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<title>Start a read transaction on an historical snapshot</title>
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<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
<h2>Start a read transaction on an historical snapshot</h2>
</div>
<blockquote><pre>
int sqlite3_snapshot_open(
sqlite3 *db,
const char *zSchema,
sqlite3_snapshot *pSnapshot
);
</pre></blockquote>
<p>
The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> interface either starts a new read
transaction or upgrades an existing one for schema S of
<a href="../c3ref/sqlite3.html">database connection</a> D such that the read transaction refers to
historical <a href="../c3ref/snapshot.html">snapshot</a> P, rather than the most recent change to the
database. The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open()</a> interface returns SQLITE_OK
on success or an appropriate <a href="../rescode.html">error code</a> if it fails.</p>
<p>In order to succeed, the database connection must not be in
<a href="../c3ref/get_autocommit.html">autocommit mode</a> when <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> is called. If there
is already a read transaction open on schema S, then the database handle
must have no active statements (SELECT statements that have been passed
to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()).
SQLITE_ERROR is returned if either of these conditions is violated, or
if schema S does not exist, or if the snapshot object is invalid.</p>
<p>A call to sqlite3_snapshot_open() will fail to open if the specified
snapshot has been overwritten by a <a href="../wal.html#ckpt">checkpoint</a>. In this case
SQLITE_ERROR_SNAPSHOT is returned.</p>
<p>If there is already a read transaction open when this function is
invoked, then the same read transaction remains open (on the same
database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_ERROR_SNAPSHOT
is returned. If another error code - for example SQLITE_PROTOCOL or an
SQLITE_IOERR error code - is returned, then the final state of the
read transaction is undefined. If SQLITE_OK is returned, then the
read transaction is now open on database snapshot P.</p>
<p>A call to <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> will fail if the
database connection D does not know that the database file for
schema S is in <a href="../wal.html">WAL mode</a>. A database connection might not know
that the database file is in <a href="../wal.html">WAL mode</a> if there has been no prior
I/O on that database connection, or if the database entered <a href="../wal.html">WAL mode</a>
after the most recent I/O on the database connection.
(Hint: Run "<a href="../pragma.html#pragma_application_id">PRAGMA application_id</a>" against a newly opened
database connection in order to make it ready to use snapshots.)</p>
<p>The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open()</a> interface is only available when the
<a href="../compile.html#enable_snapshot">SQLITE_ENABLE_SNAPSHOT</a> compile-time option is used.
</p><p>See also lists of
<a href="../c3ref/objlist.html">Objects</a>,
<a href="../c3ref/constlist.html">Constants</a>, and
<a href="../c3ref/funclist.html">Functions</a>.</p>
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