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+#
+# Run this Tcl script to generate the sqlite.html file.
+#
+set rcsid {$Id: arch.tcl,v 1.16 2004/10/10 17:24:54 drh Exp $}
+source common.tcl
+header {Architecture of SQLite}
+puts {
+<h2>The Architecture Of SQLite</h2>
+
+<h3>Introduction</h3>
+
+<table align="right" border="1" cellpadding="15" cellspacing="1">
+<tr><th>Block Diagram Of SQLite</th></tr>
+<tr><td><img src="arch2.gif"></td></tr>
+</table>
+<p>This document describes the architecture of the SQLite library.
+The information here is useful to those who want to understand or
+modify the inner workings of SQLite.
+</p>
+
+<p>
+A block diagram showing the main components of SQLite
+and how they interrelate is shown at the right.  The text that
+follows will provide a quick overview of each of these components.
+</p>
+
+
+<p>
+This document describes SQLite version 3.0.  Version 2.8 and
+earlier are similar but the details differ.
+</p>
+
+<h3>Interface</h3>
+
+<p>Much of the public interface to the SQLite library is implemented by
+functions found in the <b>main.c</b>, <b>legacy.c</b>, and
+<b>vdbeapi.c</b> source files
+though some routines are
+scattered about in other files where they can have access to data 
+structures with file scope.  The
+<b>sqlite3_get_table()</b> routine is implemented in <b>table.c</b>.
+<b>sqlite3_mprintf()</b> is found in <b>printf.c</b>.
+<b>sqlite3_complete()</b> is in <b>tokenize.c</b>.
+The Tcl interface is implemented by <b>tclsqlite.c</b>.  More
+information on the C interface to SQLite is
+<a href="capi3ref.html">available separately</a>.<p>
+
+<p>To avoid name collisions with other software, all external
+symbols in the SQLite library begin with the prefix <b>sqlite3</b>.
+Those symbols that are intended for external use (in other words,
+those symbols which form the API for SQLite) begin
+with <b>sqlite3_</b>.</p>
+
+<h3>Tokenizer</h3>
+
+<p>When a string containing SQL statements is to be executed, the
+interface passes that string to the tokenizer.  The job of the tokenizer
+is to break the original string up into tokens and pass those tokens
+one by one to the parser.  The tokenizer is hand-coded in C in 
+the file <b>tokenize.c</b>.
+
+<p>Note that in this design, the tokenizer calls the parser.  People
+who are familiar with YACC and BISON may be used to doing things the
+other way around -- having the parser call the tokenizer.  The author
+of SQLite 
+has done it both ways and finds things generally work out nicer for
+the tokenizer to call the parser.  YACC has it backwards.</p>
+
+<h3>Parser</h3>
+
+<p>The parser is the piece that assigns meaning to tokens based on
+their context.  The parser for SQLite is generated using the
+<a href="http://www.hwaci.com/sw/lemon/">Lemon</a> LALR(1) parser
+generator.  Lemon does the same job as YACC/BISON, but it uses
+a different input syntax which is less error-prone.
+Lemon also generates a parser which is reentrant and thread-safe.
+And lemon defines the concept of a non-terminal destructor so
+that it does not leak memory when syntax errors are encountered.
+The source file that drives Lemon is found in <b>parse.y</b>.</p>
+
+<p>Because
+lemon is a program not normally found on development machines, the
+complete source code to lemon (just one C file) is included in the
+SQLite distribution in the "tool" subdirectory.  Documentation on
+lemon is found in the "doc" subdirectory of the distribution.
+</p>
+
+<h3>Code Generator</h3>
+
+<p>After the parser assembles tokens into complete SQL statements,
+it calls the code generator to produce virtual machine code that
+will do the work that the SQL statements request.  There are many
+files in the code generator:
+<b>attach.c</b>,
+<b>auth.c</b>,
+<b>build.c</b>,
+<b>delete.c</b>,
+<b>expr.c</b>,
+<b>insert.c</b>,
+<b>pragma.c</b>,
+<b>select.c</b>,
+<b>trigger.c</b>,
+<b>update.c</b>,
+<b>vacuum.c</b>
+and <b>where.c</b>.
+In these files is where most of the serious magic happens.
+<b>expr.c</b> handles code generation for expressions.
+<b>where.c</b> handles code generation for WHERE clauses on
+SELECT, UPDATE and DELETE statements.  The files <b>attach.c</b>,
+<b>delete.c</b>, <b>insert.c</b>, <b>select.c</b>, <b>trigger.c</b>
+<b>update.c</b>, and <b>vacuum.c</b> handle the code generation
+for SQL statements with the same names.  (Each of these files calls routines
+in <b>expr.c</b> and <b>where.c</b> as necessary.)  All other
+SQL statements are coded out of <b>build.c</b>.
+The <b>auth.c</b> file implements the functionality of
+<b>sqlite3_set_authorizer()</b>.</p>
+
+<h3>Virtual Machine</h3>
+
+<p>The program generated by the code generator is executed by
+the virtual machine.  Additional information about the virtual
+machine is <a href="opcode.html">available separately</a>.
+To summarize, the virtual machine implements an abstract computing
+engine specifically designed to manipulate database files.  The
+machine has a stack which is used for intermediate storage.
+Each instruction contains an opcode and
+up to three additional operands.</p>
+
+<p>The virtual machine itself is entirely contained in a single
+source file <b>vdbe.c</b>.  The virtual machine also has
+its own header files: <b>vdbe.h</b> that defines an interface
+between the virtual machine and the rest of the SQLite library and
+<b>vdbeInt.h</b> which defines structure private the virtual machine.
+The <b>vdbeaux.c</b> file contains utilities used by the virtual
+machine and interface modules used by the rest of the library to
+construct VM programs.  The <b>vdbeapi.c</b> file contains external
+interfaces to the virtual machine such as the 
+<b>sqlite3_bind_...</b> family of functions.  Individual values
+(strings, integer, floating point numbers, and BLOBs) are stored
+in an internal object named "Mem" which is implemented by
+<b>vdbemem.c</b>.</p>
+
+<p>
+SQLite implements SQL functions using callbacks to C-language routines.
+Even the built-in SQL functions are implemented this way.  Most of
+the built-in SQL functions (ex: <b>coalesce()</b>, <b>count()</b>,
+<b>substr()</b>, and so forth) can be found in <b>func.c</b>.
+Date and time conversion functions are found in <b>date.c</b>.
+</p>
+
+<h3>B-Tree</h3>
+
+<p>An SQLite database is maintained on disk using a B-tree implementation
+found in the <b>btree.c</b> source file.  A separate B-tree is used for
+each table and index in the database.  All B-trees are stored in the
+same disk file.  Details of the file format are recorded in a large
+comment at the beginning of <b>btree.c</b>.</p>
+
+<p>The interface to the B-tree subsystem is defined by the header file
+<b>btree.h</b>.
+</p>
+
+<h3>Page Cache</h3>
+
+<p>The B-tree module requests information from the disk in fixed-size
+chunks.  The default chunk size is 1024 bytes but can vary between 512
+and 65536 bytes.
+The page cache is responsible for reading, writing, and
+caching these chunks.
+The page cache also provides the rollback and atomic commit abstraction
+and takes care of locking of the database file.  The
+B-tree driver requests particular pages from the page cache and notifies
+the page cache when it wants to modify pages or commit or rollback
+changes and the page cache handles all the messy details of making sure
+the requests are handled quickly, safely, and efficiently.</p>
+
+<p>The code to implement the page cache is contained in the single C
+source file <b>pager.c</b>.  The interface to the page cache subsystem
+is defined by the header file <b>pager.h</b>.
+</p>
+
+<h3>OS Interface</h3>
+
+<p>
+In order to provide portability between POSIX and Win32 operating systems,
+SQLite uses an abstraction layer to interface with the operating system.
+The interface to the OS abstraction layer is defined in
+<b>os.h</b>.  Each supported operating system has its own implementation:
+<b>os_unix.c</b> for Unix, <b>os_win.c</b> for windows, and so forth.
+Each of these operating-specific implements typically has its own
+header file: <b>os_unix.h</b>, <b>os_win.h</b>, etc.
+</p>
+
+<h3>Utilities</h3>
+
+<p>
+Memory allocation and caseless string comparison routines are located
+in <b>util.c</b>.
+Symbol tables used by the parser are maintained by hash tables found
+in <b>hash.c</b>.  The <b>utf.c</b> source file contains Unicode
+conversion subroutines.
+SQLite has its own private implementation of <b>printf()</b> (with
+some extensions) in <b>printf.c</b> and its own random number generator
+in <b>random.c</b>.
+</p>
+
+<h3>Test Code</h3>
+
+<p>
+If you count regression test scripts,
+more than half the total code base of SQLite is devoted to testing.
+There are many <b>assert()</b> statements in the main code files.
+In additional, the source files <b>test1.c</b> through <b>test5.c</b>
+together with <b>md5.c</b> implement extensions used for testing
+purposes only.  The <b>os_test.c</b> backend interface is used to
+simulate power failures to verify the crash-recovery mechanism in
+the pager.
+</p>
+
+}
+footer $rcsid