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MLton's <a href="ForeignFunctionInterface">ForeignFunctionInterface</a> allows programs to <em>export</em> SML functions to be called from C. Suppose you would like export from SML a function of type <tt>real * char -> int</tt> as the C function <tt>foo</tt>. MLton extends the syntax of SML to allow expressions like the following:
<pre>_export "foo": (real * char -> int) -> unit;
</pre>The above expression exports a C function named <tt>foo</tt>, with prototype
<pre>Int32 foo (Real64 x0, Char x1);
</pre>The <tt>_export</tt> expression denotes a function of type <tt>(real * char -> int) -> unit</tt> that when called with a function <tt>f</tt>, arranges for the exported <tt>foo</tt> function to call <tt>f</tt> when <tt>foo</tt> is called. So, for example, the following exports and defines <tt>foo</tt>.
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> e = _export <B><FONT COLOR="#BC8F8F">"foo"</FONT></B>: (real * char -> int) -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = e (<B><FONT COLOR="#A020F0">fn</FONT></B> (x, c) => <B><FONT COLOR="#5F9EA0">13</FONT></B> + Real.floor x + Char.ord c)
</PRE>
<p>
</p>
<p>
The general form of an <tt>_export</tt> expression is
</p>
<pre>_export "C function name" attr... : cFuncTy -> unit;
</pre><p>
The type and the semicolon are not optional. As with <tt>_import</tt>, a sequence of attributes may follow the function name.
</p>
<p>
MLton's <tt>-export-header</tt> option generates a C header file with prototypes for all of the functions exported from SML. Include this header file in your C files to type check calls to functions exported from SML. This header file includes <tt>typedef</tt>s for the <a href="ForeignFunctionInterfaceTypes"> types that can be passed between SML and C</a>.
</p>
<h2 id="head-0f01ed56a1e32a05e5ef96e4d779f34784af9a96">Example</h2>
<p>
Suppose that <tt>export.sml</tt> is
</p>
<p>
<pre class=code><B><FONT COLOR="#A020F0">val</FONT></B> e = _export <B><FONT COLOR="#BC8F8F">"f"</FONT></B>: (int * real * char -> char) -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = e (<B><FONT COLOR="#A020F0">fn</FONT></B> (i, r, _) =>
(print (concat [<B><FONT COLOR="#BC8F8F">"i = "</FONT></B>, Int.toString i,
<B><FONT COLOR="#BC8F8F">" r = "</FONT></B>, Real.toString r, <B><FONT COLOR="#BC8F8F">"\n"</FONT></B>])
; #<B><FONT COLOR="#BC8F8F">"g"</FONT></B>))
<B><FONT COLOR="#A020F0">val</FONT></B> g = _import <B><FONT COLOR="#BC8F8F">"g"</FONT></B> public: unit -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = g ()
<B><FONT COLOR="#A020F0">val</FONT></B> _ = g ()
<B><FONT COLOR="#A020F0">val</FONT></B> e = _export <B><FONT COLOR="#BC8F8F">"f2"</FONT></B>: (Word8.word -> word array) -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = e (<B><FONT COLOR="#A020F0">fn</FONT></B> w =>
Array.tabulate (<B><FONT COLOR="#5F9EA0">10</FONT></B>, <B><FONT COLOR="#A020F0">fn</FONT></B> _ => Word.fromLargeWord (Word8.toLargeWord w)))
<B><FONT COLOR="#A020F0">val</FONT></B> g2 = _import <B><FONT COLOR="#BC8F8F">"g2"</FONT></B> public: unit -> word array;
<B><FONT COLOR="#A020F0">val</FONT></B> a = g2 ()
<B><FONT COLOR="#A020F0">val</FONT></B> _ = print (concat [<B><FONT COLOR="#BC8F8F">"0wx"</FONT></B>, Word.toString (Array.sub (a, <B><FONT COLOR="#5F9EA0">0</FONT></B>)), <B><FONT COLOR="#BC8F8F">"\n"</FONT></B>])
<B><FONT COLOR="#A020F0">val</FONT></B> e = _export <B><FONT COLOR="#BC8F8F">"f3"</FONT></B>: (unit -> unit) -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = e (<B><FONT COLOR="#A020F0">fn</FONT></B> () => print <B><FONT COLOR="#BC8F8F">"hello\n"</FONT></B>);
<B><FONT COLOR="#A020F0">val</FONT></B> g3 = _import <B><FONT COLOR="#BC8F8F">"g3"</FONT></B> public: unit -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = g3 ()
<I><FONT COLOR="#B22222">(* This example demonstrates mutual recursion between C and SML. *)</FONT></I>
<B><FONT COLOR="#A020F0">val</FONT></B> e = _export <B><FONT COLOR="#BC8F8F">"f4"</FONT></B>: (int -> unit) -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> g4 = _import <B><FONT COLOR="#BC8F8F">"g4"</FONT></B> public: int -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = e (<B><FONT COLOR="#A020F0">fn</FONT></B> i => <B><FONT COLOR="#A020F0">if</FONT></B> i = <B><FONT COLOR="#5F9EA0">0</FONT></B> <B><FONT COLOR="#A020F0">then</FONT></B> () <B><FONT COLOR="#A020F0">else</FONT></B> g4 (i - <B><FONT COLOR="#5F9EA0">1</FONT></B>))
<B><FONT COLOR="#A020F0">val</FONT></B> _ = g4 <B><FONT COLOR="#5F9EA0">13</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> (_, zzzSet) = _symbol <B><FONT COLOR="#BC8F8F">"zzz"</FONT></B> alloc: (unit -> int) * (int -> unit);
<B><FONT COLOR="#A020F0">val</FONT></B> () = zzzSet <B><FONT COLOR="#5F9EA0">42</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> g5 = _import <B><FONT COLOR="#BC8F8F">"g5"</FONT></B> public: unit -> unit;
<B><FONT COLOR="#A020F0">val</FONT></B> _ = g5 ()
<B><FONT COLOR="#A020F0">val</FONT></B> _ = print <B><FONT COLOR="#BC8F8F">"success\n"</FONT></B>
</PRE>
</p>
<p>
Create the header file with <tt>-export-header</tt>.
</p>
<pre>% mlton -default-ann 'allowFFI true' \
-export-header export.h \
-stop tc \
export.sml
</pre><p>
<tt>export.h</tt> now contains the following C prototypes.
</p>
<pre>Int8 f (Int32 x0, Real64 x1, Int8 x2);
Pointer f2 (Word8 x0);
void f3 ();
void f4 (Int32 x0);
extern Int32 zzz;
</pre><p>
Use <tt>export.h</tt> in a C program, <tt>ffi-export.c</tt>, as follows.
</p>
<p>
<pre class=code>#<B><FONT COLOR="#5F9EA0">include</FONT></B> <B><FONT COLOR="#BC8F8F"><stdio.h></FONT></B>
#<B><FONT COLOR="#5F9EA0">include</FONT></B> <B><FONT COLOR="#BC8F8F">"export.h"</FONT></B>
<I><FONT COLOR="#B22222">/* Functions in C are by default PUBLIC symbols */</FONT></I>
<B><FONT COLOR="#228B22">void</FONT></B> <B><FONT COLOR="#0000FF">g</FONT></B> () {
Char8 c;
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g starting\n"</FONT></B>);
c = f (13, 17.15, <B><FONT COLOR="#BC8F8F">'a'</FONT></B>);
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g done char = %c\n"</FONT></B>, c);
}
Pointer <B><FONT COLOR="#0000FF">g2</FONT></B> () {
Pointer res;
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g2 starting\n"</FONT></B>);
res = f2 (0xFF);
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g2 done\n"</FONT></B>);
<B><FONT COLOR="#A020F0">return</FONT></B> res;
}
<B><FONT COLOR="#228B22">void</FONT></B> <B><FONT COLOR="#0000FF">g3</FONT></B> () {
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g3 starting\n"</FONT></B>);
f3 ();
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g3 done\n"</FONT></B>);
}
<B><FONT COLOR="#228B22">void</FONT></B> <B><FONT COLOR="#0000FF">g4</FONT></B> (Int32 i) {
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g4 (%d)\n"</FONT></B>, i);
f4 (i);
}
<B><FONT COLOR="#228B22">void</FONT></B> <B><FONT COLOR="#0000FF">g5</FONT></B> () {
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g5 ()\n"</FONT></B>);
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"zzz = %i\n"</FONT></B>, zzz);
fprintf (stderr, <B><FONT COLOR="#BC8F8F">"g5 done\n"</FONT></B>);
}
</PRE>
</p>
<p>
Compile <tt>ffi-export.c</tt> and <tt>export.sml</tt>.
<pre>% gcc -c ffi-export.c
% mlton -default-ann 'allowFFI true' \
export.sml ffi-export.o
</pre>
</p>
<p>
Finally, run <tt>export</tt>.
<pre>% ./export
g starting
...
g4 (0)
success
</pre>
</p>
<h2 id="head-a479c9c34e878d07b4d67a73a48f432ad7dc53c8">Download</h2>
<ul>
<li>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/*checkout*/mlton/tags/on-MLTONWIKIVERSION-release/doc/examples/ffi/export.sml"><img src="moin-www.png" alt="[WWW]" height="11" width="11">export.sml</a>
</p>
</li>
<li>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/*checkout*/mlton/tags/on-MLTONWIKIVERSION-release/doc/examples/ffi/ffi-export.c"><img src="moin-www.png" alt="[WWW]" height="11" width="11">ffi-export.c</a>
</p>
</li>
</ul>
</div>
<p>
<hr>
Last edited on 2006-11-02 17:34:21 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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