MPI_FILE_READ(3)

💖 MPI_FILE_READ(3) — the Open MPI glow-up 💖

okay bestie, so this one is MPI_File_read, and it literally lives in the Open MPI manual pages (third section, section 3, iykyk). It reads a file starting at the location specified by the individual file pointer — and it's blocking and noncollective, meaning it waits for you and it's just you, no squad read. 🫶

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✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

int MPI_File_read(MPI_File fh, void *buf,
     int count, MPI_Datatype datatype, MPI_Status *status)

🎀 Fortran Syntax

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

🎀 Fortran 2008 Syntax

USE mpi_f08
MPI_File_read(fh, buf, count, datatype, status, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     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, bestie. It's a handle. 💅
• count — how many elements are in your buffer, as an integer. like, the amount.
• datatype — the data type of each buffer element (a handle, not a vibe).

🦋 OUTPUT PARAMETERS 🦋

• buf — the initial address of your buffer (integer). yeah, it's where the data lands. ✨
• status — the status object (status). she's giving receipts.
• ierror — Fortran only, babes: the error status (integer).

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👑 DESCRIPTION 👑

So MPI_File_read attempts to read from the file associated with fh — starting at the current individual file pointer position that the system is out here maintaining for you — a total number of count data items having datatype type, straight into your buffer buf. 📥

The data gets taken out of those parts of the file that are specified by the current view. no cap. And then MPI_File_read stores the number of data-type elements actually read in status. Every other field of status is undefined — so don't go reading into them, she didn't say that. 🙈

🚨 okay this is the important part: it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. like, do not. that's a whole mess. 💀

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💀 ERRORS 💀

Almost all MPI routines return an error value — the C routines hand it back as the return result of the function, and the Fortran routines put it in the last argument. periodt.

Before that 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 — iconic lineup honestly:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. yeah. everybody. 😭
• 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. 🎯
• MPI_ERRORS_RETURN — returns an error code to the application. the chill 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. 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.

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🍓 COPYRIGHT 🍓

2003-2026, The Open MPI Community

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and that's the whole page, bestie — sending you off with all the love from the Open MPI girlies: this one is dated Sep 11, 2026 and it's MPI_FILE_READ(3) all the way down. slay responsibly, and may your file pointers always be where you left them 🧚‍♀️💜