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<title>Berkeley DB: DB_ENV->txn_begin</title>
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<h1>DB_ENV->txn_begin</h1>
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<h3><pre>
#include <db.h>
<p>
int
DB_ENV->txn_begin(DB_ENV *env,
DB_TXN *parent, DB_TXN **tid, u_int32_t flags);
</pre></h3>
<h1>Description</h1>
<p>The DB_ENV->txn_begin method creates a new transaction in the environment
and copies a pointer to a <a href="../api_c/txn_class.html">DB_TXN</a> that uniquely identifies it into
the memory to which <b>tid</b> refers.
Calling the <a href="../api_c/txn_abort.html">DB_TXN->abort</a>,
<a href="../api_c/txn_commit.html">DB_TXN->commit</a> or <a href="../api_c/txn_discard.html">DB_TXN->discard</a> methods will discard the returned
handle.
<p>If the <b>parent</b> argument is non-NULL, the new transaction will
be a nested transaction, with the transaction indicated by
<b>parent</b> as its parent. Transactions may be
nested to any level.
In the presence of distributed transactions and two-phase commit,
only the parental transaction, that is a transaction without
a <b>parent</b> specified, should be passed as an argument to
<a href="../api_c/txn_prepare.html">DB_TXN->prepare</a>.
<p>The <b>flags</b> value must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one or
more of the following values:
<p><dl compact>
<p><dt><a name="DB_DIRTY_READ">DB_DIRTY_READ</a><dd>All read operations performed by the transaction will read modified but
not yet committed data. Silently ignored if the <a href="../api_c/db_open.html#DB_DIRTY_READ">DB_DIRTY_READ</a>
flag was not specified when the underlying database was opened.
<p><dt><a name="DB_TXN_NOSYNC">DB_TXN_NOSYNC</a><dd>Do not synchronously flush the log when this transaction commits or
prepares. This means the transaction will exhibit the ACI (atomicity,
consistency, and isolation) properties, but not D (durability); that is,
database integrity will be maintained but it is possible that this
transaction may be undone during recovery.
<p>This behavior may be set for a Berkeley DB environment using the
<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> interface. Any value specified in this
interface overrides that setting.
<p><dt><a name="DB_TXN_NOWAIT">DB_TXN_NOWAIT</a><dd>If a lock is unavailable for any Berkeley DB operation performed in the context
of this transaction,
return DB_LOCK_NOTGRANTED
immediately instead of blocking on the lock.
<p><dt><a name="DB_TXN_SYNC">DB_TXN_SYNC</a><dd>Synchronously flush the log when this transaction commits or prepares.
This means the transaction will exhibit all of the ACID (atomicity,
consistency, isolation, and durability) properties.
<p>This behavior is the default for Berkeley DB environments unless the
<a href="../api_c/env_set_flags.html#DB_TXN_NOSYNC">DB_TXN_NOSYNC</a> flag was specified to the <a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a>
interface. Any value specified in this interface overrides that
setting.
</dl>
<p><b>Note: A transaction may not span threads; that is, each transaction must
begin and end in the same thread, and each transaction may be used only
by a single thread.</b>
<p><b>Note: Cursors may not span transactions; that is, each cursor must be
opened and closed within a single transaction.</b>
<p><b>Note: A parent transaction may not issue any Berkeley DB operations -- except for
DB_ENV->txn_begin, <a href="../api_c/txn_abort.html">DB_TXN->abort</a> and <a href="../api_c/txn_commit.html">DB_TXN->commit</a> -- while it has
active child transactions (child transactions that have not yet been
committed or aborted).</b>
<p>The DB_ENV->txn_begin method returns a non-zero error value on failure and 0 on success.
<h1>Errors</h1>
<p>The DB_ENV->txn_begin method may fail and return a non-zero error for the following conditions:
<p><dl compact>
<p><dt>ENOMEM<dd>The maximum number of concurrent transactions has been reached.
</dl>
<p>The DB_ENV->txn_begin method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the DB_ENV->txn_begin method may fail and
return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
in which case all subsequent Berkeley DB calls will fail in the same way.
<h1>Class</h1>
<a href="../api_c/env_class.html">DB_ENV</a>, <a href="../api_c/txn_class.html">DB_TXN</a>
<h1>See Also</h1>
<a href="../api_c/txn_list.html">Transaction Subsystem and Related Methods</a>
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