217 lines
8.4 KiB
HTML
217 lines
8.4 KiB
HTML
<!DOCTYPE html>
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<title>8+3 Filenames</title>
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<body>
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<h1 align="center">SQLite And 8+3 Filenames</h1>
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<p>
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The default configuration of SQLite assumes the underlying filesystem
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supports long filenames.
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</p>
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<p>
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SQLite does not impose any naming requirements on database files.
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SQLite will happily work with a database file that has any filename extension
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or with no extension at all.
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When auxiliary files are needed for a <a href="lockingv3.html#rollback">rollback journal</a> or
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a <a href="wal.html">write-ahead log</a> or for one of the other kinds of
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<a href="tempfiles.html">temporary disk files</a>, then the name for the auxiliary file is normally
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constructed by appending a suffix onto the end of the database file name.
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For example, if the original database is call "<tt>app.db</tt>" then
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the <a href="lockingv3.html#rollback">rollback journal</a> will be called "<tt>app.db-journal</tt>"
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and the <a href="wal.html">write-ahead log</a> will be called "<tt>app.db-wal</tt>".
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This approach to auxiliary file naming works great on systems that
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support long filenames. But on systems that impose 8+3 filename
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constraints, the auxiliary files do not fit the 8+3 format even though
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the original database file does.
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</p>
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<h2>Changing Filesystems</h2>
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<p>
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The recommended fix for this problem is to select a different
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filesystem. These days, there is a huge selection of high-performance,
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reliable, patent-free filesystems that support long filenames.
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Where possible, it is recommended that embedded devices use one
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of these other filesystems. This will avoid compatibility issues
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and the danger of
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<a href="shortnames.html#db83corrupt">database corruption caused by inconsistent use of 8+3 filenames</a>.
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</p>
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<h2>Adjusting SQLite To Use 8+3 Filenames</h2>
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<p>
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Some devices are compelled to use an older filesystem with 8+3
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filename restrictions for backwards compatibility, or due
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to other non-technical factors. In such situations, SQLite can be
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coerced into using auxiliary files that fit the 8+3 pattern as follows:
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</p>
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<ol>
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<li><p>Compile the SQLite library with the either the compile-time
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options <a href="compile.html#enable_8_3_names">SQLITE_ENABLE_8_3_NAMES=1</a> or
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<a href="compile.html#enable_8_3_names">SQLITE_ENABLE_8_3_NAMES=2</a>.
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Support for 8+3 filenames is not included in SQLite by default
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because it does introduce some overhead. The overhead is tiny,
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but even so, we do not want to burden the billions of SQLite
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applications that do not need 8+3 filename support.</p>
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<li><p>If the <a href="compile.html#enable_8_3_names">SQLITE_ENABLE_8_3_NAMES=1</a> option
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is used, then SQLite is capable of using 8+3 filenames but that
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capabilities is disabled and must be enabled separately for each
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database connection by using
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using <a href="uri.html">URI filenames</a> when <a href="c3ref/open.html">opening</a> or
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<a href="lang_attach.html">ATTACH-ing</a> the database files and include the
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"<tt>8_3_names=1</tt>" query parameter in the URI. If SQLite
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is compiled with
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<a href="compile.html#enable_8_3_names">SQLITE_ENABLE_8_3_NAMES=2</a> then
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8+3 filenames are enabled by default and this step can be
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skipped.</p>
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<li><p>Make sure that database filenames follow the 8+3 filename
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format and that they do not have an empty name or extension.
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In other words, the database filename must contain between
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1 and 8 characters in the base name and between 1 and 3 characters
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in the extension. Blank extensions are not allowed.</p>
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</ol>
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<p>
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When the steps above are used, SQLite will shorten filename extensions
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by only using the last 3 characters of
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the extension. Thus, for example, a file that would normally be called
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"<tt>app.db-journal</tt>" is shortened to just "<tt>app.nal</tt>".
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Similarly, "<tt>app.db-wal</tt>" will become "<tt>app.wal</tt>" and
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"<tt>app.db-shm</tt>" becomes "<tt>app.shm</tt>".
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</p>
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<p>
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Note that it is very important that the database filename have some kind
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of extension. If there is no extension, then SQLite creates auxiliary
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filenames by appending to the base name of the file. Thus, a database
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named "<tt>db01</tt>" would have a <a href="lockingv3.html#rollback">rollback journal</a> file named
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"<tt>db01-journal</tt>". And as this filename has no extension to shorten
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to 3 characters, it will be used as-is, and will violate 8+3 naming rules.
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</p>
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<a name="db83corrupt"></a>
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<h2>Database Corruption Warning</h2>
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<p>
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If a database file is accessed using 8+3 naming rather than the default
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long filename, then it must be consistently accessed
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using 8+3 naming by every database
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connection every time it is opened, or else there is a risk of database
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corruption.
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The auxiliary <a href="lockingv3.html#rollback">rollback journal</a> and <a href="wal.html">write-ahead log</a> files are essential
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to SQLite for being about to recover from a crash. If an application is
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using 8+3 names and crashes, then the information needed to safely recover
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from the crash is stored in files with the "<tt>.nal</tt>" or
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"<tt>.wal</tt>" extension. If the next application to open the database
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does not specify the "<tt>8_3_names=1</tt>" URI parameter, then SQLite
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will use the long filenames to try to locate the rollback journal or
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write-ahead log files. It will not find them, since they were saved using
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8+3 names by the application that crashed, and hence the database will not
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be properly recovered and will likely go corrupt.
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</p>
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<p>
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Using a database file with 8+3 filenames in some cases while in
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other cases using long filenames is equivalent to
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<a href="howtocorrupt.html#delhotjrnl">deleting a hot journal</a>.
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</p>
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