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<title>The GHC Commentary - Alien Functions</title>
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<h1>The GHC Commentary - Alien Functions</h1>
<p>
GHC implements experimental (by now it is actually quite well tested)
support for access to foreign functions and generally the interaction
between Haskell code and code written in other languages. Code
generation in this context can get quite tricky. This section attempts
to cast some light on this aspect of the compiler.
<h4>FFI Stub Files</h4>
<p>
For each Haskell module that contains a <code>foreign export
dynamic</code> declaration, GHC generates a <code>_stub.c</code> file
that needs to be linked with any program that imports the Haskell
module. When asked about it <a
href="mailto:simonmar@microsoft.com">Simon Marlow</a> justified the
existence of these files as follows:
<blockquote>
The stub files contain the helper function which invokes the Haskell
code when called from C.
<p>
Each time the foreign export dynamic is invoked to create a new
callback function, a small piece of code has to be dynamically
generated (by code in <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/rts/Adjustor.c"><code>Adjustor.c</code></a>). It is the address of this dynamically generated bit of
code that is returned as the <code>Addr</code> (or <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/hslibs/lang/Ptr.lhs"><code>Ptr</code></a>).
When called from C, the dynamically generated code must somehow invoke
the Haskell function which was originally passed to the
f.e.d. function -- it does this by invoking the helper function,
passing it a <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/hslibs/lang/StablePtr.lhs"><code>StablePtr</code></a>
to the Haskell function. It's split this way for two reasons: the
same helper function can be used each time the f.e.d. function is
called, and to keep the amount of dynamically generated code to a
minimum.
</blockquote>
<p>
The stub code is generated by <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/deSugar/DsForeign.lhs"><code>DSForeign</code></a><code>.fexportEntry</code>.
<p><small>
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Last modified: Fri Aug 10 11:47:41 EST 2001
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