MPI_TOPO_TEST(3)

💅 MPI_TOPO_TEST(3) — like, the Open MPI manual page, but make it make sense 🫶

okay so bestie, gather round, because today we're talking about MPI_Topo_test from the Open MPI manual (3rd section, iykyk), and honestly? she's giving "quick vibe check for your communicator" energy and I'm obsessed. ✨

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🌸 NAME 🌸

MPI_Topo_test — literally determines the type of topology (if any) associated with a communicator. that's the whole job. no cap. 💅

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🎀 SYNTAX 🎀

✨ C Syntax ✨

#include <mpi.h>

int MPI_Topo_test(MPI_Comm comm, int *top_type)

💖 Fortran Syntax 💖

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

🦋 Fortran 2008 Syntax 🦋

USE mpi_f08
MPI_Topo_test(comm, status, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(OUT) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
             DOUBLE PRECISION MPI_Wtick()
             DOUBLE PRECISION MPI_Wtime()

not me pointing out that those last two MPI_Wtick() / MPI_Wtime() lines are just sitting there in the Fortran 2008 block — but they're in the original, so they stay. we don't erase the lore. 💀

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🍓 INPUT PARAMETER 🍓

• comm — the communicator (a handle). that's your main character right there 👑

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🌈 OUTPUT PARAMETERS 🌈

• top_type — the topology type of communicator comm (a choice, bestie, so it's one of the options below 💅).
• ierror — Fortran only: error status (integer). yes, only Fortran gets this one, that's just how it is 🙈.

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⭐ DESCRIPTION ⭐

okay so here's the tea: the function MPI_Topo_test returns the type of topology that is assigned to a communicator. simple. elegant. iconic. ✨

the output value top_type is one of these bad boys:

• MPI_GRAPH — graph topology 🕸️
• MPI_CART — Cartesian topology 📐
• MPI_DIST_GRAPH — distributed graph topology 🌐
• MPI_UNDEFINED — no topology at all, she's free, she's unbothered 🦋

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💀 ERRORS 💀

ngl this is the long part, but it matters, so stay with me 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.

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.

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. yes, all of them. dramatic, but that's her vibe 💀
• 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. returns the receipts 📬

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 bank on 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.

🧚 SEE ALSO 🧚

• MPI_Graph_create
• MPI_Cart_create

like, go read those next, they're the friends of this function, fr fr 💜

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community — protecting the whole squad since forever 🫶

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and that's the page!! mwah 💋 this cutie was brought to you by the fine folks at Open MPI, dated Sep 11, 2026 — slay, absolute icons, periodt. 🌟💖