MPI_GROUP_RANK(3)

💖 MPI_Group_rank — from the Open MPI manual, section 3 💖

okay bestie, gather round, because today we're learning MPI_Group_rank and honestly? it's a serve. this one returns the rank of the calling process in the given group. that's it. that's the whole vibe. 🎀

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✨ NAME ✨

MPI_Group_rank — returns the rank of the calling process in the given group. no cap, that's literally what it does. 👑

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🌸 SYNTAX 🌸

the C era 💅

#include <mpi.h>

int MPI_Group_rank(MPI_Group group, int *rank)

the Fortran era 🦋

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

the Fortran 2008 era ✨

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 — the group (handle). that's the friend we're asking about 🫶

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💜 OUTPUT PARAMETERS 💜

• rank — rank of the calling process in group, or MPI_UNDEFINED if the process is not a member (integer). so if you're not in the group, the answer is literally MPI_UNDEFINED. not a rank. not a zero. undefined. the group said "who r u" 💀
• ierror — Fortran only: error status (integer). 🙈

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🎀 DESCRIPTION 🎀

so MPI_Group_rank hands you back, as the output parameter rank, the rank of the calling process in group. and if the process is not a member of group, then it returns MPI_UNDEFINED instead. that's the whole story. short, sweet, iconic. ⭐

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🔥 ERRORS 🔥

okay this is the long section, but it's important so don't scroll, bestie. 😭

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 that error value comes back to you, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. and if there's no communication object 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. and 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. phew. 🫠

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. yeah. everybody. gone. dramatic but valid 💀
• 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. so like, it knows where it is at all times, we love a self-aware king 👑
• MPI_ERRORS_RETURN — returns an error code to the application. polite. respectful. 🎀

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, it's not promising you anything. don't be surprised. 😭

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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and that's the whole page, bestie!! MPI_Group_rank — short function, huge energy. go find your rank, you're a 10 in every group fr fr 💅✨

signed off with love from the Open MPI crew — page dated Sep 10, 2026, MPI_GROUP_RANK(3), section 3 of the Open MPI manual. 🦋🌸🎀