MPI_GROUP_SIZE(3)

💖 NAME 💖

MPI_Group_size — bestie, it literally returns the size of a group. That's it, that's the whole job. 👑

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✨ SYNTAX ✨

🍓 C Syntax

#include <mpi.h>

int MPI_Group_size(MPI_Group group, int *size)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GROUP_SIZE(GROUP, SIZE, IERROR)
     INTEGER GROUP, SIZE, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Group_size(group, size, ierror)
     TYPE(MPI_Group), INTENT(IN) :: group
     INTEGER, INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🌸 INPUT PARAMETERS 🌸

• group — the Group, as a handle. that's the vibe, that's the whole input 💅

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🫶 OUTPUT PARAMETERS 🫶

• size — how many processes are in the group (an integer, iconic).
• ierror — Fortran only bestie: the error status (integer).

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👑 DESCRIPTION 👑

so MPI_Group_size hands you back, in size, the number of processes chillin' in the group. no cap.

like, if group = MPI_GROUP_EMPTY, then the call returns size = 0 — empty group, empty vibes, still valid 🫶

but on the other hand (and this is the part you do NOT test your luck with), a call with group = MPI_GROUP_NULL is erroneous. it's giving undefined, it's giving not it. don't do it. 💀

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🔥 ERRORS 🔥

Almost all MPI routines return an error value, bestie. C routines drop it as the return result of the function, and Fortran routines put it in that last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) gets 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. dramatic much? yes. 💀
• 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. the chill one, lowkey. ✨

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

⚠️ note that MPI does not guarantee that an MPI program can continue past an error. so like, don't assume you can just glide past it, bestie.

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. 📚

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💜 Copyright 💜

2003-2026, The Open MPI Community

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okay that's the whole page bestie, you ate 🎀 this was the MPI_GROUP_SIZE(3) entry from the Open MPI manual, dated Sep 10, 2026, brought to you by The Open MPI Community (copyright 2003-2026) — ily, go read your group sizes responsibly 💖✨🦋