MPI_ADD_ERROR_CLASS(3)

💖 NAME 💖

okay bestie, welcome to MPI_ADD_ERROR_CLASS(3), a little Open MPI page that does exactly one iconic thing: it creates a brand new error class and hands you back its value. that's it. that's the whole vibe. ✨

🌈 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 brand new error class, and yes it's an integer. she's fresh out the box 💅
• ierror — Fortran girlies only: your error status, also an integer. 💖

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💅 DESCRIPTION 💅

so like, MPI_Add_error_class creates a new, local error class. local is the key word here, remember that, it's literally the whole plot of the NOTES section. 👑

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🔥 NOTES 🔥

okay so this is where it gets real. because this function is local, the same value of errorclass may not be returned on all processes that make this call — even if they call the function concurrently. so yeah, the exact same error on different processes may not cause the same value of errorclass to be returned. it's giving chaos, ngl. 😭

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. and here's the slay: 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. consistency queen behavior, we love to see it. ✨

no function is provided to free error classes, because honestly it's not expected that an application will create them in significant numbers. so they just... stay. iconic, but permanent. 🦋

the value returned is always greater than or equal to MPI_ERR_LASTCODE. no cap, always. 💯

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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. fr fr.

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. bestie that was a lot, but it's all important, i promise. 🫶

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, but effective. 💀
• 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. main character energy on every level. 👑
• MPI_ERRORS_RETURN — returns an error code to the application. the polite one. 🎀

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 count on it, okay? 🙈

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_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, you ate it 💖 written by the Open MPI girlies, dated Sep 10, 2026, and filed right under MPI_ADD_ERROR_CLASS(3) — periodt. ✨🌸🦋