MPI_Session_create_errhandler(3)

💖 MPI_Session_create_errhandler 💖

okay bestie, so this one lives in the MPI manual (yes, that MPI, the Message Passing Interface one, the ✨iconic✨ one), and it's literally just: make me a custom error handler for my session. that's it. that's the whole vibe. you bring the function, MPI brings the handle. slay. 🎀

🌸 NAME 🌸

MPI_Session_create_errhandler — create an error handler for use with an MPI session, bestie. it's giving "custom error handling, but make it ✨yours✨". 💅

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✨ SYNOPSIS ✨

int MPI_Session_create_errhandler(MPI_Session_errhandler_function *session_errhandler_fn,
MPI_Errhandler *errhandler)

🦋 INPUT PARAMETERS 🦋

• session_errhandler_fn — your very own user-defined error handling procedure (a function, bestie, you write it, you own it, main character energy fr fr 👑)

🍓 OUTPUT PARAMETERS 🍓

• errhandler — the MPI error handler (a handle). this is what you get back, and it's literally the whole point. 💖

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🧚 THREAD AND INTERRUPT SAFETY 🧚

okay so good news and then a lil caveat, don't panic:

• ✅ this routine is thread-safe. that means multiple threads can use it at the same time, no cap, and you do not need to bring your own thread locks. no pthread_mutex girlie, put it away. 🫶
• ⚠️ but — and this is the important bit — the routine is not interrupt safe. why? typically it's because of memory allocation routines like malloc, or other non-MPICH runtime routines, which are themselves not interrupt-safe. so like, the interrupt-safety vibe check is failed, and it's not even this routine's fault, it's the allocator's. 💀

not me being thread-safe but not interrupt safe... the duality. 🙈

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🎀 NOTES FOR FORTRAN 🎀

for my Fortran besties, some house rules:

• every MPI routine in Fortran (except MPI_WTIME and MPI_WTICK — they're the special little exceptions, periodt) gets an extra argument called ierr tacked onto the end of the argument list. like a lil bonus parameter. ✨
• ierr is an integer, and it means exactly the same thing as the return value of the routine in C. same energy, different language. 💜
• in Fortran, MPI routines are subroutines, and you invoke them with the call statement. yes, call. that's the move. 🕺
• all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. every single one. ain't no structs here bestie. 🌈

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💀 ERRORS 💀

okay so, real talk. all MPI routines (except MPI_Wtime and MPI_Wtick, the eternal exceptions, we love them) return an error value. in C it's the value of the function, and in Fortran it's that last argument we just talked about. before the value comes back to you, the current MPI error handler gets called. 🔥

by default? that error handler aborts the MPI job. like, full stop, entire job, gone, bye. 😭

you can change the error handler with:

• MPI_Comm_set_errhandler (for communicators)
• MPI_File_set_errhandler (for files)
• MPI_Win_set_errhandler (for RMA windows)

the MPI-1 routine MPI_Errhandler_set may be used, but its use is deprecated — so like, it works but it's giving ancient. maybe don't. 🙈

if you want error values to just be returned to you instead of nuking everything, use the predefined error handler MPI_ERRORS_RETURN. 💖

important caveat, and I need you to actually read this one: MPI does not guarantee that an MPI program can continue past an error. however, MPI implementations will attempt to continue whenever possible. so it's a try, not a promise. no cap. ⚠️

the actual error codes:

• MPI_SUCCESS — no error; MPI routine completed successfully. we love a clean run. ✨
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). one of your args is sus, but we can't tell you which one exactly. 💀
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the "miscellaneous" bucket, bestie. go ask MPI_Error_string for the tea. 🍓

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🌸 that's the whole page! this was the MPI manual, and it's dated 4/10/2026 — go forth and handle your errors custom-style. you're gonna eat. 💅✨👑