.

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

**See also:**

MPI\_Session\_init &lt;#mpi-session-init&gt;

# [Copyright](#Copyright)

2003-2026, The Open MPI Community

July 6, 2026

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Document: MPI_Session_get_num_psets(3) from openmpi-cuda-12.4 5.0.10-r2 (Chainguard OS)
Source: https://github.com/open-mpi/ompi