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<a class = mltona href="Home">MLton MLTONWIKIVERSION</a>
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CallingFromSMLToCFunctionPointer
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Just as MLton can <a href="CallingFromSMLToC">directly call C functions</a>, it is possible to make indirect function calls; that is, function calls through a function pointer. MLton extends the syntax of SML to allow expressions like the following:
<pre>_import * : MLton.Pointer.t -> real * char -> int;
</pre><p>
This expression denotes a function of type
<pre>MLton.Pointer.t -> real * char -> int</pre>whose behavior is implemented by calling the C function at the address denoted by the <tt>MLton.Pointer.t</tt> argument, and supplying the C function two arguments, a <tt>double</tt> and an <tt>int</tt>. The C function pointer may be obtained, for example, by the dynamic linking loader (<tt>dlopen</tt>, <tt>dlsym</tt>, ...).
</p>
<p>
The general form of an indirect <tt>_import</tt> expression is:
</p>
<pre>_import * attr... : cPtrTy -> cFuncTy;
</pre><p>
The type and the semicolon are not optional.
</p>
<h2 id="head-0f01ed56a1e32a05e5ef96e4d779f34784af9a96">Example</h2>
<p>
This example uses <tt>dlopen</tt> and friends (imported using normal <tt>_import</tt>) to dynamically load the math library (<tt>libm</tt>) and call the <tt>cos</tt> function. Suppose <tt>iimport.sml</tt> contains the following.
</p>
<p>
<pre class=code><B><FONT COLOR="#0000FF">signature</FONT></B> DYN_LINK =
<B><FONT COLOR="#0000FF">sig</FONT></B>
<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> hndl
</FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> mode
</FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> fptr
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> dlopen : string * mode -> hndl
<B><FONT COLOR="#A020F0">val</FONT></B> dlsym : hndl * string -> fptr
<B><FONT COLOR="#A020F0">val</FONT></B> dlclose : hndl -> unit
<B><FONT COLOR="#A020F0">val</FONT></B> RTLD_LAZY : mode
<B><FONT COLOR="#A020F0">val</FONT></B> RTLD_NOW : mode
<B><FONT COLOR="#0000FF">end</FONT></B>
<B><FONT COLOR="#0000FF">structure</FONT></B> DynLink :> DYN_LINK =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> hndl </FONT></B>=<B><FONT COLOR="#228B22"> MLton.Pointer.t
</FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> mode </FONT></B>=<B><FONT COLOR="#228B22"> Word32.word
</FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> fptr </FONT></B>=<B><FONT COLOR="#228B22"> MLton.Pointer.t
<I><FONT COLOR="#B22222">(* These symbols come from a system libray, so the default import scope
* of external is correct.
*)</FONT></I>
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> dlopen =
_import <B><FONT COLOR="#BC8F8F">"dlopen"</FONT></B> : string * mode -> hndl;
<B><FONT COLOR="#A020F0">val</FONT></B> dlerror =
_import <B><FONT COLOR="#BC8F8F">"dlerror"</FONT></B>: unit -> MLton.Pointer.t;
<B><FONT COLOR="#A020F0">val</FONT></B> dlsym =
_import <B><FONT COLOR="#BC8F8F">"dlsym"</FONT></B> : hndl * string -> fptr;
<B><FONT COLOR="#A020F0">val</FONT></B> dlclose =
_import <B><FONT COLOR="#BC8F8F">"dlclose"</FONT></B> : hndl -> Int32.int;
<B><FONT COLOR="#A020F0">val</FONT></B> RTLD_LAZY = <B><FONT COLOR="#5F9EA0">0wx00001</FONT></B> <I><FONT COLOR="#B22222">(* Lazy function call binding. *)</FONT></I>
<B><FONT COLOR="#A020F0">val</FONT></B> RTLD_NOW = <B><FONT COLOR="#5F9EA0">0wx00002</FONT></B> <I><FONT COLOR="#B22222">(* Immediate function call binding. *)</FONT></I>
<B><FONT COLOR="#A020F0">val</FONT></B> dlerror = <B><FONT COLOR="#A020F0">fn</FONT></B> () =>
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> addr = dlerror ()
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">if</FONT></B> addr = MLton.Pointer.null
<B><FONT COLOR="#A020F0">then</FONT></B> NONE
<B><FONT COLOR="#A020F0">else</FONT></B> <B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">fun</FONT></B> loop (index, cs) =
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> w = MLton.Pointer.getWord8 (addr, index)
<B><FONT COLOR="#A020F0">val</FONT></B> c = Byte.byteToChar w
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">if</FONT></B> c = #<B><FONT COLOR="#BC8F8F">"\000"</FONT></B>
<B><FONT COLOR="#A020F0">then</FONT></B> SOME (implode (rev cs))
<B><FONT COLOR="#A020F0">else</FONT></B> loop (index + <B><FONT COLOR="#5F9EA0">1</FONT></B>, c::cs)
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">in</FONT></B>
loop (<B><FONT COLOR="#5F9EA0">0</FONT></B>, [])
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> dlopen = <B><FONT COLOR="#A020F0">fn</FONT></B> (filename, mode) =>
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> filename = filename ^ <B><FONT COLOR="#BC8F8F">"\000"</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> hndl = dlopen (filename, mode)
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">if</FONT></B> hndl = MLton.Pointer.null
<B><FONT COLOR="#A020F0">then</FONT></B> <B><FONT COLOR="#A020F0">raise</FONT></B> Fail (<B><FONT COLOR="#A020F0">case</FONT></B> dlerror () <B><FONT COLOR="#A020F0">of</FONT></B>
NONE => <B><FONT COLOR="#BC8F8F">"???"</FONT></B>
| SOME s => s)
<B><FONT COLOR="#A020F0">else</FONT></B> hndl
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> dlsym = <B><FONT COLOR="#A020F0">fn</FONT></B> (hndl, symbol) =>
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> symbol = symbol ^ <B><FONT COLOR="#BC8F8F">"\000"</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> fptr = dlsym (hndl, symbol)
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">case</FONT></B> dlerror () <B><FONT COLOR="#A020F0">of</FONT></B>
NONE => fptr
| SOME s => <B><FONT COLOR="#A020F0">raise</FONT></B> Fail s
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> dlclose = <B><FONT COLOR="#A020F0">fn</FONT></B> hndl =>
<B><FONT COLOR="#A020F0">if</FONT></B> MLton.Platform.OS.host = MLton.Platform.OS.Darwin
<B><FONT COLOR="#A020F0">then</FONT></B> () <I><FONT COLOR="#B22222">(* Darwin reports the following error message if you
* try to close a dynamic library.
* "dynamic libraries cannot be closed"
* So, we disable dlclose on Darwin.
*)</FONT></I>
<B><FONT COLOR="#A020F0">else</FONT></B>
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> res = dlclose hndl
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">if</FONT></B> res = <B><FONT COLOR="#5F9EA0">0</FONT></B>
<B><FONT COLOR="#A020F0">then</FONT></B> ()
<B><FONT COLOR="#A020F0">else</FONT></B> <B><FONT COLOR="#A020F0">raise</FONT></B> Fail (<B><FONT COLOR="#A020F0">case</FONT></B> dlerror () <B><FONT COLOR="#A020F0">of</FONT></B>
NONE => <B><FONT COLOR="#BC8F8F">"???"</FONT></B>
| SOME s => s)
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#0000FF">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> dll =
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#0000FF">open</FONT></B> MLton.Platform.OS
<B><FONT COLOR="#A020F0">in</FONT></B>
<B><FONT COLOR="#A020F0">case</FONT></B> host <B><FONT COLOR="#A020F0">of</FONT></B>
Cygwin => <B><FONT COLOR="#BC8F8F">"cygwin1.dll"</FONT></B>
| Darwin => <B><FONT COLOR="#BC8F8F">"libm.dylib"</FONT></B>
| _ => <B><FONT COLOR="#BC8F8F">"libm.so"</FONT></B>
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> hndl = DynLink.dlopen (dll, DynLink.RTLD_LAZY)
<B><FONT COLOR="#0000FF">local</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> double_to_double =
_import * : DynLink.fptr -> real -> real;
<B><FONT COLOR="#A020F0">val</FONT></B> cos_fptr = DynLink.dlsym (hndl, <B><FONT COLOR="#BC8F8F">"cos"</FONT></B>)
<B><FONT COLOR="#0000FF">in</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> cos = double_to_double cos_fptr
<B><FONT COLOR="#0000FF">end</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> _ = print (concat [<B><FONT COLOR="#BC8F8F">" Math.cos(2.0) = "</FONT></B>, Real.toString (Math.cos <B><FONT COLOR="#5F9EA0">2.0</FONT></B>), <B><FONT COLOR="#BC8F8F">"\n"</FONT></B>,
<B><FONT COLOR="#BC8F8F">"libm.so::cos(2.0) = "</FONT></B>, Real.toString (cos <B><FONT COLOR="#5F9EA0">2.0</FONT></B>), <B><FONT COLOR="#BC8F8F">"\n"</FONT></B>])
<B><FONT COLOR="#A020F0">val</FONT></B> _ = DynLink.dlclose hndl
</PRE>
</p>
<p>
Compile and run <tt>iimport.sml</tt>.
<pre>% mlton -default-ann 'allowFFI true' \
-target-link-opt linux -ldl \
-target-link-opt solaris -ldl \
iimport.sml
% iimport
Math.cos(2.0) = ~0.416146836547
libm.so::cos(2.0) = ~0.416146836547
</pre>
</p>
<p>
This example also shows the <tt>-target-link-opt</tt> option, which uses the switch when linking only when on the specified platform. Compile with <tt>-verbose 1</tt> to see in more detail what's being passed to <tt>gcc</tt>.
</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/iimport.sml"><img src="moin-www.png" alt="[WWW]" height="11" width="11">iimport.sml</a>
</p>
</li>
</ul>
</div>
<p>
<hr>
Last edited on 2006-11-02 17:49:16 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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