MPI_ADD_ERROR_CLASS(3)

💖 MPI_Add_error_class — bestie edition 💖

Okay so this is a page from the Open MPI manual, and it's about MPI_Add_error_class, which lowkey does exactly what it says on the tin: it makes a brand-new error class and hands you back its value. Iconic behaviour, honestly. Let's get into it 🫶

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Add_error_class(int *errorclass)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ADD_ERROR_CLASS(ERRORCLASS, IERROR)
     INTEGER ERRORCLASS, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Add_error_class(errorclass, ierror)
     INTEGER, INTENT(OUT) :: errorclass
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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⭐ OUTPUT PARAMETERS ⭐

• errorclass — the new error class, which is an integer. She's fresh, she's new, she's yours 💅
• ierror — Fortran only, bestie: this is the error status (integer).

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💜 DESCRIPTION 💜

So MPI_Add_error_class creates a new, local error class. That's it, that's the whole vibe. Local means local, and we'll get into why that matters in a sec 👀

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🌈 NOTES 🌈

Okay this part is important, so pay attention 🧚

Because this function is local, the same value of errorclass may not be returned on all processes that make this call — even if they all call the function concurrently. Yes, really. So the same error on different processes may not cause the same value of errorclass to be returned. Kinda chaotic of MPI, but we move 🫠

To reduce the potential for confusion, MPI_Add_error_string may be used on multiple processes to associate the same error string with the newly created errorclass. So even though errorclass may not be consistent across processes, using MPI_Add_error_string will ensure the error string associated with it will be the same everywhere. Slay, that's your workaround 💖

Also no cap: no function is provided to free error classes, as it is not expected that an application will create them in significant numbers. So don't go feral creating millions of them, mmkay?

And the value returned is always greater than or equal to MPI_ERR_LASTCODE. Always. Periodt.

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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. Standard behaviour, nothing wild there.

But here's the tea 🍵: 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. And that 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
• the 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.

👑 The three predefined error handlers 👑

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. Like, 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.
• MPI_ERRORS_RETURN — returns an error code to the application. Chillest of the three, fr fr.

🦋 DIY error handlers 🦋

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 assume you can just walk it off, 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_Add_error_code
• MPI_Add_error_string
• MPI_Error_class
• MPI_Error_string

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🌸 COPYRIGHT 🌸

2003-2026, The Open MPI Community

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And that's the whole page, bestie! 💖 This lil gem is from the Open MPI manual, MPI_ADD_ERROR_CLASS(3), dated Sep 11, 2026 — go forth and create error classes responsibly, you absolute icon 👑✨🍓