MPI_COMM_RANK(3)

🌸 NAME 🌸

okay so bestie, this is the MPI_COMM_RANK(3) page from Open MPI, and it’s literally about one thing: MPI_Comm_rank — determines the rank of the calling process in the communicator. that’s the whole vibe. 💅

✨ SYNTAX ✨

🧵 C Syntax

#include <mpi.h>

int MPI_Comm_rank(MPI_Comm comm, int *rank)

🧵 Fortran Syntax

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

🧵 Fortran 2008 Syntax

USE mpi_f08
MPI_Comm_rank(comm, rank, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(OUT) :: rank
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🎀 INPUT PARAMETERS 🎀

• comm — communicator (handle). that’s the little social circle we’re asking about, bestie. 🫶

💖 OUTPUT PARAMETERS 💖

• rank — rank of the calling process in group of comm (integer). this is your lil number in the group chat. 👑
• ierror — Fortran only: error status (integer). Fortran girlies get this extra arg, slay. ✨

🦋 DESCRIPTION 🦋

This function gives the rank of the process in the particular communicator’s group. fr fr, that’s the whole point. It is equivalent to accessing the communicator’s group with MPI_Comm_group, computing the rank using MPI_Group_rank, and then freeing the temporary group via MPI_Group_free. so like, same energy, less manual group-making. 💅

Many programs will be written with the manager-worker model, where one process (such as the rank-zero process) will play a supervisory role, and the other processes will serve as compute nodes. In this framework, MPI_Comm_size and MPI_Comm_rank are useful for determining the roles of the various processes of a communicator. main-character rank-zero energy, honestly. 🌟

💀 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. yes, even the chill ones. 😭

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. so yeah, the error-handling lore is deep. 🫠

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? yes. effective? also 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. very “know your context” of it all. 🎀
• MPI_ERRORS_RETURN — returns an error code to the application. the “hand it back to you” option. ✨

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, do not treat an MPI error as a soft little oopsie. it can be real. 😭

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. iykyk. 📚

🌈 SEE ALSO 🌈

• MPI_Comm_group
• MPI_Comm_size
• MPI_Comm_compare

💜 COPYRIGHT 💜

2003-2026, The Open MPI Community

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💖 cute lil sign-off: source/version Open MPI, footer date Sep 11, 2026, page MPI_COMM_RANK(3) — copyright stays 2003-2026, The Open MPI Community. byeee bestie, go find your rank and slay! 🎀✨