MPI_FILE_PREALLOCATE(3)

💅 MPI_FILE_PREALLOCATE(3)

okay bestie, so this is a page from the Open MPI manual set, and it's literally about grabbing storage space in a file before you need it. main character energy for your file system, no cap 🎀

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✨ NAME ✨

MPI_File_preallocate — preallocates a specified amount of storage space at the beginning of a file (collective). yes girl, collective, meaning everybody's in on it 🫶

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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 (it's a handle, bestie). your lil ticket to the file 💖

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⭐ INPUT PARAMETER ⭐

• size — how many bytes you want to preallocate the file to (integer). that's the size. that's it. iconic 💅

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🌈 OUTPUT PARAMETER 🌈

• ierror — Fortran only: the error status (integer). Fortran girlies get the extra arg, we respect it 🫶

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👑 DESCRIPTION 👑

So here's the tea ☕: MPI_File_preallocate ensures that storage space is allocated for the first size bytes of the file associated with fh. and ngl, MPI_File_preallocate can be a very time-consuming operation — like, go get a snack, it's doing stuff 😭

MPI_File_preallocate is collective — all processes in the group must pass identical values for size. no negotiating, no "well I wanted a different number", everybody agrees or it's a whole thing 💀

Regions of the file that have previously been written are unaffected. For newly allocated regions of the file, MPI_File_preallocate has the same effect as writing undefined data. (yes, undefined. don't come for me, that's what it says 🙈)

• 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 is unchanged.

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. it's giving "bad idea" 💀

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🧚 NOTES 🧚

When using 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. 📐

Sun MPI I/O does not necessarily allocate file space for such new regions. You may reserve file space either by using MPI_File_preallocate or by performing a read or write to certain bytes. so like... there's options, pick your fighter 💖

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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. 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. 🫠

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 fatal 💀
• 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. 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. iykyk ✨

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🍓 COPYRIGHT 🍓

2003-2026, The Open MPI Community

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and that's the whole page, bestie! 🎀 this one's dated Sep 11, 2026, from the Open MPI girlies, and it lives at MPI_FILE_PREALLOCATE(3) in the manual — go preallocate something, slay, periodt 💜🦋