MPI_FILE_WRITE_AT(3)

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version 5.0.11 · 17 versions
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💅 MPI_FILE_WRITE_AT(3) 💅

okay so this is from the Open MPI manual, bestie — third section, MPI_FILE_WRITE_AT, and we're about to get so into it. grab your iced coffee, we're writing bytes at offsets 🫶

MPI_File_write_at — Writes a file at an explicitly specified offset (blocking, noncollective). no cap, that's literally the whole vibe of this function.

✨ SYNTAX ✨

🌸 C Syntax 🌸

#include <mpi.h>

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

🎀 Fortran Syntax 🎀

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_FILE_WRITE_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_write_at(fh, offset, buf, count, datatype, status, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
     TYPE(*), DIMENSION(..), INTENT(IN) :: 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). it's giving "your ticket to the file," lowkey.
  • offset — File offset (integer). this is the exact spot, main character energy.
  • buf — Initial address of buffer (choice). where your data's coming from ✨
  • count — Number of elements in buffer (integer).
  • datatype — Data type of each buffer element (handle).

🩷 OUTPUT PARAMETERS 🩷

  • status — Status object (status).
  • ierror — Fortran only: Error status (integer).

👑 DESCRIPTION 👑

so like — MPI_File_write_at attempts to write into the file associated with fh (at the offset position) a total number of count data items having datatype type from the user's buffer buf. obsessed with the clarity, honestly.

the offset is in etype units relative to the current view. translation bestie: holes are not counted when locating an offset. the data gets written into those parts of the file specified by the current view. MPI_File_write_at stores the number of datatype elements actually written in status. all other fields of status are undefined — so don't go poking around in them, fr fr 🙈

⚠️ it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. not a vibe. do not 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. classic.

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. that's the whole flowchart, bestie 🧚

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

  • MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. yeah. everyone. 😭
  • 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, it might just be over. 💀

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.

🌈 SEE ALSO 🌈

  • MPI_File_iwrite_at
  • MPI_File_write_at_all
  • MPI_File_write_at_all_begin
  • MPI_File_write_at_all_end

the whole friend group, honestly — go say hi 👋

2003-2026, The Open MPI Community — iconic, they've been at it for a minute 💜

✨ and that's the page, bestie! ✨ this particular drop is from Open MPI, dated Sep 11, 2026 — so go forth, write at your offsets, and slay 💅🦋🌸

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