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+<div class=fancy>
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+The Checksum VFS Shim
+</div>
+<div class="fancy_toc">
+<a onclick="toggle_toc()">
+<span class="fancy_toc_mark" id="toc_mk">&#x25ba;</span>
+Table Of Contents
+</a>
+<div id="toc_sub"><div class="fancy-toc1"><a href="#overview">1. Overview</a></div>
+<div class="fancy-toc1"><a href="#compiling">2. Compiling</a></div>
+<div class="fancy-toc1"><a href="#loading">3. Loading</a></div>
+<div class="fancy-toc1"><a href="#usage">4. Usage</a></div>
+<div class="fancy-toc1"><a href="#verification_of_checksums">5. Verification Of Checksums</a></div>
+<div class="fancy-toc1"><a href="#controlling_checksum_verification">6. Controlling Checksum Verification</a></div>
+</div>
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+</div>
+
+
+
+
+
+<h1 id="overview"><span>1. </span>Overview</h1>
+
+<p>The checksum VFS extension is a <a href="vfs.html#shim">VFS shim</a> that adds an 8-byte
+checksum to the end of every page in an SQLite database. The checksum
+is added as each page is written and verified as each page is read.
+The checksum is intended to help detect database corruption caused by
+random bit-flips in the mass storage device.
+
+</p><p>The checksum VFS extension requires
+SQLite version 3.32.0 (2020-05-22) or later. It will not
+work with earlier versions of SQLite.
+
+</p><h1 id="compiling"><span>2. </span>Compiling</h1>
+
+<p>The checksum VFS module is a <a href="loadext.html">loadable extension</a>. It is not
+included in the <a href="amalgamation.html">amalgamation</a>. It must be added to SQLite
+either at compile-time or at run-time. The source code to
+the checksum VFS module is in the
+<a href="https://sqlite.org/src/file/ext/misc/cksumvfs.c">ext/misc/cksumvfs.c</a>
+source file in the
+<a href="https://sqlite.org/src">SQLite source tree</a>.
+
+</p><p>To build the checksum VFS module into a run-time loadable
+extension, use commands similar to the following:
+
+</p><ul>
+<li>(linux) &rarr; <tt>gcc -fPIC -shared cksumvfs.c -o cksumvfs.so</tt>
+</li><li>(mac) &rarr; <tt>clang -fPIC -dynamiclib cksumvfs.c -o cksumvfs.dylib</tt>
+</li><li>(windows) &rarr; <tt>cl cksumvfs.c -link -dll -out:cksumvfs.dll</tt>
+</li></ul>
+
+<p>You may want to add additional compiler options, of course,
+according to the needs of your project.
+
+</p><p>To statically link this extension with your product,
+compile it like any other C-language module but add the
+"-DSQLITE_CKSUMVFS_STATIC" option so that this module knows that
+it is being statically linked rather than dynamically linked.
+
+</p><h1 id="loading"><span>3. </span>Loading</h1>
+
+<p>To load this extension as a shared library, you first have to
+bring up a dummy SQLite database connection to use as the argument
+to the <a href="c3ref/load_extension.html">sqlite3_load_extension()</a> API call. Then you invoke the
+<a href="c3ref/load_extension.html">sqlite3_load_extension()</a> API and shutdown the dummy database
+connection. All subsequent database connections that are opened
+will include this extension. For example:
+
+</p><div class="codeblock"><pre>sqlite3 *db;
+sqlite3_open(":memory:", &db);
+sqlite3_load_extension(db, "./cksumvfs");
+sqlite3_close(db);
+</pre></div>
+
+<p>If this extension is compiled with -DSQLITE_CKSUMVFS_STATIC and
+statically linked against the application, initialize it using
+a single API call as follows:
+
+</p><div class="codeblock"><pre>sqlite3_cksumvfs_init();
+</pre></div>
+
+<p>Cksumvfs is a <a href="vfs.html#shim">VFS shim</a>. When loaded, "cksmvfs" becomes the new
+default VFS, and it uses the prior default VFS as the next VFS
+down in the stack. This is normally what you want. However, in
+complex situations where multiple VFS shims are being loaded,
+it might be important to ensure that cksumvfs is loaded in the
+correct order so that it sequences itself into the default VFS
+Shim stack in the right order.
+
+</p><h1 id="usage"><span>4. </span>Usage</h1>
+
+<p>Open database connections using the <a href="c3ref/open.html">sqlite3_open()</a> or
+<a href="c3ref/open.html">sqlite3_open_v2()</a> interfaces, as normal. Ordinary database files
+(without a checksum) will operate normally. Databases with
+checksums will return an SQLITE_IOERR_DATA error if a page is
+encountered that contains an invalid checksum.
+
+</p><p>Checksumming only works on databases that have a <a href="fileformat2.html#resbyte">reserve bytes</a>
+value of exactly 8. The default value for reserve-bytes is 0.
+Hence, newly created database files will omit the checksum by
+default. To create a database that includes a checksum, change
+the reserve-bytes value to 8 by running code similar to this:
+
+</p><div class="codeblock"><pre>int n = 8;
+sqlite3_file_control(db, 0, SQLITE_FCNTL_RESERVE_BYTES, &n);
+</pre></div>
+
+<p>If you do this immediately after creating a new database file,
+before anything else has been written into the file, then that
+might be all that you need to do. Otherwise, the API call
+above should be followed by:
+
+</p><div class="codeblock"><pre>sqlite3_exec(db, "VACUUM", 0, 0, 0);
+</pre></div>
+
+<p>It never hurts to run the VACUUM, even if you don't need it.
+If the database is in WAL mode, you should shutdown and
+reopen all database connections before continuing.
+
+</p><p>From the CLI, use the ".filectrl reserve_bytes 8" command,
+followed by "VACUUM;".
+
+</p><p>Note that SQLite allows the number of reserve-bytes to be
+increased but not decreased. So if a database file already
+has a reserve-bytes value greater than 8, there is no way to
+activate checksumming on that database, other than to dump
+and restore the database file. Note also that other extensions
+might also make use of the reserve-bytes. Checksumming will
+be incompatible with those other extensions.
+
+</p><h1 id="verification_of_checksums"><span>5. </span>Verification Of Checksums</h1>
+
+<p>If any checksum is incorrect, the "PRAGMA quick_check" command
+will find it. To verify that checksums are actually enabled
+and running, use SQL like the following:
+
+</p><div class="codeblock"><pre>SELECT count(*), verify_checksum(data)
+ FROM sqlite_dbpage
+ GROUP BY 2;
+</pre></div>
+
+<p>There are three possible outputs from the verify_checksum()
+function: 1, 0, and NULL. 1 is returned if the checksum is
+correct. 0 is returned if the checksum is incorrect. NULL
+is returned if the page is unreadable. If checksumming is
+enabled, the read will fail if the checksum is wrong, so the
+usual result from verify_checksum() on a bad checksum is NULL.
+
+</p><p>If everything is OK, the query above should return a single
+row where the second column is 1. Any other result indicates
+either that there is a checksum error, or checksum validation
+is disabled.
+
+</p><h1 id="controlling_checksum_verification"><span>6. </span>Controlling Checksum Verification</h1>
+
+<p>The cksumvfs extension implements a new PRAGMA statement that can
+be used to disable, re-enable, or query the status of checksum
+verification:
+
+</p><div class="codeblock"><pre>PRAGMA checksum_verification; -- query status
+PRAGMA checksum_verification=OFF; -- disable verification
+PRAGMA checksum_verification=ON; -- re-enable verification
+</pre></div>
+
+<p>The "checksum_verification" pragma will return "1" (true) or "0"
+(false) if checksum verification is enabled or disabled, respectively.
+"Verification" in this context means the feature that causes
+SQLITE_IOERR_DATA errors if a checksum mismatch is detected while
+reading. Checksums are always kept up-to-date as long as the
+<a href="fileformat2.html#resbyte">reserve bytes</a> value of the database is 8, regardless of the setting
+of this pragma. Checksum verification can be disabled (for example)
+to do forensic analysis of a database that has previously reported
+a checksum error.
+
+</p><p>The "checksum_verification" pragma will always respond with "0" if
+the database file does not have a <a href="fileformat2.html#resbyte">reserve bytes</a> value of 8. The
+pragma will return no rows at all if the cksumvfs extension is
+not loaded.
+</p><p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/cksumvfs.in?m=c9b2514ff040a5792">2020-12-10 10:53:35</a> UTC </small></i></p>
+