MPI_T_event_register_callback(3)

💖 MPI_T_event_register_callback — bestie edition 💖

okay so we're in the MPI manual, and this lil function is the one that glues your own custom function onto an event-registration handle you already grabbed. it's giving "matchmaker but for callbacks". 🎀

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✨ NAME ✨

MPI_T_event_register_callback — associates a user-defined function with an allocated event-registration handle 🫶

like. it literally just introduces your function to the handle. iconic behavior.

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🌸 SYNOPSIS 🌸

int MPI_T_event_register_callback(MPI_T_event_registration event_registration,
MPI_T_cb_safety cb_safety, MPI_Info info,
void *user_data,
MPI_T_event_cb_function event_cb_function)

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💅 INPUT PARAMETERS 💅

• event_registration — the event registration (handle). yes girl, the handle. 👑
• cb_safety — maximum callback safety level (integer). it's the ceiling on how safe the callback is allowed to be, no cap.
• info — info object (handle).
• user_data — pointer to a user-controlled buffer. default notes say None. so if you don't have anything to hand over, it's literally nothing. 🙈
• event_cb_function — pointer to user-defined callback function. the description column just says "function". your function. main character. ⭐

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🧚 THREAD AND INTERRUPT SAFETY 🧚

so this routine is thread-safe, periodt. that means multiple threads can use it at the same time and you do NOT need to bring your own thread locks. she handles it herself. 💅

but — and this is the important lil plot twist — the routine is not interrupt safe. typically that's because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so like, don't be sending signals at her mid-allocation, bestie. 😭

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💀 ERRORS 💀

okay so every single MPI routine (except MPI_Wtime and MPI_Wtick, the two little rebels) returns an error value. C routines put it as the value of the function, and Fortran routines put it in the last argument. no cap.

before that value comes back to you, the current MPI error handler gets called. and by default? that error handler aborts the MPI job. like, the whole thing. gone. so maybe pay attention to this part. 🔥

you can swap the error handler out with:

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

and yes, the MPI-1 routine MPI_Errhandler_set may be used — but its use is deprecated, so like... it's giving flop era. use the new ones. 🙈

there's also the predefined error handler MPI_ERRORS_RETURN, which you can use to make error values get returned instead. that's the one if you'd rather handle things yourself.

note bestie: 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 maybe. she'll try her best. 💖

the actual error codes:

• MPI_SUCCESS — no error; MPI routine completed successfully. slay. ✨
• MPI_ERR_INFO — invalid Info.
• MPI_T_ERR_INVALID_HANDLE — the handle is invalid. 💀
• MPI_T_ERR_NOT_INITIALIZED — the MPI tool information interface is not initialized. did you even turn it on. 🥲
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code.

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that's the whole page, bestie, you ate it 🍓

cute lil sign-off: this was the MPI_T_event_register_callback(3) page from the MPI manual, dated 4/10/2026 💜🧚