MPI_FILE_PREALLOCATE(3)				     Open MPI				   MPI_FILE_PREALLOCATE(3)

MPI_File_preallocate — Preallocates a specified amount of storage space at the beginning of a file (collective).

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

INPUT/OUTPUT PARAMETER
       • fh: File handle (handle).

INPUT PARAMETER
       • size: Size to preallocate file, in bytes (integer).

OUTPUT PARAMETER
       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_File_preallocate  ensures  that  storage  space  is	allocated  for	the  first  size bytes of the file
       associated with fh.  MPI_File_preallocate can be a very time-consuming operation.

       MPI_File_preallocate is collective; all processes in the group must pass identical values for size. 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. 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 <#mpi-file-set-size>. If MPI_MODE_SEQUENTIAL mode was specified when the file was opened,
       it is erroneous to call this routine.

NOTES
       When  using  the collective routine MPI_File_set_size <#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 <#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.

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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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.

       • 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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				   MPI_FILE_PREALLOCATE(3)
