MPI_FILE_IWRITE_AT(3)

💖 MPI_FILE_IWRITE_AT(3) 💖

okay so this is Open MPI’s manual page for MPI_FILE_IWRITE_AT(3), and bestie, it’s giving “write into a file at a very specific offset, but make it async.” grab your iced coffee and let’s get into it 🧚✨

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🌸 NAME 🌸

MPI_File_iwrite_at — writes a file at an offset you explicitly specify, and it’s nonblocking and noncollective. no cap.

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🧚 SYNTAX 🧚

C Syntax ✨

          #include <mpi.h>

          int MPI_File_iwrite_at(MPI_File fh, MPI_Offset offset,
               const void *buf, int count, MPI_Datatype datatype, MPI_Request *request)

Fortran Syntax 🎀

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

Fortran 2008 Syntax 🦋

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

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💌 INPUT/OUTPUT PARAMETER 💌

• fh — file handle (handle). main character energy, honestly 👑

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💖 INPUT PARAMETERS 💖

• offset — file offset (integer). yes bestie, you literally pick the spot 📍
• buf — initial address of buffer (choice). this is where your data is sitting pretty 🫶
• count — number of elements in buffer (integer).
• datatype — data type of each buffer element (handle).

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✨ OUTPUT PARAMETERS ✨

• request — request object (handle). hold onto this for your MPI_Wait / MPI_Test moment later 💅
• ierror — Fortran only: error status (integer).

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🦋 DESCRIPTION 🦋

okay so MPI_File_iwrite_at is literally the nonblocking version of MPI_File_write_at. it 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. iconic behavior, fr fr.

the offset is in etype units relative to the current view. that is, holes are not counted when locating an offset. the data is written into those parts of the file specified by the current view.

MPI_File_iwrite_at stores the number of datatype elements actually written in status. all other fields of status are undefined — do not trust them, periodt 🙈

the request structure can be passed to MPI_Wait or MPI_Test, which will return a status with the number of bytes actually accessed.

⚠️ it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was open. like, do not do that, bestie. that’s the vibe check failing 💀

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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. dramatic? yes. effective? also yes 😭
• 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: MPI does not guarantee that an MPI program can continue past an error. so like… don’t assume it’s gonna bounce back, bestie 🫶

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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kisses and good luck with your I/O, bestie 💖 — this page is from Open MPI, dated Sep 11, 2026, and its manual-page name is MPI_FILE_IWRITE_AT(3).