MPI_FILE_PREALLOCATE(3)

💖 NAME 💖

MPI_File_preallocate — bestie, this one preallocates a specified amount of storage space at the beginning of a file, and it's collective 💅

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✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

int MPI_File_preallocate(MPI_File fh, MPI_Offset size)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_FILE_PREALLOCATE(FH, SIZE, IERROR)
     INTEGER FH, IERROR
     INTEGER(KIND=MPI_OFFSET_KIND)   SIZE

🎀 Fortran 2008 Syntax

USE mpi_f08
MPI_File_preallocate(fh, size, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🫶 INPUT/OUTPUT PARAMETER 🫶

• fh — the file handle, bestie (handle).

🌸 INPUT PARAMETER 🌸

• size — how much file space you wanna preallocate, in bytes (integer).

🌈 OUTPUT PARAMETER 🌈

• ierror — Fortran only, it's the error status (integer).

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💅 DESCRIPTION 💅

okay so. MPI_File_preallocate makes sure storage space is actually allocated for the first size bytes of the file that's chillin' with fh. And ngl, MPI_File_preallocate can be a very time-consuming operation 😭 so don't just casually spam it, bestie.

MPI_File_preallocate is collective — every process in the group has to pass identical values for size, no cap. Regions of the file that have already been written? Unaffected, they're fine, they're thriving 💖. For regions of the file that are newly allocated, MPI_File_preallocate has the same effect as writing undefined data. If size is larger than the current file size, the file size increases to size. If size is less than or equal to the current file size, the file size stays unchanged, periodt.

The treatment of file pointers, pending nonblocking accesses, and file consistency is the same as with MPI_File_set_size.

🚨 If MPI_MODE_SEQUENTIAL mode was specified when the file was opened, it is erroneous to call this routine. Like, don't. That's the vibe check failing.

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📝 NOTES 📝

When you use the collective routine MPI_File_set_size on a UNIX file, if the size that is set is smaller than the current file size, the file is truncated at the position defined by size. If the size is set to be larger than the current file size, the file size becomes the set size. When the file size is increased this way with MPI_File_set_size, new regions are created in the file with displacements between the old file size and the larger, newly set file size. Iconic behaviour, honestly.

Sun MPI I/O does not necessarily allocate file space for such new regions. So you can reserve file space either by using MPI_File_preallocate or by performing a read or write to certain bytes. Your call, bestie 🦋

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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. That's just how they are.

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 💀 but make it effective.
• 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, no cap.

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 👑

Copyright 2003-2026, The Open MPI Community

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okay that's the whole page bestie, you ate that 💅✨ and for the sign-off: this was the Open MPI manual, MPI_FILE_PREALLOCATE(3), dated Sep 10, 2026 — go forth and preallocate, queen 🎀🦋💖