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<h1 align=center>An Introduction To The SQLite C/C++ Interface</h1>
<p>
This article provides an overview to the C/C++ interface
to SQLite.
</p>
<p>
Early versions of SQLite were very easy to learn since they only
supported 5 C/C++ interfaces. But as SQLite has grown in capability,
new C/C++ interfaces have been added so that now there
are over 200 distinct APIs. This can be overwhelming to a new programmer.
Fortunately, most of the C/C++ interfaces in SQLite are very specialized
and never need to be considered. Despite having so many
entry points, the core API is still relatively simple and easy to code to.
This article aims to provide all of the background information needed to
easily understand how SQLite works.
</p>
<p>
A separate document, <a href="c3ref/intro.html">The SQLite C/C++ Interface</a>,
provides detailed
specifications for all of the various C/C++ APIs for SQLite. Once
the reader
understands the basic principles of operation for SQLite,
<a href="c3ref/intro.html">that document</a> should be used as a reference
guide. This article is intended as introduction only and is neither a
complete nor authoritative reference for the SQLite API.
</p>
<h2>1.0 Core Objects And Interfaces</h2>
<p>
The principal task of an SQL database engine is to evaluate statements
of SQL. In order to accomplish this purpose, the developer needs
to know about two objects:
</p>
<p><ul>
<li> The <a href="c3ref/sqlite3.html">database connection</a> object: sqlite3 </li>
<li> The <a href="c3ref/stmt.html">prepared statement</a> object: sqlite3_stmt </li>
</ul></p>
<p>
Strictly speaking, the <a href="c3ref/stmt.html">prepared statement</a> object is not required since
the convenience wrapper interfaces, <a href="c3ref/exec.html">sqlite3_exec</a> or
<a href="c3ref/free_table.html">sqlite3_get_table</a>, can be used and these convenience wrappers
encapsulate and hide the <a href="c3ref/stmt.html">prepared statement</a> object.
Nevertheless, an understanding of
<a href="c3ref/stmt.html">prepared statements</a> is needed to make full use of SQLite.
</p>
<p>
The <a href="c3ref/sqlite3.html">database connection</a> and <a href="c3ref/stmt.html">prepared statement</a> objects are controlled
by a small set of C/C++ interface routine listed below.
</p>
<p><ul>
<li> <a href="c3ref/open.html">sqlite3_open()</a> </li>
<li> <a href="c3ref/prepare.html">sqlite3_prepare()</a> </li>
<li> <a href="c3ref/step.html">sqlite3_step()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column()</a> </li>
<li> <a href="c3ref/finalize.html">sqlite3_finalize()</a> </li>
<li> <a href="c3ref/close.html">sqlite3_close()</a> </li>
</ul></p>
<p>
The six C/C++ interface routines and two objects listed above form the core
functionality of SQLite. The developer who understands them
will have a good foundation for using SQLite.
</p>
<p>
Note that the list of routines above is conceptual rather than actual.
Many of these routines come in multiple versions.
For example, the list above shows a single routine
named <a href="c3ref/open.html">sqlite3_open()</a> when in fact there are three separate routines
that accomplish the same thing in slightly different ways:
<a href="c3ref/open.html">sqlite3_open()</a>, <a href="c3ref/open.html">sqlite3_open16()</a> and <a href="c3ref/open.html">sqlite3_open_v2()</a>.
The list mentions <a href="c3ref/column_blob.html">sqlite3_column()</a>
when in fact no such routine exists.
The "sqlite3_column()" shown in the list is place holders for
an entire family of routines to be used for extracting column
data in various datatypes.
</p>
<p>
Here is a summary of what the core interfaces do:
</p>
<table border="0" cellspacing="15">
<tr><td valign="top" align="right"><a href="c3ref/open.html">sqlite3_open()</a></td>
<td valign="top">
This routine
opens a connection to an SQLite database file and returns a
<a href="c3ref/sqlite3.html">database connection</a> object. This is often the first SQLite API
call that an application makes and is a prerequisite for most other
SQLite APIs. Many SQLite interfaces require a pointer to
the <a href="c3ref/sqlite3.html">database connection</a> object as their first parameter and can
be thought of as methods on the <a href="c3ref/sqlite3.html">database connection</a> object.
This routine is the constructor for the <a href="c3ref/sqlite3.html">database connection</a> object.
</td>
<tr><td valign="top" align="right"><a href="c3ref/prepare.html">sqlite3_prepare()</a></td>
<td valign="top">
This routine
converts SQL text into a <a href="c3ref/stmt.html">prepared statement</a> object and returns a pointer
to that object. This interface requires a <a href="c3ref/sqlite3.html">database connection</a> pointer
created by a prior call to <a href="c3ref/open.html">sqlite3_open()</a> and a text string containing
the SQL statement to be prepared. This API does not actually evaluate
the SQL statement. It merely prepares the SQL statement for evaluation.
<p>Think of each SQL statement as a small computer program. The purpose
of <a href="c3ref/prepare.html">sqlite3_prepare()</a> is to compile that program into object code.
The <a href="c3ref/stmt.html">prepared statement</a> is the object code. The <a href="c3ref/step.html">sqlite3_step()</a> interface
then runs the object code to get a result.
<p>Note that the use of <a href="c3ref/prepare.html">sqlite3_prepare()</a> is not recommended for new
applications. A newer alternative routine <a href="c3ref/prepare.html">sqlite3_prepare_v2()</a> should
be used instead.</p>
</td>
<tr><td valign="top" align="right"><a href="c3ref/step.html">sqlite3_step()</a></td>
<td valign="top">
This routine is used to evaluate a <a href="c3ref/stmt.html">prepared statement</a> that has been
previously created by the <a href="c3ref/prepare.html">sqlite3_prepare()</a> interface. The statement
is evaluated up to the point where the first row of results are available.
To advance to the second row of results, invoke <a href="c3ref/step.html">sqlite3_step()</a> again.
Continue invoking <a href="c3ref/step.html">sqlite3_step()</a> until the statement is complete.
Statements that do not return results (ex: INSERT, UPDATE, or DELETE
statements) run to completion on a single call to <a href="c3ref/step.html">sqlite3_step()</a>.
</td>
<tr><td valign="top" align="right"><a href="c3ref/column_blob.html">sqlite3_column()</a></td>
<td valign="top">
This routine returns a single column from the current row of a result
set for a <a href="c3ref/stmt.html">prepared statement</a> that is being evaluated by <a href="c3ref/step.html">sqlite3_step()</a>.
Each time <a href="c3ref/step.html">sqlite3_step()</a> stops with a new result set row, this routine
can be called multiple times to find the values of all columns in that row.
As noted above, there really is no such thing as a "sqlite3_column()"
function in the SQLite API. Instead, what we here call "sqlite3_column()"
is a place-holder for an entire family of functions that return
a value from the result set in various data types. There are also routines
in this family that return the size of the result (if it is a string or
BLOB) and the number of columns in the result set.
<p><ul>
<li> <a href="c3ref/column_blob.html">sqlite3_column_blob()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_bytes()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_bytes16()</a> </li>
<li> <a href="c3ref/column_count.html">sqlite3_column_count()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_double()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_int()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_int64()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_text()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_text16()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_type()</a> </li>
<li> <a href="c3ref/column_blob.html">sqlite3_column_value()</a> </li>
</ul></p>
</td>
<tr><td valign="top" align="right"><a href="c3ref/finalize.html">sqlite3_finalize()</a></td>
<td valign="top">
This routine destroys a <a href="c3ref/stmt.html">prepared statement</a> created by a prior call
to <a href="c3ref/prepare.html">sqlite3_prepare()</a>. Every prepared statement must be destroyed using
a call to this routine in order to avoid memory leaks.
</td>
<tr><td valign="top" align="right"><a href="c3ref/close.html">sqlite3_close()</a></td>
<td valign="top">
This routine closes a <a href="c3ref/sqlite3.html">database connection</a> previously opened by a call
to <a href="c3ref/open.html">sqlite3_open()</a>. All <a href="c3ref/stmt.html">prepared statements</a> associated with the
connection should be <a href="c3ref/finalize.html">finalized</a> prior to closing the
connection.
</td>
</table>
<h3>1.1 Typical Usage Of Core Routines And Objects</h3>
<p>
An application that wants to use SQLite will typically use
<a href="c3ref/open.html">sqlite3_open()</a> to create a single <a href="c3ref/sqlite3.html">database connection</a>
during initialization.
Note that <a href="c3ref/open.html">sqlite3_open()</a> can be used to either open existing database
files or to create and open new database files.
While many applications use only a single <a href="c3ref/sqlite3.html">database connection</a>, there is
no reason why an application cannot call <a href="c3ref/open.html">sqlite3_open()</a> multiple times
in order to open multiple <a href="c3ref/sqlite3.html">database connections</a> - either to the same
database or to different databases. Sometimes a multi-threaded application
will create separate <a href="c3ref/sqlite3.html">database connections</a> for each threads.
Note too that is not necessary to open separate database connections in
order to access two or more databases. A single <a href="c3ref/sqlite3.html">database connection</a>
can be made to access two or more databases at one time using the
<a href="lang_attach.html">ATTACH</a> SQL command.
</p>
<p>
Many applications destroy their <a href="c3ref/sqlite3.html">database connections</a> using calls to
<a href="c3ref/close.html">sqlite3_close()</a> at shutdown. Or, for example, an application that
uses SQLite as its <a href="appfileformat.html">application file format</a> might
open <a href="c3ref/sqlite3.html">database connections</a> in response to a File/Open menu action
and then destroy the corresponding <a href="c3ref/sqlite3.html">database connection</a> in response
to the File/Close menu.
</p>
<p>
To run an SQL statement, the application follows these steps:
</p>
<p><ol>
<li> Create a <a href="c3ref/stmt.html">prepared statement</a> using <a href="c3ref/prepare.html">sqlite3_prepare()</a>. </li>
<li> Evaluate the <a href="c3ref/stmt.html">prepared statement</a> by calling <a href="c3ref/step.html">sqlite3_step()</a> one
or more times. </li>
<li> For queries, extract results by calling
<a href="c3ref/column_blob.html">sqlite3_column()</a> in between
two calls to <a href="c3ref/step.html">sqlite3_step()</a>. </li>
<li> Destroy the <a href="c3ref/stmt.html">prepared statement</a> using <a href="c3ref/finalize.html">sqlite3_finalize()</a>. </li>
</ol></p>
<p>
The foregoing is all one really needs to know in order to use SQLite
effectively. All the rest is just ornamentation and detail.
</p>
<h2>2.0 Convenience Wrappers Around Core Routines</h2>
<p>
The <a href="c3ref/exec.html">sqlite3_exec()</a> interface is a convenience wrapper that carries out
all four of the above steps with a single function call. A callback
function passed into <a href="c3ref/exec.html">sqlite3_exec()</a> is used to process each row of
the result set. The <a href="c3ref/free_table.html">sqlite3_get_table()</a> is another convenience wrapper
that does all four of the above steps. The <a href="c3ref/free_table.html">sqlite3_get_table()</a> interface
differs from <a href="c3ref/exec.html">sqlite3_exec()</a> in that it stores the results of queries
in heap memory rather than invoking a callback.
</p>
<p>
It is important to realize that neither <a href="c3ref/exec.html">sqlite3_exec()</a> nor
<a href="c3ref/free_table.html">sqlite3_get_table()</a> do anything that cannot be accomplished using
the core routines. In fact, these wrappers are implemented purely in
terms of the core routines.
</p>
<h2>3.0 Binding Parameters and Reusing Prepared Statements</h2>
<p>
In prior discussion, it was assumed that each SQL statement is prepared
once, evaluated, then destroyed. However, the SQLite allows the same
<a href="c3ref/stmt.html">prepared statement</a> to be evaluated multiple times. This is accomplished
using the following routines:
</p>
<p><ul>
<li> <a href="c3ref/reset.html">sqlite3_reset()</a> </li>
<li> <a href="c3ref/bind_blob.html">sqlite3_bind()</a> </li>
</ul></p>
<p>
After a <a href="c3ref/stmt.html">prepared statement</a> has been evaluated by one or more calls to
<a href="c3ref/step.html">sqlite3_step()</a>, it can be reset in order to be evaluated again by a
call to <a href="c3ref/reset.html">sqlite3_reset()</a>.
Using <a href="c3ref/reset.html">sqlite3_reset()</a> on an existing <a href="c3ref/stmt.html">prepared statement</a> rather than
creating a new <a href="c3ref/stmt.html">prepared statement</a> avoids unnecessary calls to
<a href="c3ref/prepare.html">sqlite3_prepare()</a>.
In many SQL statements, the time needed
to run <a href="c3ref/prepare.html">sqlite3_prepare()</a> equals or exceeds the time needed by
<a href="c3ref/step.html">sqlite3_step()</a>. So avoiding calls to <a href="c3ref/prepare.html">sqlite3_prepare()</a> can result
in a significant performance improvement.
</p>
<p>
Usually, though, it is not useful to evaluate exactly the same SQL
statement more than once. More often, one wants to evaluate similar
statements. For example, you might want to evaluate an INSERT statement
multiple times though with different values to insert. To accommodate
this kind of flexibility, SQLite allows SQL statements to contain
<a href="lang_expr.html#varparam">parameters</a>
which are "bound" to values prior to being evaluated. These values can
later be changed and the same <a href="c3ref/stmt.html">prepared statement</a> can be evaluated
a second time using the new values.
</p>
<p>
In SQLite, wherever it is valid to include a string literal, one can use
a <a href="lang_expr.html#varparam">parameter</a> in one of the following forms:
</p>
<p><ul>
<li> <b>?</b> </li>
<li> <b>?</b><i>NNN</i> </li>
<li> <b>:</b><i>AAA</i> </li>
<li> <b>$</b><i>AAA</i> </li>
<li> <b>@</b><i>AAA</i> </li>
</ul></p>
<p>
In the examples above, <i>NNN</i> is an integer value and
<i>AAA</i> is an identifier.
A parameter initially has a value of NULL.
Prior to calling <a href="c3ref/step.html">sqlite3_step()</a> for the first time or immediately
after <a href="c3ref/reset.html">sqlite3_reset()</a>, the application can invoke one of the
<a href="c3ref/bind_blob.html">sqlite3_bind()</a> interfaces to attach values
to the parameters. Each call to <a href="c3ref/bind_blob.html">sqlite3_bind()</a>
overrides prior bindings on the same parameter.
</p>
<p>
An application is allowed to prepare multiple SQL statements in advance
and evaluate them as needed.
There is no arbitrary limit to the number of outstanding
<a href="c3ref/stmt.html">prepared statements</a>.
</p>
<h2>4.0 Configuring SQLite</h2>
<p>
The default configuration for SQLite works great for most applications.
But sometimes developers want to tweak the setup to try to squeeze out
a little more performance, or take advantage of some obscure feature.
<p>
The <a href="c3ref/config.html">sqlite3_config()</a> interface is used to make global, process-wide
configuration changes for SQLite. The <a href="c3ref/config.html">sqlite3_config()</a> interface must
be called before any <a href="c3ref/sqlite3.html">database connections</a> are created. The
<a href="c3ref/config.html">sqlite3_config()</a> interface allows the programmer to do things like:
<ul>
<li>Adjust how SQLite does <a href="malloc.html">memory allocation</a>, including setting up
alternative memory allocators appropriate for safety-critical
real-time embedded systems and application-defined memory allocators.
<li>Set up a process-wide <a href="errlog.html">error log</a>.
<li>Specify an application-defined page cache.
<li>Adjust the use of mutexes so that they are appropriate for various
<a href="threadsafe.html">threading models</a>, or substitute an
application-defined mutex system.
</ul>
<p>
After process-wide configuration is complete and <a href="c3ref/sqlite3.html">database connections</a>
have been created, individual database connections can be configured using
calls to <a href="c3ref/limit.html">sqlite3_limit()</a> and <a href="c3ref/db_config.html">sqlite3_db_config()</a>.
<h2>5.0 Extending SQLite</h2>
<p>
SQLite includes interfaces that can be used to extend its functionality.
Such routines include:
</p>
<p><ul>
<li> <a href="c3ref/create_collation.html">sqlite3_create_collation()</a> </li>
<li> <a href="c3ref/create_function.html">sqlite3_create_function()</a> </li>
<li> <a href="c3ref/create_module.html">sqlite3_create_module()</a> </li>
<li> <a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a> </li>
</ul></p>
<p>
The <a href="c3ref/create_collation.html">sqlite3_create_collation()</a> interface is used to create new
<a href="datatype3.html#collation">collating sequences</a> for sorting text.
The <a href="c3ref/create_module.html">sqlite3_create_module()</a> interface is used to register new
<a href="vtab.html">virtual table</a> implementations.
The <a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a> interface creates new <a href="vfs.html">VFSes</a>.
</p>
<p>
The <a href="c3ref/create_function.html">sqlite3_create_function()</a> interface creates new SQL functions -
either scalar or aggregate. The new function implementation typically
makes use of the following additional interfaces:
</p>
<p><ul>
<li> <a href="c3ref/aggregate_context.html">sqlite3_aggregate_context()</a> </li>
<li> <a href="c3ref/result_blob.html">sqlite3_result()</a> </li>
<li> <a href="c3ref/user_data.html">sqlite3_user_data()</a> </li>
<li> <a href="c3ref/value_blob.html">sqlite3_value()</a> </li>
</ul></p>
<p>
All of the built-in SQL functions of SQLite are created using exactly
these same interfaces. Refer to the SQLite source code, and in particular
the
<a href="http://www.sqlite.org/src/doc/trunk/src/date.c">date.c</a> and
<a href="http://www.sqlite.org/src/doc/trunk/src/func.c">func.c</a> source files
for examples.
</p>
<p>
Shared libraries or DLLs can be used as <a href="loadext.html">loadable extensions</a> to SQLite.
<h2>6.0 Other Interfaces</h2>
<p>
This article only mentions the foundational SQLite interfaces.
The SQLite library includes many other APIs implementing useful
features that are not described here.
A <a href="c3ref/funclist.html">complete list of functions</a> that form the SQLite
application programming interface is found at the
<a href="c3ref/intro.html">C/C++ Interface Specification</a>.
Refer to that document for complete and authoritative information about
all SQLite interfaces.
</p>
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