MPI_FILE_READ_ORDERED_BEGIN(3)

💅 MPI_FILE_READ_ORDERED_BEGIN 💅

okay bestie, so this one lives in the Open MPI section of your man pages, and it's the beginning half of a split collective nonblocking file read — like, the "part one" of a two-parter 🎀

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🌸 SYNTAX 🌸

✨ C Syntax ✨

#include <mpi.h>

int MPI_File_read_ordered_begin(MPI_File fh, void *buf,
     int count, MPI_Datatype datatype)

✨ Fortran Syntax ✨

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_FILE_READ_ORDERED_BEGIN(FH, BUF, COUNT, DATATYPE, IERROR)
     <type>  BUF(*)
     INTEGER FH, COUNT, DATATYPE, IERROR

✨ Fortran 2008 Syntax ✨

USE mpi_f08
MPI_File_read_ordered_begin(fh, buf, count, datatype, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🦋 INPUT/OUTPUT PARAMETER 🦋

• fh — the file handle, bestie (handle).

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🌈 INPUT PARAMETERS 🌈

• count — how many elements are chillin' in your buffer (integer).
• datatype — the data type of each element in the buffer (handle).

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💜 OUTPUT PARAMETERS 💜

• buf — the initial address of your buffer (choice).
• ierror — Fortran only, bb: error status (integer).

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🎀 DESCRIPTION 🎀

MPI_File_read_ordered_begin is the beginning part of a split collective, nonblocking routine, and the way that it works is that every single process in the communicator group associated with the file handle fh has to call it — no ghosting, no skipping, all of you 🫶. Each process is allowed to pass different argument values for the datatype and count arguments, so like, you don't all have to match.

Every process attempts to read, from the file associated with fh, a total number of count data items that have type datatype into the user's buffer buf. And here's the iconic ordered part 👑: for each process, the location in the file where the data is read from is the position where the shared file pointer would be after all the processes whose ranks within the group are less than that of this process had already read their data. It's giving "wait your turn in line" energy, fr fr 💅.

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⭐ NOTES ⭐

All the nonblocking collective routines for data access are "split" into two routines, each one with either _begin or _end as a suffix. Those split collective routines are subject to the semantic rules described in Section 9.4.5 of the MPI-2 standard — iykyk, go read it 📖✨.

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💀 ERRORS 💀

Almost all MPI routines return an error value, bestie: C routines as the return result of the function, and Fortran routines in the last argument.

Before that error value gets returned, the current MPI error handler associated with the communication object (like a communicator, window, or file) is called. If there's no communication object 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 🚨. That 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.

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. absolutely no survivors 😭
• 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.
• MPI_ERRORS_RETURN — returns an error code to the application.

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

ngl, note that MPI does not guarantee that an MPI program can continue past an error. like, bestie, do not count on it 🙈

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 📚.

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's a wrap, bestie 💖 that last line was giving Sep 11, 2026 and MPI_FILE_READ_ORDERED_BEGIN(3), so go forth and read your files in order, periodt ✨🦋🌸