[ghc-hetmet.git] / ghc / docs / users_guide / ffi-chap.xml

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<!-- FFI docs as a chapter -->

<chapter id="ffi">
<title>Foreign function interface (FFI)</title>

  <para>GHC (mostly) conforms to the Haskell 98 Foreign Function Interface
  Addendum 1.0, whose definition is available from <ulink
  url="http://haskell.org/"><literal>http://haskell.org/</literal></ulink >.
  The FFI support in GHC diverges from the Addendum in the following ways:
  </para>

  <itemizedlist>
    <listitem>
      <para>Syntactic forms and library functions proposed in earlier versions
      of the FFI are still supported for backwards compatibility.</para>
    </listitem>

    <listitem>
      <para>GHC implements a number of GHC-specific extensions to the FFI
      Addendum.  These extensions are described in <xref
      linkend="sec-ffi-ghcexts"/>, but please note that programs using
      these features are not portable.  Hence, these features should be
      avoided where possible.</para>
    </listitem>
  </itemizedlist>

  <para>The FFI libraries are documented in the accompanying library
  documentation; see for example the <literal>Foreign</literal>
  module.</para>

  <sect1 id="sec-ffi-ghcexts">
    <title>GHC extensions to the FFI Addendum</title>

    <para>The FFI features that are described in this section are specific to
    GHC.  Avoid them where possible to not compromise the portability of the
    resulting code.</para>

    <sect2>
      <title>Unboxed types</title>

      <para>The following unboxed types may be used as basic foreign types
      (see FFI Addendum, Section 3.2): <literal>Int#</literal>,
      <literal>Word#</literal>, <literal>Char#</literal>,
      <literal>Float#</literal>, <literal>Double#</literal>,
      <literal>Addr#</literal>, <literal>StablePtr# a</literal>,
      <literal>MutableByteArray#</literal>, <literal>ForeignObj#</literal>,
      and <literal>ByteArray#</literal>.</para>
    </sect2>

  </sect1>

  <sect1 id="sec-ffi-ghc">
    <title>Using the FFI with GHC</title>

    <para>The following sections also give some hints and tips on the
    use of the foreign function interface in GHC.</para>

    <sect2 id="foreign-export-ghc">
      <title>Using <literal>foreign export</literal> and <literal>foreign import ccall "wrapper"</literal> with GHC</title>

      <indexterm><primary><literal>foreign export
      </literal></primary><secondary>with GHC</secondary>
      </indexterm>

      <para>When GHC compiles a module (say <filename>M.hs</filename>)
      which uses <literal>foreign export</literal> or 
      <literal>foreign import "wrapper"</literal>, it generates two
      additional files, <filename>M_stub.c</filename> and
      <filename>M_stub.h</filename>.  GHC will automatically compile
      <filename>M_stub.c</filename> to generate
      <filename>M_stub.o</filename> at the same time.</para>

      <para>For a plain <literal>foreign export</literal>, the file
      <filename>M_stub.h</filename> contains a C prototype for the
      foreign exported function, and <filename>M_stub.c</filename>
      contains its definition.  For example, if we compile the
      following module:</para>

<programlisting>
module Foo where

foreign export ccall foo :: Int -> IO Int

foo :: Int -> IO Int
foo n = return (length (f n))

f :: Int -> [Int]
f 0 = []
f n = n:(f (n-1))</programlisting>

      <para>Then <filename>Foo_stub.h</filename> will contain
      something like this:</para>

<programlisting>
#include "HsFFI.h"
extern HsInt foo(HsInt a0);</programlisting>

      <para>and <filename>Foo_stub.c</filename> contains the
      compiler-generated definition of <literal>foo()</literal>.  To
      invoke <literal>foo()</literal> from C, just <literal>#include
      "Foo_stub.h"</literal> and call <literal>foo()</literal>.</para>

      <sect3 id="using-own-main"> 
        <title>Using your own <literal>main()</literal></title>

        <para>Normally, GHC's runtime system provides a
        <literal>main()</literal>, which arranges to invoke
        <literal>Main.main</literal> in the Haskell program.  However,
        you might want to link some Haskell code into a program which
        has a main function written in another language, say C.  In
        order to do this, you have to initialize the Haskell runtime
        system explicitly.</para>

        <para>Let's take the example from above, and invoke it from a
        standalone C program.  Here's the C code:</para>

<programlisting>
#include &lt;stdio.h&gt;
#include "HsFFI.h"

#ifdef __GLASGOW_HASKELL__
#include "foo_stub.h"
#endif

#ifdef __GLASGOW_HASKELL__
extern void __stginit_Foo ( void );
#endif

int main(int argc, char *argv[])
{
  int i;

  hs_init(&amp;argc, &amp;argv);
#ifdef __GLASGOW_HASKELL__
  hs_add_root(__stginit_Foo);
#endif

  for (i = 0; i &lt; 5; i++) {
    printf("%d\n", foo(2500));
  }

  hs_exit();
  return 0;
}</programlisting>

        <para>We've surrounded the GHC-specific bits with
        <literal>#ifdef __GLASGOW_HASKELL__</literal>; the rest of the
        code should be portable across Haskell implementations that
        support the FFI standard.</para>

        <para>The call to <literal>hs_init()</literal>
        initializes GHC's runtime system.  Do NOT try to invoke any
        Haskell functions before calling
        <literal>hs_init()</literal>: strange things will
        undoubtedly happen.</para>

        <para>We pass <literal>argc</literal> and
        <literal>argv</literal> to <literal>hs_init()</literal>
        so that it can separate out any arguments for the RTS
        (i.e. those arguments between
        <literal>+RTS...-RTS</literal>).</para>

        <para>Next, we call
        <function>hs_add_root</function><indexterm><primary><function>hs_add_root</function></primary>
        </indexterm>, a GHC-specific interface which is required to
        initialise the Haskell modules in the program.  The argument
        to <function>hs_add_root</function> should be the name of the
        initialization function for the "root" module in your program
        - in other words, the module which directly or indirectly
        imports all the other Haskell modules in the program.  In a
        standalone Haskell program the root module is normally
        <literal>Main</literal>, but when you are using Haskell code
        from a library it may not be.  If your program has multiple
        root modules, then you can call
        <function>hs_add_root</function> multiple times, one for each
        root.  The name of the initialization function for module
        <replaceable>M</replaceable> is
        <literal>__stginit_<replaceable>M</replaceable></literal>, and
        it may be declared as an external function symbol as in the
        code above.</para>

        <para>After we've finished invoking our Haskell functions, we
        can call <literal>hs_exit()</literal>, which
        terminates the RTS.  It runs any outstanding finalizers and
        generates any profiling or stats output that might have been
        requested.</para>

        <para>There can be multiple calls to
        <literal>hs_init()</literal>, but each one should be matched
        by one (and only one) call to
        <literal>hs_exit()</literal><footnote><para>The outermost
        <literal>hs_exit()</literal> will actually de-initialise the
        system.  NOTE that currently GHC's runtime cannot reliably
        re-initialise after this has happened.</para>
        </footnote>.</para>

        <para>NOTE: when linking the final program, it is normally
        easiest to do the link using GHC, although this isn't
        essential.  If you do use GHC, then don't forget the flag
        <option>-no-hs-main</option><indexterm><primary><option>-no-hs-main</option></primary>
          </indexterm>, otherwise GHC will try to link
        to the <literal>Main</literal> Haskell module.</para>
      </sect3>

      <sect3 id="foreign-export-dynamic-ghc">
        <title>Using <literal>foreign import ccall "wrapper"</literal> with GHC</title>

        <indexterm><primary><literal>foreign import
        ccall "wrapper"</literal></primary><secondary>with GHC</secondary>
        </indexterm>

        <para>When <literal>foreign import ccall "wrapper"</literal> is used
        in a Haskell module, The C stub file <filename>M_stub.c</filename>
        generated by GHC contains small helper functions used by the code
        generated for the imported wrapper, so it must be linked in to the
        final program.  When linking the program, remember to include
        <filename>M_stub.o</filename> in the final link command line, or
        you'll get link errors for the missing function(s) (this isn't
        necessary when building your program with <literal>ghc
        &ndash;&ndash;make</literal>, as GHC will automatically link in the
        correct bits).</para>
      </sect3>
    </sect2>
    
    <sect2 id="glasgow-foreign-headers">
      <title>Using function headers</title>

      <indexterm><primary>C calls, function headers</primary></indexterm>

      <para>When generating C (using the <option>-fvia-C</option>
      directive), one can assist the C compiler in detecting type
      errors by using the <option>-&num;include</option> directive
      (<xref linkend="options-C-compiler"/>) to provide
      <filename>.h</filename> files containing function
      headers.</para>

      <para>For example,</para>

<programlisting>
#include "HsFFI.h"

void         initialiseEFS (HsInt size);
HsInt        terminateEFS (void);
HsForeignObj emptyEFS(void);
HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x);
HsInt        lookupEFS (HsForeignObj a, HsInt i);
</programlisting>

      <para>The types <literal>HsInt</literal>,
      <literal>HsForeignObj</literal> etc. are described in the H98 FFI
      Addendum.</para>

      <para>Note that this approach is only
      <emphasis>essential</emphasis> for returning
      <literal>float</literal>s (or if <literal>sizeof(int) !=
      sizeof(int *)</literal> on your architecture) but is a Good
      Thing for anyone who cares about writing solid code.  You're
      crazy not to do it.</para>

<para>
What if you are importing a module from another package, and
a cross-module inlining exposes a foreign call that needs a supporting
<option>-&num;include</option>?  If the imported module is from the same package as
the module being compiled, you should supply all the <option>-&num;include</option>
that you supplied when compiling the imported module.  If the imported module comes
from another package, you won't necessarily know what the appropriate 
<option>-&num;include</option> options are; but they should be in the package 
configuration, which GHC knows about.  So if you are building a package, remember
to put all those <option>-&num;include</option> options into the package configuration.
See the <literal>c_includes</literal> field in <xref linkend="package-management"/>.
</para>

<para>
It is also possible, according the FFI specification, to put the 
<option>-&num;include</option> option in the foreign import 
declaration itself:
<programlisting>
  foreign import "foo.h f" f :: Int -> IO Int
</programlisting>
When compiling this module, GHC will generate a C file that includes
the specified <option>-&num;include</option>.  However, GHC
<emphasis>disables</emphasis> cross-module inlining for such foreign
calls, because it doesn't transport the <option>-&num;include</option>
information across module boundaries.  (There is no fundamental reason for this;
it was just tiresome to implement.  The wrapper, which unboxes the arguments
etc, is still inlined across modules.)  So if you want the foreign call itself
to be inlined across modules, use the command-line and package-configuration
<option>-&num;include</option> mechanism.
</para>

      <sect3 id="finding-header-files">
        <title>Finding Header files</title>

        <para>Header files named by the <option>-&num;include</option>
        option or in a <literal>foreign import</literal> declaration
        are searched for using the C compiler's usual search path.
        You can add directories to this search path using the
        <option>-I</option> option (see <xref
        linkend="c-pre-processor"/>).</para>

        <para>Note: header files are ignored unless compiling via C.
        If you had been compiling your code using the native code
        generator (the default) and suddenly switch to compiling via
        C, then you can get unexpected errors about missing include
        files.  Compiling via C is enabled automatically when certain
        options are given (eg. <option>-O</option> and
        <option>-prof</option> both enable
        <option>-fvia-C</option>).</para>
      </sect3>

    </sect2>

    <sect2>
      <title>Memory Allocation</title>

      <para>The FFI libraries provide several ways to allocate memory
      for use with the FFI, and it isn't always clear which way is the
      best.  This decision may be affected by how efficient a
      particular kind of allocation is on a given compiler/platform,
      so this section aims to shed some light on how the different
      kinds of allocation perform with GHC.</para>

      <variablelist>
        <varlistentry>
          <term><literal>alloca</literal> and friends</term>
          <listitem>
            <para>Useful for short-term allocation when the allocation
            is intended to scope over a given <literal>IO</literal>
            computation.  This kind of allocation is commonly used
            when marshalling data to and from FFI functions.</para>

            <para>In GHC, <literal>alloca</literal> is implemented
            using <literal>MutableByteArray#</literal>, so allocation
            and deallocation are fast: much faster than C's
            <literal>malloc/free</literal>, but not quite as fast as
            stack allocation in C.  Use <literal>alloca</literal>
            whenever you can.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><literal>mallocForeignPtr</literal></term>
          <listitem>
            <para>Useful for longer-term allocation which requires
            garbage collection.  If you intend to store the pointer to
            the memory in a foreign data structure, then
            <literal>mallocForeignPtr</literal> is
            <emphasis>not</emphasis> a good choice, however.</para>

            <para>In GHC, <literal>mallocForeignPtr</literal> is also
            implemented using <literal>MutableByteArray#</literal>.
            Although the memory is pointed to by a
            <literal>ForeignPtr</literal>, there are no actual
            finalizers involved (unless you add one with
            <literal>addForeignPtrFinalizer</literal>), and the
            deallocation is done using GC, so
            <literal>mallocForeignPtr</literal> is normally very
            cheap.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><literal>malloc/free</literal></term>
          <listitem>
            <para>If all else fails, then you need to resort to
            <literal>Foreign.malloc</literal> and
            <literal>Foreign.free</literal>.  These are just wrappers
            around the C functions of the same name, and their
            efficiency will depend ultimately on the implementations
            of these functions in your platform's C library.  We
            usually find <literal>malloc</literal> and
            <literal>free</literal> to be significantly slower than
            the other forms of allocation above.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><literal>Foreign.Marshal.Pool</literal></term>
          <listitem>
            <para>Pools are currently implemented using
            <literal>malloc/free</literal>, so while they might be a
            more convenient way to structure your memory allocation
            than using one of the other forms of allocation, they
            won't be any more efficient.  We do plan to provide an
            improved-performance implementation of Pools in the
            future, however.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
  </sect1>
</chapter>

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