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<h3 class="section">4.2 Lazy Alien Resolution</h3>

<p>On linkage-table ports SBCL is able to deal with forward-references to
aliens &ndash; which is to say, compile and load code referring to aliens
before the shared object containing the alien in question has been
loaded.
</p>
<p>This is handled by <code>ENSURE-DYNAMIC-FOREIGN-SYMBOL-ADDRESS</code>, which
first tries to resolve the address in the loaded shared objects, but
failing that records the alien as undefined and returns the address of
a read/write/execute protected guard page for variables, and address
of <code>undefined_alien_function</code> for routines. These are in turn
responsible for catching attempts to access the undefined alien, and
signalling the appropriate error.
</p>
<p>These placeholder addresses get recorded in the linkage-table.
</p>
<p>When new shared objects are loaded <code>UPDATE-LINKAGE-TABLE</code> is
called, which in turn attempts to resolve all currently undefined
aliens, and registers the correct addresses for them in the
linkage-table.
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sbcl-doc 2:1.4.5-1 / usr / share / doc / sbcl / sbcl-internals / Lazy-Alien-Resolution.html
