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<title>The GNU Fortran Compiler: Data consistency and durability</title>
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<a name="Data-consistency-and-durability"></a>
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Next: <a href="Files-opened-without-an-explicit-ACTION_003d-specifier.html#Files-opened-without-an-explicit-ACTION_003d-specifier" accesskey="n" rel="next">Files opened without an explicit ACTION= specifier</a>, Previous: <a href="Thread_002dsafety-of-the-runtime-library.html#Thread_002dsafety-of-the-runtime-library" accesskey="p" rel="prev">Thread-safety of the runtime library</a>, Up: <a href="Compiler-Characteristics.html#Compiler-Characteristics" accesskey="u" rel="up">Compiler Characteristics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Data-consistency-and-durability-1"></a>
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<h3 class="section">5.4 Data consistency and durability</h3>
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<a name="index-consistency_002c-durability"></a>
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<p>This section contains a brief overview of data and metadata
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consistency and durability issues when doing I/O.
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</p>
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<p>With respect to durability, GNU Fortran makes no effort to ensure that
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data is committed to stable storage. If this is required, the GNU
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Fortran programmer can use the intrinsic <code>FNUM</code> to retrieve the
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low level file descriptor corresponding to an open Fortran unit. Then,
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using e.g. the <code>ISO_C_BINDING</code> feature, one can call the
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underlying system call to flush dirty data to stable storage, such as
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<code>fsync</code> on POSIX, <code>_commit</code> on MingW, or <code>fcntl(fd,
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F_FULLSYNC, 0)</code> on Mac OS X. The following example shows how to call
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fsync:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> ! Declare the interface for POSIX fsync function
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interface
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function fsync (fd) bind(c,name="fsync")
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use iso_c_binding, only: c_int
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integer(c_int), value :: fd
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integer(c_int) :: fsync
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end function fsync
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end interface
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! Variable declaration
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integer :: ret
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! Opening unit 10
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open (10,file="foo")
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! ...
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! Perform I/O on unit 10
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! ...
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! Flush and sync
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flush(10)
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ret = fsync(fnum(10))
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! Handle possible error
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if (ret /= 0) stop "Error calling FSYNC"
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</pre></div>
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<p>With respect to consistency, for regular files GNU Fortran uses
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buffered I/O in order to improve performance. This buffer is flushed
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automatically when full and in some other situations, e.g. when
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closing a unit. It can also be explicitly flushed with the
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<code>FLUSH</code> statement. Also, the buffering can be turned off with the
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<code>GFORTRAN_UNBUFFERED_ALL</code> and
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<code>GFORTRAN_UNBUFFERED_PRECONNECTED</code> environment variables. Special
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files, such as terminals and pipes, are always unbuffered. Sometimes,
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however, further things may need to be done in order to allow other
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processes to see data that GNU Fortran has written, as follows.
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</p>
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<p>The Windows platform supports a relaxed metadata consistency model,
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where file metadata is written to the directory lazily. This means
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that, for instance, the <code>dir</code> command can show a stale size for a
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file. One can force a directory metadata update by closing the unit,
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or by calling <code>_commit</code> on the file descriptor. Note, though,
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that <code>_commit</code> will force all dirty data to stable storage, which
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is often a very slow operation.
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</p>
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<p>The Network File System (NFS) implements a relaxed consistency model
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called open-to-close consistency. Closing a file forces dirty data and
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metadata to be flushed to the server, and opening a file forces the
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client to contact the server in order to revalidate cached
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data. <code>fsync</code> will also force a flush of dirty data and metadata
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to the server. Similar to <code>open</code> and <code>close</code>, acquiring and
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releasing <code>fcntl</code> file locks, if the server supports them, will
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also force cache validation and flushing dirty data and metadata.
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</p>
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Next: <a href="Files-opened-without-an-explicit-ACTION_003d-specifier.html#Files-opened-without-an-explicit-ACTION_003d-specifier" accesskey="n" rel="next">Files opened without an explicit ACTION= specifier</a>, Previous: <a href="Thread_002dsafety-of-the-runtime-library.html#Thread_002dsafety-of-the-runtime-library" accesskey="p" rel="prev">Thread-safety of the runtime library</a>, Up: <a href="Compiler-Characteristics.html#Compiler-Characteristics" accesskey="u" rel="up">Compiler Characteristics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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