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<h3 class="section">9.2 Binding and unbinding</h3>

<p>Binding goes like this: the binding stack pointer (bsp) is bumped, old
value and symbol are stored at bsp - 1, new value is stored in symbol&rsquo;s
value slot or the tls. On multithreaded builds, <code>TLS-INDEX</code> is
stored on the binding stack in place of the symbol.
</p>
<p>Unbinding: the symbol&rsquo;s value is restored from bsp - 1, value and
symbol at bsp - 1 are set to zero, and finally bsp is decremented.
</p>
<p>The <code>UNBIND-TO-HERE</code> VOP assists in unwinding the stack. It
iterates over the bindings on the binding stack until it reaches the
prescribed point. For each binding with a non-zero symbol it does an
<code>UNBIND</code>.
</p>
<p>How can a binding&rsquo;s symbol be zero? <code>BIND</code> is not pseudo atomic
(for performance reasons) and it can be interrupted by a signal. If
the signal hits after the bsp is incremented but before the values on
the stack are set the symbol is zero because a thread starts with a
zeroed tls plus <code>UNBIND</code> and <code>UNBIND-TO-HERE</code> both zero the
binding being unbound.
</p>
<p>Zeroing the binding&rsquo;s symbol would not be enough as the binding&rsquo;s
value can be moved or garbage collected and if the above interrupt
initiates gc (or be <code>SIG_STOP_FOR_GC</code>) it will be greeted by a
garbage pointer.
</p>
<p>Furthermore, <code>BIND</code> must always write the value to the binding
stack first and the symbol second because the symbol being non-zero
means validity to <code>UNBIND-TO-HERE</code>. For similar reasons
<code>UNBIND</code> also zeroes the symbol first. But if it is interrupted
by a signal that does an async unwind then <code>UNBIND-TO-HERE</code> can
be triggered when the symbol is zeroed but the value is not. In this
case <code>UNBIND-TO-HERE</code> must zero out the value to avoid leaving
garbage around that may wreck the ship on the next <code>BIND</code>.
</p>
<p>In other words, the invariant is that the binding stack above bsp only
contains zeros. This makes <code>BIND</code> safe in face of gc triggered at
any point during its execution.
</p>
<p>On platforms with the <code>UNWIND-TO-FRAME-AND-CALL-VOP</code> feature, it&rsquo;s
possible to restart frames in the debugger, unwinding the binding stack.
To know how much to unwind, <code>BIND-SENTINEL</code> in the beginning of a
function puts the current frame pointer on the binding stack with
<code>UNBOUND-MARKER-WIDETAG</code> instead of the symbol/tls-index.
<code>UNBIND-SENTINEL</code> removes it before returning. The debugger then
search for <code>UNBOUND-MARKER-WIDETAG</code> with the value being equal to
the desired frame, and calls <code>UNBIND-TO-HERE</code>. Consequently,
<code>UNBIND-TO-HERE</code> treats <code>UNBOUND-MARKER-WIDETAG</code> the same way
as zeros.
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sbcl-doc 2:1.4.5-1 / usr / share / doc / sbcl / sbcl-internals / Binding-and-unbinding.html
