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6 Foreign function interface (FFI)
9 <para>GHC (mostly) conforms to the Haskell 98 Foreign Function Interface
10 Addendum 1.0, whose definition is available from <ulink url="http://haskell.org/"><literal>http://haskell.org/</literal></ulink>.</para>
12 <para>To enable FFI support in GHC, give the <option>-fffi</option><indexterm><primary><option>-fffi</option></primary>
14 the <option>-fglasgow-exts</option><indexterm><primary><option>-fglasgow-exts</option></primary>
15 </indexterm> flag which implies <option>-fffi</option>
18 <para>The FFI support in GHC diverges from the Addendum in the following ways:</para>
22 <para>Syntactic forms and library functions proposed in earlier versions
23 of the FFI are still supported for backwards compatibility.</para>
27 <para>GHC implements a number of GHC-specific extensions to the FFI
28 Addendum. These extensions are described in <xref linkend="sec-ffi-ghcexts" />, but please note that programs using
29 these features are not portable. Hence, these features should be
30 avoided where possible.</para>
34 <para>The FFI libraries are documented in the accompanying library
35 documentation; see for example the <literal>Foreign</literal>
38 <sect1 id="sec-ffi-ghcexts">
39 <title>GHC extensions to the FFI Addendum</title>
41 <para>The FFI features that are described in this section are specific to
42 GHC. Avoid them where possible to not compromise the portability of the
43 resulting code.</para>
46 <title>Unboxed types</title>
48 <para>The following unboxed types may be used as basic foreign types
49 (see FFI Addendum, Section 3.2): <literal>Int#</literal>,
50 <literal>Word#</literal>, <literal>Char#</literal>,
51 <literal>Float#</literal>, <literal>Double#</literal>,
52 <literal>Addr#</literal>, <literal>StablePtr# a</literal>,
53 <literal>MutableByteArray#</literal>, <literal>ForeignObj#</literal>,
54 and <literal>ByteArray#</literal>.</para>
59 <sect1 id="sec-ffi-ghc">
60 <title>Using the FFI with GHC</title>
62 <para>The following sections also give some hints and tips on the
63 use of the foreign function interface in GHC.</para>
65 <sect2 id="foreign-export-ghc">
66 <title>Using <literal>foreign export</literal> and <literal>foreign import ccall "wrapper"</literal> with GHC</title>
68 <indexterm><primary><literal>foreign export
69 </literal></primary><secondary>with GHC</secondary>
72 <para>When GHC compiles a module (say <filename>M.hs</filename>)
73 which uses <literal>foreign export</literal> or
74 <literal>foreign import "wrapper"</literal>, it generates two
75 additional files, <filename>M_stub.c</filename> and
76 <filename>M_stub.h</filename>. GHC will automatically compile
77 <filename>M_stub.c</filename> to generate
78 <filename>M_stub.o</filename> at the same time.</para>
80 <para>For a plain <literal>foreign export</literal>, the file
81 <filename>M_stub.h</filename> contains a C prototype for the
82 foreign exported function, and <filename>M_stub.c</filename>
83 contains its definition. For example, if we compile the
84 following module:</para>
89 foreign export ccall foo :: Int -> IO Int
92 foo n = return (length (f n))
96 f n = n:(f (n-1))</programlisting>
98 <para>Then <filename>Foo_stub.h</filename> will contain
99 something like this:</para>
103 extern HsInt foo(HsInt a0);</programlisting>
105 <para>and <filename>Foo_stub.c</filename> contains the
106 compiler-generated definition of <literal>foo()</literal>. To
107 invoke <literal>foo()</literal> from C, just <literal>#include
108 "Foo_stub.h"</literal> and call <literal>foo()</literal>.</para>
110 <sect3 id="using-own-main">
111 <title>Using your own <literal>main()</literal></title>
113 <para>Normally, GHC's runtime system provides a
114 <literal>main()</literal>, which arranges to invoke
115 <literal>Main.main</literal> in the Haskell program. However,
116 you might want to link some Haskell code into a program which
117 has a main function written in another language, say C. In
118 order to do this, you have to initialize the Haskell runtime
119 system explicitly.</para>
121 <para>Let's take the example from above, and invoke it from a
122 standalone C program. Here's the C code:</para>
125 #include <stdio.h>
128 #ifdef __GLASGOW_HASKELL__
129 #include "foo_stub.h"
132 #ifdef __GLASGOW_HASKELL__
133 extern void __stginit_Foo ( void );
136 int main(int argc, char *argv[])
140 hs_init(&argc, &argv);
141 #ifdef __GLASGOW_HASKELL__
142 hs_add_root(__stginit_Foo);
145 for (i = 0; i < 5; i++) {
146 printf("%d\n", foo(2500));
153 <para>We've surrounded the GHC-specific bits with
154 <literal>#ifdef __GLASGOW_HASKELL__</literal>; the rest of the
155 code should be portable across Haskell implementations that
156 support the FFI standard.</para>
158 <para>The call to <literal>hs_init()</literal>
159 initializes GHC's runtime system. Do NOT try to invoke any
160 Haskell functions before calling
161 <literal>hs_init()</literal>: strange things will
162 undoubtedly happen.</para>
164 <para>We pass <literal>argc</literal> and
165 <literal>argv</literal> to <literal>hs_init()</literal>
166 so that it can separate out any arguments for the RTS
167 (i.e. those arguments between
168 <literal>+RTS...-RTS</literal>).</para>
171 <function>hs_add_root</function><indexterm><primary><function>hs_add_root</function></primary>
172 </indexterm>, a GHC-specific interface which is required to
173 initialise the Haskell modules in the program. The argument
174 to <function>hs_add_root</function> should be the name of the
175 initialization function for the "root" module in your program
176 - in other words, the module which directly or indirectly
177 imports all the other Haskell modules in the program. In a
178 standalone Haskell program the root module is normally
179 <literal>Main</literal>, but when you are using Haskell code
180 from a library it may not be. If your program has multiple
181 root modules, then you can call
182 <function>hs_add_root</function> multiple times, one for each
183 root. The name of the initialization function for module
184 <replaceable>M</replaceable> is
185 <literal>__stginit_<replaceable>M</replaceable></literal>, and
186 it may be declared as an external function symbol as in the
189 <para>After we've finished invoking our Haskell functions, we
190 can call <literal>hs_exit()</literal>, which
191 terminates the RTS. It runs any outstanding finalizers and
192 generates any profiling or stats output that might have been
195 <para>There can be multiple calls to
196 <literal>hs_init()</literal>, but each one should be matched
197 by one (and only one) call to
198 <literal>hs_exit()</literal><footnote><para>The outermost
199 <literal>hs_exit()</literal> will actually de-initialise the
200 system. NOTE that currently GHC's runtime cannot reliably
201 re-initialise after this has happened.</para>
204 <para>NOTE: when linking the final program, it is normally
205 easiest to do the link using GHC, although this isn't
206 essential. If you do use GHC, then don't forget the flag
207 <option>-no-hs-main</option><indexterm><primary><option>-no-hs-main</option></primary>
208 </indexterm>, otherwise GHC will try to link
209 to the <literal>Main</literal> Haskell module.</para>
212 <sect3 id="foreign-export-dynamic-ghc">
213 <title>Using <literal>foreign import ccall "wrapper"</literal> with GHC</title>
215 <indexterm><primary><literal>foreign import
216 ccall "wrapper"</literal></primary><secondary>with GHC</secondary>
219 <para>When <literal>foreign import ccall "wrapper"</literal> is used
220 in a Haskell module, The C stub file <filename>M_stub.c</filename>
221 generated by GHC contains small helper functions used by the code
222 generated for the imported wrapper, so it must be linked in to the
223 final program. When linking the program, remember to include
224 <filename>M_stub.o</filename> in the final link command line, or
225 you'll get link errors for the missing function(s) (this isn't
226 necessary when building your program with <literal>ghc
227 ––make</literal>, as GHC will automatically link in the
228 correct bits).</para>
232 <sect2 id="glasgow-foreign-headers">
233 <title>Using function headers</title>
235 <indexterm><primary>C calls, function headers</primary></indexterm>
237 <para>When generating C (using the <option>-fvia-C</option>
238 directive), one can assist the C compiler in detecting type
239 errors by using the <option>-#include</option> directive
240 (<xref linkend="options-C-compiler"/>) to provide
241 <filename>.h</filename> files containing function
244 <para>For example,</para>
249 void initialiseEFS (HsInt size);
250 HsInt terminateEFS (void);
251 HsForeignObj emptyEFS(void);
252 HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x);
253 HsInt lookupEFS (HsForeignObj a, HsInt i);
256 <para>The types <literal>HsInt</literal>,
257 <literal>HsForeignObj</literal> etc. are described in the H98 FFI
260 <para>Note that this approach is only
261 <emphasis>essential</emphasis> for returning
262 <literal>float</literal>s (or if <literal>sizeof(int) !=
263 sizeof(int *)</literal> on your architecture) but is a Good
264 Thing for anyone who cares about writing solid code. You're
265 crazy not to do it.</para>
268 What if you are importing a module from another package, and
269 a cross-module inlining exposes a foreign call that needs a supporting
270 <option>-#include</option>? If the imported module is from the same package as
271 the module being compiled, you should supply all the <option>-#include</option>
272 that you supplied when compiling the imported module. If the imported module comes
273 from another package, you won't necessarily know what the appropriate
274 <option>-#include</option> options are; but they should be in the package
275 configuration, which GHC knows about. So if you are building a package, remember
276 to put all those <option>-#include</option> options into the package configuration.
277 See the <literal>c_includes</literal> field in <xref linkend="package-management"/>.
281 It is also possible, according the FFI specification, to put the
282 <option>-#include</option> option in the foreign import
285 foreign import "foo.h f" f :: Int -> IO Int
287 When compiling this module, GHC will generate a C file that includes
288 the specified <option>-#include</option>. However, GHC
289 <emphasis>disables</emphasis> cross-module inlining for such foreign
290 calls, because it doesn't transport the <option>-#include</option>
291 information across module boundaries. (There is no fundamental reason for this;
292 it was just tiresome to implement. The wrapper, which unboxes the arguments
293 etc, is still inlined across modules.) So if you want the foreign call itself
294 to be inlined across modules, use the command-line and package-configuration
295 <option>-#include</option> mechanism.
298 <sect3 id="finding-header-files">
299 <title>Finding Header files</title>
301 <para>Header files named by the <option>-#include</option>
302 option or in a <literal>foreign import</literal> declaration
303 are searched for using the C compiler's usual search path.
304 You can add directories to this search path using the
305 <option>-I</option> option (see <xref
306 linkend="c-pre-processor"/>).</para>
308 <para>Note: header files are ignored unless compiling via C.
309 If you had been compiling your code using the native code
310 generator (the default) and suddenly switch to compiling via
311 C, then you can get unexpected errors about missing include
312 files. Compiling via C is enabled automatically when certain
313 options are given (eg. <option>-O</option> and
314 <option>-prof</option> both enable
315 <option>-fvia-C</option>).</para>
321 <title>Memory Allocation</title>
323 <para>The FFI libraries provide several ways to allocate memory
324 for use with the FFI, and it isn't always clear which way is the
325 best. This decision may be affected by how efficient a
326 particular kind of allocation is on a given compiler/platform,
327 so this section aims to shed some light on how the different
328 kinds of allocation perform with GHC.</para>
332 <term><literal>alloca</literal> and friends</term>
334 <para>Useful for short-term allocation when the allocation
335 is intended to scope over a given <literal>IO</literal>
336 computation. This kind of allocation is commonly used
337 when marshalling data to and from FFI functions.</para>
339 <para>In GHC, <literal>alloca</literal> is implemented
340 using <literal>MutableByteArray#</literal>, so allocation
341 and deallocation are fast: much faster than C's
342 <literal>malloc/free</literal>, but not quite as fast as
343 stack allocation in C. Use <literal>alloca</literal>
344 whenever you can.</para>
349 <term><literal>mallocForeignPtr</literal></term>
351 <para>Useful for longer-term allocation which requires
352 garbage collection. If you intend to store the pointer to
353 the memory in a foreign data structure, then
354 <literal>mallocForeignPtr</literal> is
355 <emphasis>not</emphasis> a good choice, however.</para>
357 <para>In GHC, <literal>mallocForeignPtr</literal> is also
358 implemented using <literal>MutableByteArray#</literal>.
359 Although the memory is pointed to by a
360 <literal>ForeignPtr</literal>, there are no actual
361 finalizers involved (unless you add one with
362 <literal>addForeignPtrFinalizer</literal>), and the
363 deallocation is done using GC, so
364 <literal>mallocForeignPtr</literal> is normally very
370 <term><literal>malloc/free</literal></term>
372 <para>If all else fails, then you need to resort to
373 <literal>Foreign.malloc</literal> and
374 <literal>Foreign.free</literal>. These are just wrappers
375 around the C functions of the same name, and their
376 efficiency will depend ultimately on the implementations
377 of these functions in your platform's C library. We
378 usually find <literal>malloc</literal> and
379 <literal>free</literal> to be significantly slower than
380 the other forms of allocation above.</para>
385 <term><literal>Foreign.Marshal.Pool</literal></term>
387 <para>Pools are currently implemented using
388 <literal>malloc/free</literal>, so while they might be a
389 more convenient way to structure your memory allocation
390 than using one of the other forms of allocation, they
391 won't be any more efficient. We do plan to provide an
392 improved-performance implementation of Pools in the
393 future, however.</para>
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