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<a name="The-deferral-mechanism"></a>
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<hr>
<a name="The-deferral-mechanism-1"></a>
<h3 class="section">7.2 The deferral mechanism</h3>

<a name="Pseudo-atomic-sections"></a>
<h4 class="subsection">7.2.1 Pseudo atomic sections</h4>

<p>Some operations, such as allocation, consist of several steps and
temporarily break for instance gc invariants. Interrupting said
operations is therefore dangerous to one&rsquo;s health. Blocking the
signals for each allocation is out of question as the overhead of the
two <code>sigsetmask</code> system calls would be enormous. Instead, pseudo
atomic sections are implemented with a simple flag.
</p>
<p>When a deferrable signal is delivered to a thread within a pseudo
atomic section the pseudo-atomic-interrupted flag is set, the signal
and its context are stored, and all deferrable signals blocked. This
is to guarantee that there is at most one pending handler in
SBCL. While the signals are blocked, the responsibility of keeping
track of other pending signals lies with the OS.
</p>
<p>On leaving the pseudo atomic section, the pending handler is run and
the signals are unblocked.
</p>
<a name="WITHOUT_002dINTERRUPTS"></a>
<h4 class="subsection">7.2.2 <code>WITHOUT-INTERRUPTS</code></h4>

<p>Similar to pseudo atomic, <code>WITHOUT-INTERRUPTS</code> defers deferrable
signals in its thread until the end of its body, provided it is not
nested in another <code>WITHOUT-INTERRUPTS</code>.
</p>
<p>Not so frequently used as pseudo atomic, <code>WITHOUT-INTERRUPTS</code>
benefits less from the deferral mechanism.
</p>
<a name="Stop-the-world"></a>
<h4 class="subsection">7.2.3 Stop the world</h4>

<p>Something of a special case, a signal that is blockable but not
deferrable by <code>WITHOUT-INTERRUPTS</code> is <code>SIG_STOP_FOR_GC</code>. It
is deferred by pseudo atomic and <code>WITHOUT-GCING</code>.
</p>
<a name="When-are-signals-handled_003f"></a>
<h4 class="subsection">7.2.4 When are signals handled?</h4>

<p>At once or as soon as the mechanism that deferred them allows.
</p>
<p>First, if something is deferred by pseudo atomic then it is run at the
end of pseudo atomic without exceptions. Even when both a GC request
or a <code>SIG_STOP_FOR_GC</code> and a deferrable signal such as
SIG_INTERRUPT_THREAD interrupts the pseudo atomic section.
</p>
<p>Second, an interrupt deferred by WITHOUT-INTERRUPTS is run when the
interrupts are enabled again. GC cannot interfere.
</p>
<p>Third, if GC or <code>SIG_STOP_FOR_GC</code> is deferred by
<code>WITHOUT-GCING</code> then the GC or stopping for GC will happen when
GC is not inhibited anymore. Interrupts cannot delay a gc.
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sbcl-doc 2:1.4.5-1 / usr / share / doc / sbcl / sbcl-internals / The-deferral-mechanism.html
