MPI_GROUP_RANK(3)

💅 MPI_Group_rank — like, where do I even sit in this group?? 👑

Okay bestie, welcome to the Open MPI page for MPI_Group_rank — yes, the manual is literally called MPI_GROUP_RANK(3) and it's giving very much "small function, big feelings" energy. Let's get into it. 🫶

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✨ NAME ✨

MPI_Group_rank — returns the rank of the calling process in the given group. That's it. That's the whole vibe check. 💫

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🌸 SYNTAX 🌸

💻 C Syntax

#include <mpi.h>

int MPI_Group_rank(MPI_Group group, int *rank)

🧚 Fortran Syntax

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

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Group_rank(group, rank, ierror)
     TYPE(MPI_Group), INTENT(IN) :: group
     INTEGER, INTENT(OUT) :: rank
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎀 INPUT PARAMETERS 🎀

• group — a Group (handle). Slay, hand it over. 💅

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⭐ OUTPUT PARAMETERS ⭐

• rank — the rank of the calling process in group, or MPI_UNDEFINED if the process is not a member (integer). So either you're in the group chat or you're literally not, no in-between. 😭
• ierror — Fortran only: error status (integer). The Fortran girlies get their own little error bag. 🎁

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🔥 DESCRIPTION 🔥

So the way that MPI_Group_rank works is: it returns, as the output parameter rank, the rank of the calling process in group. That's your seat number, bestie, your spot in the lineup, your main character coordinate. 👑

And if the process is not a member of group? Then MPI_UNDEFINED is returned. Not a member, not a rank. It's giving uninvited to the party and honestly? Respect. 🙈

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💀 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. Classic. 💅

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. 😤 (Yes, that's a whole lot — it's the whole lore, don't skip it.)

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. Everyone. Gone. 💀
• 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, it 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, it aborts only the local process. Selective chaos, we love to see it. 🔥
• MPI_ERRORS_RETURN — returns an error code to the application. Chillest of the three, ngl. 😌

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. Like, fr fr, an error can be the end of the story. 🫠

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 👑

Copyright 2003-2026, The Open MPI Community. Iconic, long-running, still slaying. 💖

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And that's the whole page, bestie! This one was dated Sep 11, 2026 and brought to you by the Open MPI girlies and their MPI_GROUP_RANK(3) era. Go find your rank. Periodt. ✨🎀🦋