| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1999-2014 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Funding Free Software", the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| "GNU Free Documentation License". |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>The GNU Fortran Compiler: Data consistency and durability</title> |
| |
| <meta name="description" content="The GNU Fortran Compiler: Data consistency and durability"> |
| <meta name="keywords" content="The GNU Fortran Compiler: Data consistency and durability"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Compiler-Characteristics.html#Compiler-Characteristics" rel="up" title="Compiler Characteristics"> |
| <link href="Extensions.html#Extensions" rel="next" title="Extensions"> |
| <link href="Thread_002dsafety-of-the-runtime-library.html#Thread_002dsafety-of-the-runtime-library" rel="prev" title="Thread-safety of the runtime library"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Data-consistency-and-durability"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="Data-consistency-and-durability-1"></a> |
| <h3 class="section">5.4 Data consistency and durability</h3> |
| <a name="index-consistency_002c-durability"></a> |
| |
| <p>This section contains a brief overview of data and metadata |
| consistency and durability issues when doing I/O. |
| </p> |
| <p>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 <code>FNUM</code> to retrieve the |
| low level file descriptor corresponding to an open Fortran unit. Then, |
| using e.g. the <code>ISO_C_BINDING</code> feature, one can call the |
| underlying system call to flush dirty data to stable storage, such as |
| <code>fsync</code> on POSIX, <code>_commit</code> on MingW, or <code>fcntl(fd, |
| F_FULLSYNC, 0)</code> on Mac OS X. The following example shows how to call |
| fsync: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> ! 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" |
| </pre></div> |
| |
| <p>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 |
| <code>FLUSH</code> statement. Also, the buffering can be turned off with the |
| <code>GFORTRAN_UNBUFFERED_ALL</code> and |
| <code>GFORTRAN_UNBUFFERED_PRECONNECTED</code> 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. |
| </p> |
| <p>The Windows platform supports a relaxed metadata consistency model, |
| where file metadata is written to the directory lazily. This means |
| that, for instance, the <code>dir</code> command can show a stale size for a |
| file. One can force a directory metadata update by closing the unit, |
| or by calling <code>_commit</code> on the file descriptor. Note, though, |
| that <code>_commit</code> will force all dirty data to stable storage, which |
| is often a very slow operation. |
| </p> |
| <p>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 |
| data. <code>fsync</code> will also force a flush of dirty data and metadata |
| to the server. Similar to <code>open</code> and <code>close</code>, acquiring and |
| releasing <code>fcntl</code> file locks, if the server supports them, will |
| also force cache validation and flushing dirty data and metadata. |
| </p> |
| |
| |
| |
| <hr> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
