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<title>Using SQLite In Multi-Threaded Applications</title>
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Using SQLite In Multi-Threaded Applications
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<h1 id="overview"><span>1. </span>Overview</h1>
<p>SQLite supports 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>
<h1 id="compile_time_selection_of_threading_mode"><span>2. </span>Compile-time selection of threading mode</h1>
<p>
Use the <a href="compile.html#threadsafe">SQLITE_THREADSAFE</a> compile-time parameter to select 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 the value
of SQLITE_THREADSAFE set at compile-time. It does not reflect changes
to the threading mode made at runtime via the <a href="c3ref/config.html">sqlite3_config()</a>
interface or by flags given as the third argument to <a href="c3ref/open.html">sqlite3_open_v2()</a>.
</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>
<h1 id="start_time_selection_of_threading_mode"><span>3. </span>Start-time selection of threading mode</h1>
<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_covering_index_scan.html#sqliteconfigsinglethread">SQLITE_CONFIG_SINGLETHREAD</a> verb
puts SQLite into single-thread mode, the <a href="c3ref/c_config_covering_index_scan.html#sqliteconfigmultithread">SQLITE_CONFIG_MULTITHREAD</a>
verb sets multi-thread mode, and the <a href="c3ref/c_config_covering_index_scan.html#sqliteconfigserialized">SQLITE_CONFIG_SERIALIZED</a> verb
sets serialized mode.
</p>
<h1 id="run_time_selection_of_threading_mode"><span>4. </span>Run-time selection of threading mode</h1>
<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>
<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/threadsafe.in?m=fc623895ed39582cf">2022-01-20 21:38:08</a> UTC </small></i></p>
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