NAME

       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 (see FORTRAN 77 NOTES)

       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

C++ Syntax

       #include <mpi.h>
       void MPI::File::Preallocate(MPI::Offset size)

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. If MPI_MODE_SEQUENTIAL mode was specified  when  the  file
       was opened, it is erroneous to call this routine.

FORTRAN 77 NOTES

       The MPI standard prescribes portable Fortran syntax for the SIZE argument only for Fortran
       90.  FORTRAN 77 users may use the non-portable syntax

            INTEGER*MPI_OFFSET_KIND SIZE

       where MPI_OFFSET_KIND is a constant defined in mpif.h and gives the length of the declared
       integer in bytes.

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.

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran  routines in the last argument. C++ functions do not return errors. If the default
       error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then  on  error  the  C++  exception
       mechanism will be used to throw an MPI::Exception object.

       Before  the  error value is returned, the current MPI error handler is called. For MPI I/O
       function errors, the default error handler is set to MPI_ERRORS_RETURN. The error  handler
       may    be   changed   with   MPI_File_set_errhandler;   the   predefined   error   handler
       MPI_ERRORS_ARE_FATAL may be used to  make  I/O  errors  fatal.  Note  that  MPI  does  not
       guarantee that an MPI program can continue past an error.