5.4 Data consistency and durability
This section contains a brief overview of data and metadata consistency and durability issues when doing I/O.
With respect to durability, GNU Fortran makes no effort to ensure that
data is committed to stable storage. If this is required, the GNU
Fortran programmer can use the intrinsic
FNUM to retrieve the
low level file descriptor corresponding to an open Fortran unit. Then,
using e.g. the
ISO_C_BINDING feature, one can call the
underlying system call to flush dirty data to stable storage, such as
fsync on POSIX,
_commit on MingW, or
F_FULLSYNC, 0) on Mac OS X. The following example shows how to call
! Declare the interface for POSIX fsync function interface function fsync (fd) bind(c,name="fsync") use iso_c_binding, only: c_int integer(c_int), value :: fd integer(c_int) :: fsync end function fsync end interface ! Variable declaration integer :: ret ! Opening unit 10 open (10,file="foo") ! ... ! Perform I/O on unit 10 ! ... ! Flush and sync flush(10) ret = fsync(fnum(10)) ! Handle possible error if (ret /= 0) stop "Error calling FSYNC"
With respect to consistency, for regular files GNU Fortran uses
buffered I/O in order to improve performance. This buffer is flushed
automatically when full and in some other situations, e.g. when
closing a unit. It can also be explicitly flushed with the
FLUSH statement. Also, the buffering can be turned off with the
GFORTRAN_UNBUFFERED_PRECONNECTED environment variables. Special
files, such as terminals and pipes, are always unbuffered. Sometimes,
however, further things may need to be done in order to allow other
processes to see data that GNU Fortran has written, as follows.
The Windows platform supports a relaxed metadata consistency model,
where file metadata is written to the directory lazily. This means
that, for instance, the
dir command can show a stale size for a
file. One can force a directory metadata update by closing the unit,
or by calling
_commit on the file descriptor. Note, though,
_commit will force all dirty data to stable storage, which
is often a very slow operation.
The Network File System (NFS) implements a relaxed consistency model
called open-to-close consistency. Closing a file forces dirty data and
metadata to be flushed to the server, and opening a file forces the
client to contact the server in order to revalidate cached
fsync will also force a flush of dirty data and metadata
to the server. Similar to
close, acquiring and
fcntl file locks, if the server supports them, will
also force cache validation and flushing dirty data and metadata.