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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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<title>Using SQLite In Multi-Threaded Applications</title>
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<h2>SQLite And Multiple Threads</h2>
<p>SQLite support three different threading modes:</p>
<ol>
<li><p><b>Single-thread</b>.
In this mode, all mutexes are disabled and SQLite is unsafe to use in
more than a single thread at once.</p></li>
<li><p><b>Multi-thread</b>.
In this mode, SQLite can be safely used by multiple threads provided that
no single database connection is used simultaneously in two or more threads.
</p></li>
<li><p><b>Serialized</b>.
In serialized mode, SQLite can be safely used by multiple threads with no
restriction.</p></li>
</ol>
<p>
The threading mode can be selected at compile-time (when the SQLite
library is being compiled from source code) or at start-time (when the
application that intends to use SQLite is initializing) or at
run-time (when a new SQLite database connection is being created).
Generally speaking, run-time overrides start-time and start-time
overrides compile-time. Except, single-thread mode cannot be
overridden once selected.
</p>
<p>
The default mode is serialized.
</p>
<h3>Compile-time selection of threading mode</h3>
<p>
Use the <a href="compile.html#threadsafe">SQLITE_THREADSAFE</a> compile-time parameter to selected the
threading mode. If no <a href="compile.html#threadsafe">SQLITE_THREADSAFE</a> compile-time parameter is
present, then serialized mode is used.
This can be made explicit with
<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=1</a>.
With
<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=0</a> the threading mode is
single-thread. With
<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=2</a> the threading mode is
multi-thread.
</p>
<p>
The return value of the <a href="c3ref/threadsafe.html">sqlite3_threadsafe()</a> interface is determined
by the compile-time threading mode selection. If single-thread mode
is selected at compile-time, then <a href="c3ref/threadsafe.html">sqlite3_threadsafe()</a> returns false.
If either the multi-thread or serialized modes are selected, then
<a href="c3ref/threadsafe.html">sqlite3_threadsafe()</a> returns true. The <a href="c3ref/threadsafe.html">sqlite3_threadsafe()</a>
interface predates the multi-thread mode and start-time and run-time
mode selection and so is unable to distinguish
between multi-thread and serialized mode nor is it able to report start-time
or run-time mode changes.
</p>
<p>
If single-thread mode is selected at compile-time, then critical
mutexing logic is omitted from the build and it is impossible to
enable either multi-thread or serialized modes at start-time or
run-time.
</p>
<h3>Start-time selection of threading mode</h3>
<p>
Assuming that the compile-time threading mode is not single-thread, then
the threading mode can be changed during initialization using the
<a href="c3ref/config.html">sqlite3_config()</a> interface. The <a href="c3ref/c_config_getmalloc.html#sqliteconfigsinglethread">SQLITE_CONFIG_SINGLETHREAD</a> verb
puts SQLite into single-thread mode, the <a href="c3ref/c_config_getmalloc.html#sqliteconfigmultithread">SQLITE_CONFIG_MULTITHREAD</a>
verb sets multi-thread mode, and the <a href="c3ref/c_config_getmalloc.html#sqliteconfigserialized">SQLITE_CONFIG_SERIALIZED</a> verb
sets serialized mode.
</p>
<h3>Run-time selection of threading mode</h3>
<p>If single-thread mode has not been selected at compile-time or start-time,
then individual database connections can be created as either multi-thread
or serialized. It is not possible to downgrade an individual database
connection to single-thread mode. Nor is it possible to escalate an
individual database connection if the compile-time or start-time mode
is single-thread.</p>
<p>The threading mode for an individual database connection is determined
by flags given as the third argument to <a href="c3ref/open.html">sqlite3_open_v2()</a>. The
<a href="c3ref/c_open_autoproxy.html">SQLITE_OPEN_NOMUTEX</a> flag causes the database connection to be in the
multi-thread mode and the <a href="c3ref/c_open_autoproxy.html">SQLITE_OPEN_FULLMUTEX</a> flag causes the connection
to be in serialized mode. If neither flag is specified or if
<a href="c3ref/open.html">sqlite3_open()</a> or <a href="c3ref/open.html">sqlite3_open16()</a> are used instead of
<a href="c3ref/open.html">sqlite3_open_v2()</a>, then the default
mode determined by the compile-time and start-time settings is used.
</p>
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