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<a name="g_t_005f_005fenv_005flock_002c-_005f_005fenv_005funlock_002d_002d_002dlock-environ-variable"></a>
<h3 class="section">2.16 <code>__env_lock</code>, <code>__env_unlock</code>&mdash;lock environ variable</h3>
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<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;envlock.h&gt;
void __env_lock (struct _reent *<var>reent</var>);
void __env_unlock (struct _reent *<var>reent</var>);

</pre></div>
<p><strong>Description</strong><br>
The <code>setenv</code> family of routines call these functions when they need to
modify the environ variable.  The version of these routines supplied in the
library use the lock API defined in sys/lock.h.  If multiple threads of
execution can call <code>setenv</code>, or if <code>setenv</code> can be called reentrantly,
then you need to define your own versions of these functions in order to
safely lock the memory pool during a call.  If you do not, the memory pool
may become corrupted.
</p>
<p>A call to <code>setenv</code> may call <code>__env_lock</code> recursively; that is,
the sequence of calls may go <code>__env_lock</code>, <code>__env_lock</code>,
<code>__env_unlock</code>, <code>__env_unlock</code>.  Any implementation of these
routines must be careful to avoid causing a thread to wait for a lock
that it already holds.
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