MPI_FILE_READ_AT(3)

MPI_File_read_at 💅 — Open MPI manual page, bestie

okay so this is the MPI_FILE_READ_AT(3) page from the Open MPI manual. It’s literally about reading a file at an offset you explicitly picked, and the vibe is blocking + noncollective — main character energy only, no cap. 💅

🌸 NAME 🌸

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

✨ 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

🎀 INPUT PARAMETERS 🎀

• fh — file handle (handle). This is lowkey your VIP pass to the file, no cap.
• offset — file offset (integer). The exact spot, bestie — it’s giving “I know where I’m going.”
• count — number of elements in buffer (integer).
• datatype — data type of each buffer element (handle).

🫶 OUTPUT PARAMETERS 🫶

• buf — initial address of buffer (choice). This is where the read data lands, iconic.
• status — status object (status). Receipts, basically.
• ierror — Fortran only: error status (integer).

💅 DESCRIPTION 💅

MPI_File_read_at attempts to 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. 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. MPI_File_read_at stores the number of datatype elements actually read in status. All other fields of status are undefined — so don’t go reading them, bestie.

🚨 hard no: it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. Like, don’t do it.

😭 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. Ngl, that’s the basics.

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 — and they’re iconic in different ways:

• 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, 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. Fr fr.

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.

👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

💌 cute sign-off: source/version is Open MPI, date is Sep 11, 2026. That’s the whole footer vibe, bestie — now go read at your explicitly specified offset. Periodt. 🫶✨