MPI_IS_THREAD_MAIN(3)

💅 MPI_IS_THREAD_MAIN(3) — the Open MPI glow-up 🦋

okay bestie, so you know how Open MPI has its own little corner of the man-page universe? yeah, this is one of those — it's from Open MPI, and it's about the cutest lil identity check in the whole MPI roster. let's get into it ✨

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🌸 NAME 🌸

MPI_Is_thread_main — literally just answers the question "is THIS thread the main character?" i.e. determines if the thread called MPI_Init 💅

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📜 SYNTAX 📜

✨ C Syntax ✨

#include <mpi.h>

int MPI_Is_thread_main(int *flag)

🧚 Fortran Syntax 🧚

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_IS_THREAD_MAIN(FLAG, IERROR)
     LOGICAL FLAG
     INTEGER IERROR

🦋 Fortran 2008 Syntax 🦋

USE mpi_f08
MPI_Is_thread_main(flag, ierror)
     LOGICAL, INTENT(OUT) :: flag
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎀 OUTPUT PARAMETERS 🎀

• flag — TRUE if the calling thread is the main thread (it's a boolean, bestie, so like... yes/no energy). 💖
• ierror — Fortran only: the error status, as an integer. 🙈

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👑 DESCRIPTION 👑

So MPI_Is_thread_main is the function a thread calls to find out whether the caller is the main thread — and by "main thread" we mean the icon, the legend, the moment: the actual thread that called MPI_Init or MPI_Init_thread ⭐🔥

like, it's a vibe check for threads. that's the whole thing. and it ate. 💅

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💀 ERRORS 💀

ngl, almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines stash it in that last argument. 📬

Before the error value even comes back to you, the current MPI error handler associated with the communication object (think: communicator, window, file) gets 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 — meaning before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when you're using the Sessions Model exclusively — the error raises the initial error handler. And you can change that initial error handler by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or the 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. periodt. 💥

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🌈 the three predefined error handlers Open MPI ships with 🌈

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. yeah. everybody. 😭
• 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. 💀
• MPI_ERRORS_RETURN — returns an error code to the application. polite. respectful. 🫶

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🧚 making your own error handler (yes, you can) 🧚

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, no promises, 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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⭐ SEE ALSO: ⭐

• MPI_Init
• MPI_Init_thread

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💜 COPYRIGHT 💜

2003-2026, The Open MPI Community

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🎀 that's the whole page, bestie! source + version: Open MPI, dated Sep 11, 2026 — go forth and know which thread you are 💅✨🦋