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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>.
-
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-Last modified: Fri Aug 10 11:47:41 EST 2001
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