MPI_FILE_IREAD(3)

💖 NAME 💖

MPI_File_iread — bestie she reads a file starting at the location specified by the individual file pointer. She's nonblocking. She's noncollective. That's the whole vibe. 🦋

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_File_iread(MPI_File fh, void  *buf, int  count,
     MPI_Datatype  datatype, MPI_Request  *request)

🌸 Fortran Syntax

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

🌸 Fortran 2008 Syntax

USE mpi_f08
MPI_File_iread(fh, buf, count, datatype, request, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     TYPE(*), DIMENSION(..), 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 — the file handle. It's a handle, bestie. She's giving "which file are we even talking about" energy. 💅

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🎀 INPUT PARAMETERS 🎀

• count — how many elements are chilling in the buffer. Integer. Slay.
• datatype — the data type of each buffer element. It's a handle. 🌸

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👑 OUTPUT PARAMETERS 👑

• buf — initial address of the buffer. Choice. Yes she's an output param AND she's holding your data, main character behaviour fr fr. 😭
• request — the request object. Handle. This is your little ticket to come back and check on her later. 🎟️
• ierror — Fortran only: error status. Integer. 💜

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🔥 DESCRIPTION 🔥

okay so MPI_File_iread is literally the nonblocking version of MPI_File_read. No cap. She attempts to read from the file associated with fh, and she does it at the current individual file pointer position — the one the system maintains. The total number of count data items, all having the datatype datatype, get read into your buffer buf. 🦋

The data gets taken out of those parts of the file specified by the current view. That's the vibe check right there.

Now here's the part you need to internalise: MPI_File_iread stores the number of data-type elements actually read in status. All other fields of status are undefined. So like, don't go poking at the other fields, they're not gonna tell you anything. 💀

and it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. Like, literally don't. She will not be happy. 🙈

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😭 ERRORS 😭

Almost all MPI routines return an error value, bestie. In C it comes back as the return result of the function, and in Fortran it lives 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. And 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 are iconic:

• 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, 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. Very "it depends on context" of her. 💅
• 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. So like, don't count on bouncing back. 💀

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 that's the whole page bestie, you ate that 🍓💖 this was the MPI_FILE_IREAD(3) page from Open MPI, dated Sep 10, 2026 — go read a file, nonblocking, and slay ✨