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MPI_File_preallocate.3.rst

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MPI_File_preallocate

:ref:`MPI_File_preallocate` |mdash| 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

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

:ref:`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, :ref:`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 :ref:`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 :ref:`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 :ref:`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 :ref:`MPI_File_preallocate` or by performing a read or write to certain bytes.

ERRORS