MPI_FILE_READ_AT_ALL_BEGIN(3)

💅 MPI_FILE_READ_AT_ALL_BEGIN(3) — Open MPI, bestie edition

okay so this is a page from the Open MPI manual, and we're talking about MPI_File_read_at_all_begin — the beginning half of a split collective file read where you tell it exactly where to read from. nonblocking. very "i'll start it now and finish it later" energy ✨

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🌸 NAME

MPI_File_read_at_all_begin — reads a file at explicitly specified offsets; this is the beginning part of a split collective routine, and it's nonblocking, iykyk.

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🎀 SYNTAX

✨ C Syntax

#include <mpi.h>

int MPI_File_read_at_all_begin(MPI_File fh, MPI_Offset
     offset, void *buf, int count, MPI_Datatype
     datatype)

✨ Fortran Syntax

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

✨ Fortran 2008 Syntax

USE mpi_f08
MPI_File_read_at_all_begin(fh, offset, buf, count, datatype, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💖 INPUT PARAMETERS

• fh — file handle (handle). it's literally the file you're reading from, bestie 🫶
• offset — file offset (integer). this is the "where tho" of the operation 📍
• count — number of elements in buffer (integer). how many of them 🍓
• datatype — data type of each buffer element.

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🦋 OUTPUT PARAMETERS

• buf — initial address of buffer (choice). your stuff goes here 💜
• ierror — Fortran only: error status (integer).

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👑 DESCRIPTION

so MPI_File_read_at_all_begin is the beginning part of a split collective routine. what it's attempting to do (yes, "attempts", the manpage said attempts, i'm just relaying) is read from the file associated with fh — at the offset position — a total number of count data items, having datatype type, into the user's buffer buf. 🎯

the offset is in etype units relative to the current view. and like, that means holes are not counted when locating an offset, periodt. the data is taken out of those parts of the file specified by the current view. no cap. 🌈

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⭐ NOTES

all the nonblocking collective routines for data access are "split" into two routines, each with a _begin or _end suffix. these split collective routines are subject to the semantic rules described in Section 9.4.5 of the MPI-2 standard. yes, actual section number, go read it 🔥

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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.

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. 😭

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. 💀
• 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

note that MPI does not guarantee that an MPI program can continue past an error. like, fr fr, it might just not.

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 vibe check, bestie — this page is from the Open MPI manual, dated Sep 11, 2026, filing under MPI_FILE_READ_AT_ALL_BEGIN(3). go read your file at explicitly specified offsets, queen 👑💅✨