MPI_FILE_READ_AT(3)

💖 NAME 💖

MPI_File_read_at — reads a file at an explicitly specified offset (blocking, noncollective). ✨

And hi bestie, welcome to the MPI_FILE_READ_AT(3) page of the Open MPI manual, it's giving very much "I know exactly where I'm reading from and I'm not moving the file pointer about it" 💅

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🌸 SYNTAX 🌸

🍓 C Syntax

#include <mpi.h>

int MPI_File_read_at(MPI_File fh, MPI_Offset offset,
     void *buf, int count, MPI_Datatype datatype,
     MPI_Status *status)

🍓 Fortran Syntax

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

🍓 Fortran 2008 Syntax

USE mpi_f08
MPI_File_read_at(fh, offset, buf, count, datatype, status, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
     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 (a handle). it's literally the "which file, bestie" of it all 🫶
• offset — the file offset (an integer). this is the whole at in read_at, the coordinates of the vibe 📍
• count — number of elements in the buffer (integer). slay, how many we grabbing
• datatype — the data type of each buffer element (handle). no cap ✨

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🦋 OUTPUT PARAMETERS 🦋

• buf — initial address of the buffer (choice). this is where the data lands 💖
• status — status object (status). the receipts 🧾
• ierror — Fortran only: error status (integer). (C girlies get it as the return value, don't worry)

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✨ DESCRIPTION ✨

Okay so MPI_File_read_at attempts to read from the file associated with fh — at the offset position, exactly like the name promised — a total number of count data items having datatype type into the user's buffer buf. 🎀

And the offset? It's in etype units relative to the current view. That is, holes are not counted when locating an offset. The data is taken out of those parts of the file specified by the current view. So like, the view is the map and the offset walks the map, not the raw bytes, iykyk. 🗺️

MPI_File_read_at stores the number of datatype elements actually read in status. All other fields of status are undefined. Not me warning you about reading uninitialized fields… 🙈

⚠️ It is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. Like, do not. It's not a vibe. 💀

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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. fr fr.

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. like, everybody. all of them. 💀
• 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. 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. periodt.

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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okay bestie that's the whole page, go read your files at your little offsets and slay 💖

signing off — Sep 10, 2026, from the MPI_FILE_READ_AT(3) page in the Open MPI manual, mwah 🎀🦋✨