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Overview</a></div> +<div class="fancy-toc1"><a href="#loading_an_extension">2. Loading An Extension</a></div> +<div class="fancy-toc1"><a href="#compiling_a_loadable_extension">3. Compiling A Loadable Extension</a></div> +<div class="fancy-toc1"><a href="#programming_loadable_extensions">4. Programming Loadable Extensions</a></div> +<div class="fancy-toc2"><a href="#example_extensions">4.1. Example Extensions</a></div> +<div class="fancy-toc1"><a href="#persistent_loadable_extensions">5. Persistent Loadable Extensions</a></div> +<div class="fancy-toc1"><a href="#statically_linking_a_run_time_loadable_extension">6. Statically Linking A Run-Time Loadable Extension</a></div> +<div class="fancy-toc1"><a href="#implementation_details">7. Implementation Details</a></div> +</div> +</div> +<script> +function toggle_toc(){ +var sub = document.getElementById("toc_sub") +var mk = document.getElementById("toc_mk") +if( sub.style.display!="block" ){ +sub.style.display = "block"; +mk.innerHTML = "▼"; +} else { +sub.style.display = "none"; +mk.innerHTML = "►"; +} +} +</script> +</div> + + + + + +<h1 id="overview"><span>1. </span>Overview</h1> + +<p>SQLite has the ability to load extensions (including new +<a href="appfunc.html">application-defined SQL functions</a>, +<a href="datatype3.html#collation">collating sequences</a>, <a href="vtab.html">virtual tables</a>, and <a href="vfs.html">VFSes</a>) at run-time. +This feature allows the code for extensions to be developed and +tested separately from the application and then loaded +on an as-needed basis.</p> + +<p>Extensions can also be statically linked with the application. +The code template shown below will work just as well as a statically +linked extension as it does as a run-time loadable extension except that +you should give the entry point function ("sqlite3_extension_init") +a different name to avoid name collisions if your application contains +two or more extensions.</p> + +<h1 id="loading_an_extension"><span>2. </span>Loading An Extension</h1> + +<p>An SQLite extension is a shared library or DLL. To load it, you +need to supply SQLite with the name of the file containing the +shared library or DLL and an entry point to initialize the extension. +In C code, this information is supplied using the +<a href="c3ref/load_extension.html">sqlite3_load_extension()</a> API. See the documentation on that +routine for additional information.</p> + +<p>Note that different operating systems use different filename +suffixes for their shared libraries. Windows uses ".dll", Mac uses +".dylib", and most unixes other than mac use ".so". If you want to +make your code portable, you can omit the suffix from the shared +library filename and the appropriate suffix will be added automatically +by the <a href="c3ref/load_extension.html">sqlite3_load_extension()</a> interface.</p> + +<p>There is also an SQL function that can be used to load extensions: +<a href="lang_corefunc.html#load_extension">load_extension(X,Y)</a>. It works just like the <a href="c3ref/load_extension.html">sqlite3_load_extension()</a> +C interface.</p> + +<p>Both methods for loading an extension allow you to specify +the name of an entry point for the extension. +You can leave this argument blank - passing in +a NULL pointer for the <a href="c3ref/load_extension.html">sqlite3_load_extension()</a> C-language interface +or omitting the second argument for the <a href="lang_corefunc.html#load_extension">load_extension()</a> SQL interface - +and the extension loader logic will attempt to figure out the entry point +on its own. It will first try the generic extension name +"sqlite3_extension_init". If that does not work, it constructs a +entry point using the template "sqlite3_X_init" where the X is replaced +by the lowercase equivalent of every ASCII character in the filename +after the last "/" and before the first following "." omitting the +first three characters if they happen to be "lib". So, for example, +if the filename is "/usr/lib/libmathfunc-4.8.so" the entry point name +would be "sqlite3_mathfunc_init". Or if the filename is +"./SpellFixExt.dll" then the entry point would be called +"sqlite3_spellfixext_init". + +</p><p>For security reasons, extension loading is turned off by default. +In order to use either the C-language or SQL extension loading functions, +one must first enable extension loading using the +<a href="c3ref/db_config.html">sqlite3_db_config</a>(db,<a href="c3ref/c_dbconfig_defensive.html#sqlitedbconfigenableloadextension">SQLITE_DBCONFIG_ENABLE_LOAD_EXTENSION</a>,1,NULL) +C-language API in your application.</p> + +<p>From the <a href="cli.html">command-line shell</a>, extensions can be loaded using the +".load" dot-command. For example: + +</p><blockquote><pre> +.load ./YourCode +</pre></blockquote> + +<p>Note that the command-line shell program has already enabled +extension loading for you (by calling the <a href="c3ref/enable_load_extension.html">sqlite3_enable_load_extension()</a> +interface as part of its setup) so the command above works without +any special switches, setup, or other complications.</p> + +<p>The ".load" command with one argument invokes sqlite3_load_extension() +with the zProc parameter set to NULL, causing SQLite to first look for +an entry point named "sqlite3_extension_init" and then "sqlite3_X_init" +where "X" is derived from the filename. If your extension has an entry +point with a different name, simply supply that name as the second +argument. For example:</p> + +<blockquote><pre> +.load ./YourCode nonstandard_entry_point +</pre></blockquote> + +<a name="build"></a> + +<h1 id="compiling_a_loadable_extension"><span>3. </span>Compiling A Loadable Extension</h1> + +<p>Loadable extensions are C-code. To compile them on +most unix-like operating +systems, the usual command is something like this:</p> + +<blockquote><pre> +gcc -g -fPIC -shared YourCode.c -o YourCode.so +</pre></blockquote> + +<p>Macs are unix-like, but they do not follow the usual shared library +conventions. To compile a shared library on a Mac, use a command like +this:</p> + +<blockquote><pre> +gcc -g -fPIC -dynamiclib YourCode.c -o YourCode.dylib +</pre></blockquote> + +<p>If when you try to load your library you get back an error message +that says "mach-o, but wrong architecture" then you might need to add +command-line options "-arch i386" or "arch x86_64" to gcc, depending +on how your application is built.</p> + +<p>To compile on Windows using MSVC, a command similar to the following +will usually work:</p> + +<blockquote><pre> +cl YourCode.c -link -dll -out:YourCode.dll +</pre></blockquote> + +<p>To compile for Windows using MinGW, the command line is just like it +is for unix except that the output file suffix is changed to ".dll" and +the -fPIC argument is omitted:</p> + +<blockquote><pre> +gcc -g -shared YourCode.c -o YourCode.dll +</pre></blockquote> + +<a name="write"></a> + +<h1 id="programming_loadable_extensions"><span>4. </span>Programming Loadable Extensions</h1> + +<p>A template loadable extension contains the following three elements:</p> + +<ol> +<li><p> +Use "<tt>#include <sqlite3ext.h></tt>" at the top of your source +code files instead of "<tt>#include <sqlite3.h></tt>". +</p> + +</li><li><p> +Put the macro "<tt>SQLITE_EXTENSION_INIT1</tt>" on a line by itself +right after the "<tt>#include <sqlite3ext.h></tt>" line. +</p> + +</li><li><p> +Add an extension loading entry point routine that looks like +something the following: + +</p><div class="codeblock"><pre>#ifdef _WIN32 +__declspec(dllexport) +#endif +int sqlite3_extension_init( /* <== Change this name, maybe */ + sqlite3 *db, + char **pzErrMsg, + const sqlite3_api_routines *pApi +){ + int rc = SQLITE_OK; + SQLITE_EXTENSION_INIT2(pApi); + /* insert code to initialize your extension here */ + return rc; +} +</pre></div> + +<p>You will do well to customize the name of your entry point to +correspond to the name of the shared library you will be generating, +rather than using the generic "sqlite3_extension_init" name. Giving +your extension a custom entry point name will enable you to statically +link two or more extensions into the same program without a linker +conflict, if you later decide to use static linking rather than run-time +linking. +If your shared library ends up being named "YourCode.so" or +"YourCode.dll" or "YourCode.dylib" as shown in the compiler examples +above, then the correct entry point name would be +"sqlite3_yourcode_init". +</p></li></ol> + +<p>Here is a complete template extension that you can copy/paste +to get started:</p> + +<div class="codeblock"><pre>/* Add your header comment here */ +#include <sqlite3ext.h> /* Do not use <sqlite3.h>! */ +SQLITE_EXTENSION_INIT1 + +/* Insert your extension code here */ + +#ifdef _WIN32 +__declspec(dllexport) +#endif +/* TODO: Change the entry point name so that "extension" is replaced by +** text derived from the shared library filename as follows: Copy every +** ASCII alphabetic character from the filename after the last "/" through +** the next following ".", converting each character to lowercase, and +** discarding the first three characters if they are "lib". +*/ +int sqlite3_extension_init( + sqlite3 *db, + char **pzErrMsg, + const sqlite3_api_routines *pApi +){ + int rc = SQLITE_OK; + SQLITE_EXTENSION_INIT2(pApi); + /* Insert here calls to + ** sqlite3_create_function_v2(), + ** sqlite3_create_collation_v2(), + ** sqlite3_create_module_v2(), and/or + ** sqlite3_vfs_register() + ** to register the new features that your extension adds. + */ + return rc; +} +</pre></div> + +<h2 id="example_extensions"><span>4.1. </span>Example Extensions</h2> + +<p>Many examples of complete and working loadable extensions can be +seen in the SQLite source tree in the +<a href="https://www.sqlite.org/src/file/ext/misc">ext/misc</a> subdirectory. +Each file in that directory is a separate extension. Documentation +is provided by a header comment on the file. +Here are brief notes on a few of the extensions in +the <a href="https://www.sqlite.org/src/file/ext/misc">ext/misc</a> subdirectory: + +</p><ul> +<li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/carray.c">carray.c</a> — +Implementation of the <a href="carray.html">carray table-valued function</a>. +</p></li><li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/compress.c">compress.c</a> — +Implementation of <a href="appfunc.html">application-defined SQL functions</a> compress() and +uncompress() that do zLib compression of text or blob content. +</p></li><li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/json1.c">json1.c</a> — +Implementation of <a href="json1.html">JSON SQL functions</a> and <a href="vtab.html#tabfunc2">table-valued functions</a>. +This is a larger and more complex extension. +</p></li><li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/memvfs.c">memvfs.c</a> — +Implementation of a new <a href="vfs.html">VFS</a> that stores all content in-memory. +</p></li><li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/rot13.c">rot13.c</a> — +Implementation of a <a href="https://en.wikipedia.org/wiki/ROT13">rot13()</a> +SQL function. This is a very simple example of an extension function +and is useful as a template for creating new extensions. +</p></li><li><p> +<a href="https://www.sqlite.org/src/file/ext/misc/series.c">series.c</a> — +Implementation of the generate_series <a href="vtab.html">virtual table</a> and +<a href="vtab.html#tabfunc2">table-valued function</a>. This is a relatively simple example of a +virtual table implementation which can serve as a template for writing +new virtual tables. +</p></li></ul> + +<p>Other and more complex extensions can be found in subfolders +under <a href="https://www.sqlite.org/src/file/ext">ext/</a> other than ext/misc/. + +<a name="persist"></a> + +</p><h1 id="persistent_loadable_extensions"><span>5. </span>Persistent Loadable Extensions</h1> + +<p>The default behavior for a loadable extension is that it is unloaded +from process memory when the database connection that originally invoked +<a href="c3ref/load_extension.html">sqlite3_load_extension()</a> closes. (In other words, the xDlClose method +of the <a href="c3ref/vfs.html">sqlite3_vfs</a> object is called for all extensions when a database +connection closes.) However, if the initialization procedure returns +<a href="rescode.html#ok_load_permanently">SQLITE_OK_LOAD_PERMANENTLY</a> instead of SQLITE_OK, then the extension will +not be unloaded (xDlClose will not be invoked) and the extension will remain +in process memory indefinitely. The SQLITE_OK_LOAD_PERMANENTLY return +value is useful for extensions that want to register new <a href="vfs.html">VFSes</a>. + +</p><p>To clarify: an extension for which the initialization function returns +SQLITE_OK_LOAD_PERMANENTLY continues to exist in memory after the database +connection closes. However, the extension is <em>not</em> automatically +registered with subsequent database connections. This makes it possible +to load extensions that implement new <a href="vfs.html">VFSes</a>. +To persistently load and register an extension that implements new SQL +functions, collating sequences, and/or virtual tables, such that those +added capabilities are available to all subsequent database connections, +then the initialization routine should also invoke <a href="c3ref/auto_extension.html">sqlite3_auto_extension()</a> +on a subfunction that will register those services. + +</p><p>The <a href="https://sqlite.org/src/file/ext/misc/vfsstat.c">vfsstat.c</a> extension +show an example of a loadable extension that persistently registers both +a new VFS and a new virtual table. The +<a href="https://sqlite.org/src/info/77b5b4235c9f7f11?ln=801-819">sqlite3_vfsstat_init()</a> +initialization routine in that extension is called only once, when the +extension is first loaded. It registers the new "vfslog" VFS just that +one time, and it returns SQLITE_OK_LOAD_PERMANENTLY so that the code used +to implement the "vfslog" VFS will remain in memory. The initialization routine +also invokes <a href="c3ref/auto_extension.html">sqlite3_auto_extension()</a> on a pointer to the "vstatRegister()" +function so that all subsequent database connections will invoke the +"vstatRegister()" function as they start up, and hence register the +"vfsstat" virtual table. + +</p><h1 id="statically_linking_a_run_time_loadable_extension"><span>6. </span>Statically Linking A Run-Time Loadable Extension</h1> + +<p>The exact same source code can be used for both a run-time loadable +shared library or DLL and as a module that is statically linked with your +application. This provides flexibility and allows you to reuse the same +code in different ways.</p> + +<p>To statically link your extension, simply add the -DSQLITE_CORE +compile-time option. The SQLITE_CORE macro causes the SQLITE_EXTENSION_INIT1 +and SQLITE_EXTENSION_INIT2 macros to become no-ops. Then modify your +application to invoke the entry point directly, passing in a NULL pointer +as the third "pApi" parameter.</p> + +<p>It is particularly important to use an entry point name that is +based on the extension filename, rather than the generic +"sqlite3_extension_init" entry point name, if you will be statically +linking two or more extensions. If you use the generic name, there +will be multiple definitions of the same symbol and the link will fail.</p> + +<p>If you will be opening multiple database connections in your application, +rather than invoking the extension entry points for each database +connection separately, you might want to consider using the +<a href="c3ref/auto_extension.html">sqlite3_auto_extension()</a> interface to register your extensions and +to cause them to be automatically started as each database connection +is opened. You only have to register each extension once, and you can +do so near the beginning of your main() routine. Using the +<a href="c3ref/auto_extension.html">sqlite3_auto_extension()</a> interface to register your extensions makes +your extensions work as if they were built into the core SQLite - they +automatically exist whenever you open a new database connection +without needing to be initialized. Just be sure to complete any +configuration you need to accomplish using <a href="c3ref/config.html">sqlite3_config()</a> before +registering your extensions, since the <a href="c3ref/auto_extension.html">sqlite3_auto_extension()</a> +interface implicitly calls <a href="c3ref/initialize.html">sqlite3_initialize()</a>.</p> + +<h1 id="implementation_details"><span>7. </span>Implementation Details</h1> + +<p>SQLite implements run-time extension loading using the +xDlOpen(), xDlError(), xDlSym(), and xDlClose() methods of the +<a href="c3ref/vfs.html">sqlite3_vfs</a> object. These methods are implemented using +the dlopen() library on unix (which explains why SQLite commonly +needs to be linked against the "-ldl" library on unix systems) +and using LoadLibrary() API on Windows. In a custom <a href="vfs.html">VFS</a> for +unusual systems, these methods can all be omitted, in which case +the run-time extension loading mechanism will not work (though +you will still be able to statically link the extension code, assuming +the entry pointers are uniquely named). +SQLite can be compiled with +<a href="compile.html#omit_load_extension">SQLITE_OMIT_LOAD_EXTENSION</a> to omit the extension loading code +from the build. +</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/loadext.in?m=b7044b2344d956155">2022-03-08 04:52:03</a> UTC </small></i></p> + |