MPI_Register_datarep(3)

💖 MPI_Register_datarep(3) 💖

okay bestie, welcome to the MPI manual, section 3, and today's main character is MPI_Register_datarep — no cap, she's the one who lets you decide how your data gets translated between the file's vibe and your machine's vibe ✨

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🌸 NAME 🌸

MPI_Register_datarep — Register a set of user-provided data conversion (yes, literally register your own little conversion function squad, iykyk) 💅

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✨ SYNOPSIS ✨

int MPI_Register_datarep(const char *datarep,
MPI_Datarep_conversion_function *read_conversion_fn,
MPI_Datarep_conversion_function *write_conversion_fn,
MPI_Datarep_extent_function *dtype_file_extent_fn,
void *extra_state)

int MPI_Register_datarep_c(const char *datarep,
MPI_Datarep_conversion_function_c *read_conversion_fn,
MPI_Datarep_conversion_function_c *write_conversion_fn,
MPI_Datarep_extent_function *dtype_file_extent_fn,
void *extra_state)

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🎀 INPUT PARAMETERS 🎀

group chat roll call, everybody say hi 🙈

• datarep — the data representation identifier, and she's a string, periodt. Like, the name you're registering this whole conversion situation under.
• read_conversion_fn — the function that gets invoked to convert from file representation to native representation (function). so like, file brain → your machine's brain 🧠💫
• write_conversion_fn — the function invoked to convert from native representation to file representation (function). the reverse glow-up: your machine's brain → file brain 🦋
• dtype_file_extent_fn — the function invoked to get the extent of a datatype as represented in the file (function). yes she measures, she's giving ruler energy 📏
• extra_state — extra state (None). that's it, that's the whole note 💀

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🫶 THREAD AND INTERRUPT SAFETY 🫶

This routine is thread-safe ✨ which means she can be safely used by multiple threads without needing any user-provided thread locks — she handles it, no lock drama, we love a low-maintenance icon 👑

BUT. she is not interrupt safe 😭 typically this happens because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so like, don't pull her out mid-interrupt, bestie, that's not the vibe.

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🌈 NOTES FOR FORTRAN 🌈

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and they're invoked with the call statement — yes, lowercase call, that's just how Fortran is, we respect her 🫡

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. INTEGER. not integer. okay periodt 💅

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💀 ERRORS 💀

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job 😭 like, full stop, the whole thing is over.

The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated — girl, we don't use her anymore, she's over, respectfully 🙈

The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. so like, they try their best, and that's all we can ask for 🥺

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🍓 the error codes themselves

• MPI_SUCCESS — No error; MPI routine completed successfully. she ate and she left no crumbs ✨
• 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 and we can't tell you which one 💀
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code. the "it's giving mystery" error, go ask MPI_Error_string for the tea 🍵

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🧚 SEE ALSO 🧚

okay so there's nothing else listed here bestie, this page is self-contained — she's an only child and she's thriving 💜

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tiny reminder from your favorite Unix bestie: MPI_Register_datarep is there so YOU get to define the conversion, which means the responsibility is also on YOU to get the conversion functions right. iconic AND terrifying, we love a balanced queen 👑

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so anyway, that's the whole page, we got through it together 🥹 This was MPI_Register_datarep(3) from the MPI manual — that page you're holding? dated 4/10/2026 🌸 fr fr, go slay some data conversion, bestie, and don't forget your extra_state (which is None, in case you forgot, which you didn't, because you're a genius) 💖🎀🦋