161 lines
7.7 KiB
HTML
161 lines
7.7 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1999-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU Fortran Compiler: Data consistency and durability</title>
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<meta name="description" content="The GNU Fortran Compiler: Data consistency and durability">
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<meta name="keywords" content="The GNU Fortran Compiler: Data consistency and durability">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Compiler-Characteristics.html#Compiler-Characteristics" rel="up" title="Compiler Characteristics">
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<link href="Files-opened-without-an-explicit-ACTION_003d-specifier.html#Files-opened-without-an-explicit-ACTION_003d-specifier" rel="next" title="Files opened without an explicit ACTION= specifier">
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<link href="Thread_002dsafety-of-the-runtime-library.html#Thread_002dsafety-of-the-runtime-library" rel="prev" title="Thread-safety of the runtime library">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Data-consistency-and-durability"></a>
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<div class="header">
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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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</div>
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<hr>
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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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<hr>
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<div class="header">
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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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</div>
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</body>
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</html>
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