.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright 2015-2016 Research Organization for Information Science
.\"                     and Technology (RIST). All rights reserved.
.\" $COPYRIGHT$
.TH MPI_File_preallocate 3 "Sep 30, 2023" "4.1.6" "Open MPI"
.SH NAME
\fBMPI_File_preallocate\fP \- Preallocates a specified amount of storage space  at the beginning of a file (collective).

.SH SYNTAX
.ft R
.nf
.SH C Syntax
.nf
#include <mpi.h>
int MPI_File_preallocate(MPI_File \fIfh\fP, MPI_Offset \fIsize\fP)

.fi
.SH Fortran Syntax (see FORTRAN 77 NOTES)
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_FILE_PREALLOCATE(\fIFH\fP, \fISIZE\fP, \fIIERROR\fP)
	INTEGER	\fIFH, IERROR\fP
	INTEGER(KIND=MPI_OFFSET_KIND)	\fISIZE\fP

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_File_preallocate(\fIfh\fP, \fIsize\fP, \fIierror\fP)
	TYPE(MPI_File), INTENT(IN) :: \fIfh\fP
	INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: \fIsize\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH C++ Syntax
.nf
#include <mpi.h>
void MPI::File::Preallocate(MPI::Offset \fIsize\fP)

.fi
.SH INPUT/OUTPUT PARAMETER
.ft R
.TP 1i
fh
File handle (handle).

.SH INPUT PARAMETER
.ft R
.TP 1i
size
Size to preallocate file, in bytes (integer).

.SH OUTPUT PARAMETER
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_File_preallocate ensures that storage space is allocated for the first \fIsize\fP bytes of the file associated with \fIfh\fP. 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 \fIsize\fP. 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 \fIsize\fP. If \fIsize\fP 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.

.SH FORTRAN 77 NOTES
.ft R
The MPI standard prescribes portable Fortran syntax for
the \fISIZE\fP argument only for Fortran 90.  FORTRAN 77
users may use the non-portable syntax
.sp
.nf
     INTEGER*MPI_OFFSET_KIND \fISIZE\fP
.fi
.sp
where MPI_OFFSET_KIND is a constant defined in mpif.h
and gives the length of the declared integer in bytes.

.SH NOTES
.ft R
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.
.sp
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.

.SH 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.
.sp
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.