MPI_REGISTER_DATAREP(3)

📦
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💅 MPI_REGISTER_DATAREP(3) — like, the Open MPI glow-up guide ✨

okay so bestie, welcome to MPI_Register_datarep — it's the Open MPI routine that literally defines a data representation and honestly? it's giving matchmaker energy 💖

🌸 NAME 🌸

MPI_Register_datarep — Defines data representation. periodt. 🎀

✨ SYNTAX ✨

🦋 C Syntax 🦋

#include <mpi.h>

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)

🦋 Fortran Syntax 🦋

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REGISTER_DATAREP(DATAREP, READ_CONVERSION_FN,
     WRITE_CONVERSION_FN, DTYPE_FILE_EXTENT_FN,
     EXTRA_STATE, IERROR)
     CHARACTER*(*)   DATAREP
     EXTERNAL        READ_CONVERSION_FN, WRITE_CONVERSION_FN, DTYPE_FILE_EXTENT_FN
     INTEGER IERROR
     INTEGER(KIND=MPI_ADDRESS_KIND)  EXTRA_STATE

🦋 Fortran 2008 Syntax 🦋

USE mpi_f08
MPI_Register_datarep(datarep, read_conversion_fn, write_conversion_fn,
             dtype_file_extent_fn, extra_state, ierror)
     CHARACTER(LEN=*), INTENT(IN) :: datarep
     PROCEDURE(MPI_Datarep_conversion_function) :: read_conversion_fn
     PROCEDURE(MPI_Datarep_conversion_function) :: write_conversion_fn
     PROCEDURE(MPI_Datarep_extent_function) :: dtype_file_extent_fn
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: extra_state
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🫶 INPUT PARAMETERS 🫶

  • datarep — the data representation identifier (string). it's the ✨name✨ you're gonna be calling this whole vibe by.
  • read_conversion_fn — the function it calls to convert from file representation to native representation (function). the glow-UP direction 💫
  • write_conversion_fn — the function it calls to convert from native representation to file representation (function). the glow-DOWN direction, iykyk.
  • dtype_file_extent_fn — the function it calls to get the extent of a data type as represented in the file (function). it literally measures the vibe check 📏
  • extra_state — extra state. no cap, that's the whole description. it's just... extra state. 🫡

🎀 OUTPUT PARAMETER 🎀

  • ierror — Fortran only: error status (integer). classic Fortran behavior, bestie 🍓

🔥 DESCRIPTION 🔥

so MPI_Register_datarep defines a data representation — main character behavior, honestly 👑 it associates the data representation's identifier (a string, bestie) with the functions that convert from file representation to the native representation and vice versa, plus the function that gets the extent of a data type as represented in the file, and also with "extra state," which is used for passing arguments. it's the whole little friend group 💖 once a data representation has been registered using this routine, you may specify its identifier as an argument to MPI_File_set_view, causing subsequent data-access operations to call the specified conversion functions. slay. ✨

the call associates read_conversion_fn, write_conversion_fn, and dtype_file_extent_fn with the data representation identifier datarep. then datarep can be used as an argument to MPI_File_set_view, causing subsequent data access operations to call the conversion functions to convert all data items accessed between file data representation and native representation. it's giving relay race 🏃‍♀️💨

MPI_Register_datarep is a local operation and only registers the data representation for the calling MPI process. lowkey important — it doesn't spread to all your processes, just the one doing the calling 🙈

if datarep is already defined, an error in the error class MPI_ERR_DUP_DATAREP is raised using the default file error handler. so like, no duplicate names, bestie — that's a hard no 💀

the length of a data representation string is limited to the value of MPI_MAX_DATAREP_STRING. and MPI_MAX_DATAREP_STRING must have a value of at least 64. keep your names short and cute, k? 🎀

no routines are provided to delete data representations and free the associated resources; it is not expected that an application will generate them in significant numbers. so like, there's no "unregister" button 😭 — but honestly? nobody's out here making millions of these, so we're fine.

🧚 NOTES 🧚

the Fortran version of each MPI I/O routine includes a final argument, IERROR, which is not defined in the PARAMETERS sections. this argument is used to return the error status of the routine in the manner typical for Fortran library routines. that's just, like, tradition 💅

the C version of each routine returns an error status as an integer return value.

error classes are found in mpi.h (for C) and mpif.h (for Fortran). iykyk 🦋

💀 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.

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. ngl that's a lot of error-handler choreography, but we move 🫶

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

  • MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. yeah. everybody. 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, 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, aborts only the local process. selective chaos, iconic 🌈
  • 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. fr fr, that's a real caveat, don't @ me 😭

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.

2003-2026, The Open MPI Community — ok that's the whole copyright line, keep it, it's theirs 💖

and that's the whole page bestie!! 🎀✨ footer vibes: this was Open MPI, dated Sep 11, 2026 — mwah, love that for us 🫶💜

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