MPI_FILE_READ_ORDERED(3)

💖 MPI_FILE_READ_ORDERED(3) 💖

okay bestie, gather round, this is a page from the Open MPI manual and it's all about MPI_File_read_ordered — reading a file at a location specified by a shared file pointer (blocking, collective). yes it's a whole mouthful, yes it slays, let's go ✨

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🌸 SYNOPSIS 🌸

C Syntax

#include <mpi.h>

int MPI_File_read_ordered(MPI_File fh, void *buf,
     int count, MPI_Datatype datatype,
     MPI_Status *status)

Fortran Syntax

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

Fortran 2008 Syntax

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

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🫶 INPUT PARAMETERS 🫶

• fh — the file handle, aka the lil badge that says "yes this file, that one" (handle).
• count — how many elements are chillin' in your buffer (integer).
• datatype — the data type of each and every buffer element (handle).

🎀 OUTPUT PARAMETERS 🎀

• buf — the initial address of your buffer, the cozy lil home for the incoming data (choice).
• status — the status object, where the receipts live (Status).
• ierror — Fortran only, the error status (integer).

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✨ DESCRIPTION ✨

so MPI_File_read_ordered is a collective routine, meaning it's a group activity, no solo missions here. every single process in the communicator group associated with the file handle fh has to call it. no skipping class. 🙈

the cute part: each process can pass different argument values for the datatype and count arguments. you do you, bestie. each process attempts to read, from the file associated with fh, a total number of count data items having datatype type into the user's buffer buf.

and here's the ordered tea — for each process, the location in the file at which data is read is the position at which the shared file pointer would be after all processes whose ranks within the group are less than that of this process had read their data. like, you're literally queued up behind everyone with a lower rank than you. main character energy but make it polite. 👑

MPI_File_read_ordered returns the actual number of datatype elements read in status. and the shared file pointer gets updated by the amounts of data requested by all processes of the group — communal effort, communal pointer movement. periodt. 💅

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💀 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. (yes, the classic "last argument is the vibe check" situation.)

before that error value comes back to you, the current MPI error handler associated with the communication object (e.g., communicator, window, file) gets 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 — that is, 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
• the 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. dramatic AND effective.
• 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.
• 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

note that MPI does not guarantee that an MPI program can continue past an error. like, fr fr, do not assume it survives. 😭

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 the whole page bestie, you ate that 🍓 the copyright belongs to the Open MPI Community, dated 2003-2026, and this lil doc is stamped Sep 10, 2026 — write it down, cite it, live your truth 💜🦋