MPI_FILE_READ_ALL_BEGIN(3)

💖 MPI_FILE_READ_ALL_BEGIN 💖

okay bestie, gather round, because today we're talking about MPI_File_read_all_begin from the Open MPI manual, section 3 (library calls, obviously). it's the beginning half of a split collective read. she's nonblocking, she's sparkly, she's got a sister routine ending in _end and you literally cannot have one without the other. iykyk ✨

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🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_File_read_all_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_ALL_BEGIN(FH, BUF, COUNT, DATATYPE, IERROR)
     <type>  BUF(*)
     INTEGER FH, COUNT, DATATYPE, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_File_read_all_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 (a handle, bestie). this is how we know which file we're being iconic about 🫶

🦋 INPUT PARAMETERS 🦋

• count — how many elements are chillin' in the buffer (an integer).
• datatype — the data type of each one of those buffer elements (a handle).

🌈 OUTPUT PARAMETERS 🌈

• buf — the initial address of the buffer (a choice).
• ierror — Fortran only: the error status (an integer).

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✨ DESCRIPTION ✨

so MPI_File_read_all_begin is the beginning part of a split collective operation. what it's attempting to do (and yes, the manpage says "attempts", we love an honest queen) is read from the file associated with fh — starting at the current individual file pointer position that the system is maintaining for you — a total number of count data items, of type datatype, into your buffer buf. 💅

and where does that data come from? out of those parts of the file specified by the current view. no view, no vibe check, no data. periodt.

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💀 NOTES 💀

every single nonblocking collective routine for data access is "split" into two routines, each one carrying a _begin or _end suffix. and these split collective routines? they're subject to the semantic rules laid out in Section 9.4.5 of the MPI-2 standard. so if you're out here freestyling the semantics, that's on you, not on me 🙈

lowkey important: _begin without _end is not a personality trait, it's a problem. the rules are in the standard, go read them. i'm not guessing at anything, and neither should you.

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🔥 ERRORS 🔥

like, almost every MPI routine returns an error value. C routines give it back as the return result of the function, and Fortran routines drop it into that last argument. simple as that.

before that error value comes home to you, the current MPI error handler associated with the communication object (think: communicator, window, file) gets called. and if there's no communication object attached to the MPI call at all? then the call is considered attached to MPI_COMM_SELF and it'll call the associated MPI error handler. 👑

now here's the spicy part: when MPI_COMM_SELF isn't initialized — meaning before MPI_Init / MPI_Init_thread, after MPI_Finalize, or when you're using the Sessions Model exclusively — the error raises the initial error handler. and you can change that initial error handler by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when you're using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or the info key to MPI_Comm_spawn / MPI_Comm_spawn_multiple. and if no other appropriate error handler has been set? then MPI_ERRORS_RETURN gets called for MPI I/O functions, and MPI_ERRORS_ABORT gets called for all other MPI functions. that's the rule, no cap. ⭐

Open MPI ships with three predefined error handlers you can use:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic? yes. effective? also yes. 💀
• MPI_ERRORS_ABORT — an error handler that can be invoked on a communicator, window, file, or session. when it's called on a communicator, it acts as if MPI_Abort was called on that communicator. if it's called on a window or a file, it acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. and if it's called on a session, it aborts only the local process. context is everything.
• 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 and then MPI_Comm_set_errhandler
• MPI_File_create_errhandler and then MPI_File_set_errhandler
• MPI_Session_create_errhandler and then MPI_Session_set_errhandler, or at MPI_Session_init
• MPI_Win_create_errhandler and then MPI_Win_set_errhandler

⚠️ note that MPI does not guarantee that an MPI program can continue past an error. like, at all. so don't build your whole architecture on "it'll probably be fine."

see the MPI man page for a full list of MPI error codes. and see the Error Handling section of the MPI-3.1 standard for more information. go read, bestie, it's good for you 📚

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👑 Copyright 👑

2003-2026, The Open MPI Community

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and that's the whole page, my love 🫶 she's a split collective, she needs her _end counterpart, and she is not here to be your entire I/O strategy by herself. stay hydrated, close your file handles, and remember: the vibe changes, the semantics do not. 💖✨🌸

this lil transmission came to you from the Open MPI source, dated Sep 10, 2026 — MPI_FILE_READ_ALL_BEGIN(3), mwah 💅🎀