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://www.haskell.org/"><literal>http://www.haskell.org/</literal></ulink>.</para>
+ <para>GHC (mostly) conforms to the Haskell Foreign Function Interface,
+ whose definition is part of the Haskell Report on <ulink url="http://www.haskell.org/"><literal>http://www.haskell.org/</literal></ulink>.</para>
- <para>To enable FFI support in GHC, give the <option>-XForeignFunctionInterface</option><indexterm><primary><option>-XForeignFunctionInterface</option></primary>
+ <para>FFI support is enabled by default, but can be enabled or disabled explicitly with the <option>-XForeignFunctionInterface</option><indexterm><primary><option>-XForeignFunctionInterface</option></primary>
</indexterm> flag.</para>
<para>GHC implements a number of GHC-specific extensions to the FFI
<para>The FFI libraries are documented in the accompanying library
documentation; see for example the
- <ulink url="../libraries/base/Control-Concurrent.html"><literal>Foreign</literal></ulink> module.</para>
+ <ulink url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Foreign</literal></ulink> module.</para>
<sect1 id="ffi-ghcexts">
<title>GHC extensions to the FFI Addendum</title>
</programlisting>
</para>
</sect2>
+
+ <sect2 id="ffi-prim">
+ <title>Primitive imports</title>
+ <para>
+ GHC extends the FFI with an additional calling convention
+ <literal>prim</literal>, e.g.:
+<programlisting>
+ foreign import prim "foo" foo :: ByteArray# -> (# Int#, Int# #)
+</programlisting>
+ This is used to import functions written in Cmm code that follow an
+ internal GHC calling convention. This feature is not intended for
+ use outside of the core libraries that come with GHC. For more
+ details see the GHC developer wiki.
+ </para>
+ </sect2>
+
+ <sect2 id="ffi-interruptible">
+ <title>Interruptible foreign calls</title>
+ <para>
+ This concerns the interaction of foreign calls
+ with <literal>Control.Concurrent.throwTo</literal>.
+ Normally when the target of a <literal>throwTo</literal> is
+ involved in a foreign call, the exception is not raised
+ until the call returns, and in the meantime the caller is
+ blocked. This can result in unresponsiveness, which is
+ particularly undesirable in the case of user interrupt
+ (e.g. Control-C). The default behaviour when a Control-C
+ signal is received (<literal>SIGINT</literal> on Unix) is to raise
+ the <literal>UserInterrupt</literal> exception in the main
+ thread; if the main thread is blocked in a foreign call at
+ the time, then the program will not respond to the user
+ interrupt.
+ </para>
+
+ <para>
+ The problem is that it is not possible in general to
+ interrupt a foreign call safely. However, GHC does provide
+ a way to interrupt blocking system calls which works for
+ most system calls on both Unix and Windows. A foreign call
+ can be annotated with <literal>interruptible</literal> instead
+ of <literal>safe</literal> or <literal>unsafe</literal>:
+
+<programlisting>
+foreign import ccall interruptible
+ "sleep" :: CUint -> IO CUint
+</programlisting>
+
+ <literal>interruptble</literal> behaves exactly as
+ <literal>safe</literal>, except that when
+ a <literal>throwTo</literal> is directed at a thread in an
+ interruptible foreign call, an OS-specific mechanism will be
+ used to attempt to cause the foreign call to return:
+
+ <variablelist>
+ <varlistentry>
+ <term>Unix systems</term>
+ <listitem>
+ <para>
+ The thread making the foreign call is sent
+ a <literal>SIGPIPE</literal> signal
+ using <literal>pthread_kill()</literal>. This is
+ usually enough to cause a blocking system call to
+ return with <literal>EINTR</literal> (GHC by default
+ installs an empty signal handler
+ for <literal>SIGPIPE</literal>, to override the
+ default behaviour which is to terminate the process
+ immediately).
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Windows systems</term>
+ <listitem>
+ <para>
+ [Vista and later only] The RTS calls the Win32
+ function <literal>CancelSynchronousIO</literal>,
+ which will cause a blocking I/O operation to return
+ with the
+ error <literal>ERROR_OPERATION_ABORTED</literal>.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ If the system call is successfully interrupted, it will
+ return to Haskell whereupon the exception can be raised. Be
+ especially careful when
+ using <literal>interruptible</literal> that the caller of
+ the foreign function is prepared to deal with the
+ consequences of the call being interrupted; on Unix it is
+ good practice to check for <literal>EINTR</literal> always,
+ but on Windows it is not typically necessary to
+ handle <literal>ERROR_OPERATION_ABORTED</literal>.
+ </para>
+ </sect2>
</sect1>
<sect1 id="ffi-ghc">
#include "foo_stub.h"
#endif
-#ifdef __GLASGOW_HASKELL__
-extern void __stginit_Foo ( void );
-#endif
-
int main(int argc, char *argv[])
{
int i;
hs_init(&argc, &argv);
-#ifdef __GLASGOW_HASKELL__
- hs_add_root(__stginit_Foo);
-#endif
for (i = 0; i < 5; i++) {
printf("%d\n", foo(2500));
(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. Note that the symbol name should be transformed
- according to the Z-encoding:</para>
-
<informaltable>
<tgroup cols="2" align="left" colsep="1" rowsep="1">
<thead>
<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>
+ re-initialise after this has happened,
+ see <xref linkend="ffi-divergence" />.</para>
</footnote>.</para>
<para>NOTE: when linking the final program, it is normally
// Initialize Haskell runtime
hs_init(&argc, &argv);
- // Tell Haskell about all root modules
- hs_add_root(__stginit_Foo);
-
// do any other initialization here and
// return false if there was a problem
return HS_BOOL_TRUE;
</programlisting>
<para>The initialisation routine, <literal>mylib_init</literal>, calls
- <literal>hs_init()</literal> and <literal>hs_add_root()</literal> as
+ <literal>hs_init()</literal> as
normal to initialise the Haskell runtime, and the corresponding
deinitialisation function <literal>mylib_end()</literal> calls
<literal>hs_exit()</literal> to shut down the runtime.</para>
</sect3>
- <sect3 id="hs-exit">
- <title>On the use of <literal>hs_exit()</literal></title>
-
- <para><literal>hs_exit()</literal> normally causes the termination of
- any running Haskell threads in the system, and when
- <literal>hs_exit()</literal> returns, there will be no more Haskell
- threads running. The runtime will then shut down the system in an
- orderly way, generating profiling
- output and statistics if necessary, and freeing all the memory it
- owns.</para>
-
- <para>It isn't always possible to terminate a Haskell thread forcibly:
- for example, the thread might be currently executing a foreign call,
- and we have no way to force the foreign call to complete. What's
- more, the runtime must
- assume that in the worst case the Haskell code and runtime are about
- to be removed from memory (e.g. if this is a <link linkend="win32-dlls">Windows DLL</link>,
- <literal>hs_exit()</literal> is normally called before unloading the
- DLL). So <literal>hs_exit()</literal> <emphasis>must</emphasis> wait
- until all outstanding foreign calls return before it can return
- itself.</para>
-
- <para>The upshot of this is that if you have Haskell threads that are
- blocked in foreign calls, then <literal>hs_exit()</literal> may hang
- (or possibly busy-wait) until the calls return. Therefore it's a
- good idea to make sure you don't have any such threads in the system
- when calling <literal>hs_exit()</literal>. This includes any threads
- doing I/O, because I/O may (or may not, depending on the
- type of I/O and the platform) be implemented using blocking foreign
- calls.</para>
-
- <para>The GHC runtime treats program exit as a special case, to avoid
- the need to wait for blocked threads when a standalone
- executable exits. Since the program and all its threads are about to
- terminate at the same time that the code is removed from memory, it
- isn't necessary to ensure that the threads have exited first.
- (Unofficially, if you want to use this fast and loose version of
- <literal>hs_exit()</literal>, then call
- <literal>shutdownHaskellAndExit()</literal> instead).</para>
- </sect3>
</sect2>
<sect2 id="glasgow-foreign-headers">
- <title>Using function headers</title>
+ <title>Using header files</title>
<indexterm><primary>C calls, function headers</primary></indexterm>
</varlistentry>
</variablelist>
</sect2>
+
+ <sect2 id="ffi-threads">
+ <title>Multi-threading and the FFI</title>
+
+ <para>In order to use the FFI in a multi-threaded setting, you must
+ use the <option>-threaded</option> option
+ (see <xref linkend="options-linker" />).</para>
+
+ <sect3>
+ <title>Foreign imports and multi-threading</title>
+
+ <para>When you call a <literal>foreign import</literal>ed
+ function that is annotated as <literal>safe</literal> (the
+ default), and the program was linked
+ using <option>-threaded</option>, then the call will run
+ concurrently with other running Haskell threads. If the
+ program was linked without <option>-threaded</option>,
+ then the other Haskell threads will be blocked until the
+ call returns.</para>
+
+ <para>This means that if you need to make a foreign call to
+ a function that takes a long time or blocks indefinitely,
+ then you should mark it <literal>safe</literal> and
+ use <option>-threaded</option>. Some library functions
+ make such calls internally; their documentation should
+ indicate when this is the case.</para>
+
+ <para>If you are making foreign calls from multiple Haskell
+ threads and using <option>-threaded</option>, make sure that
+ the foreign code you are calling is thread-safe. In
+ particularly, some GUI libraries are not thread-safe and
+ require that the caller only invokes GUI methods from a
+ single thread. If this is the case, you may need to
+ restrict your GUI operations to a single Haskell thread,
+ and possibly also use a bound thread (see
+ <xref linkend="haskell-threads-and-os-threads" />).</para>
+
+ <para>Note that foreign calls made by different Haskell
+ threads may execute in <emphasis>parallel</emphasis>, even
+ when the <literal>+RTS -N</literal> flag is not being used
+ (<xref linkend="parallel-options" />). The <literal>+RTS
+ -N</literal> flag controls parallel execution of Haskell
+ threads, but there may be an arbitrary number of foreign
+ calls in progress at any one time, regardless of
+ the <literal>+RTS -N</literal> value.</para>
+
+ <para>If a call is annotated as <literal>interruptible</literal>
+ and the program was multithreaded, the call may be
+ interrupted in the event that the Haskell thread receives an
+ exception. The mechanism by which the interrupt occurs
+ is platform dependent, but is intended to cause blocking
+ system calls to return immediately with an interrupted error
+ code. The underlying operating system thread is not to be
+ destroyed. See <xref linkend="ffi-interruptible"/> for more details.</para>
+ </sect3>
+
+ <sect3 id="haskell-threads-and-os-threads">
+ <title>The relationship between Haskell threads and OS
+ threads</title>
+
+ <para>Normally there is no fixed relationship between Haskell
+ threads and OS threads. This means that when you make a
+ foreign call, that call may take place in an unspecified OS
+ thread. Furthermore, there is no guarantee that multiple
+ calls made by one Haskell thread will be made by the same OS
+ thread.</para>
+
+ <para>This usually isn't a problem, and it allows the GHC
+ runtime system to make efficient use of OS thread resources.
+ However, there are cases where it is useful to have more
+ control over which OS thread is used, for example when
+ calling foreign code that makes use of thread-local state.
+ For cases like this, we provide <emphasis>bound
+ threads</emphasis>, which are Haskell threads tied to a
+ particular OS thread. For information on bound threads, see
+ the documentation
+ for the <ulink url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>
+ module.</para>
+ </sect3>
+
+ <sect3>
+ <title>Foreign exports and multi-threading</title>
+
+ <para>When the program is linked
+ with <option>-threaded</option>, then you may
+ invoke <literal>foreign export</literal>ed functions from
+ multiple OS threads concurrently. The runtime system must
+ be initialised as usual by
+ calling <literal>hs_init()</literal>, and this call must
+ complete before invoking any <literal>foreign
+ export</literal>ed functions.</para>
+ </sect3>
+
+ <sect3 id="hs-exit">
+ <title>On the use of <literal>hs_exit()</literal></title>
+
+ <para><literal>hs_exit()</literal> normally causes the termination of
+ any running Haskell threads in the system, and when
+ <literal>hs_exit()</literal> returns, there will be no more Haskell
+ threads running. The runtime will then shut down the system in an
+ orderly way, generating profiling
+ output and statistics if necessary, and freeing all the memory it
+ owns.</para>
+
+ <para>It isn't always possible to terminate a Haskell thread forcibly:
+ for example, the thread might be currently executing a foreign call,
+ and we have no way to force the foreign call to complete. What's
+ more, the runtime must
+ assume that in the worst case the Haskell code and runtime are about
+ to be removed from memory (e.g. if this is a <link linkend="win32-dlls">Windows DLL</link>,
+ <literal>hs_exit()</literal> is normally called before unloading the
+ DLL). So <literal>hs_exit()</literal> <emphasis>must</emphasis> wait
+ until all outstanding foreign calls return before it can return
+ itself.</para>
+
+ <para>The upshot of this is that if you have Haskell threads that are
+ blocked in foreign calls, then <literal>hs_exit()</literal> may hang
+ (or possibly busy-wait) until the calls return. Therefore it's a
+ good idea to make sure you don't have any such threads in the system
+ when calling <literal>hs_exit()</literal>. This includes any threads
+ doing I/O, because I/O may (or may not, depending on the
+ type of I/O and the platform) be implemented using blocking foreign
+ calls.</para>
+
+ <para>The GHC runtime treats program exit as a special case, to avoid
+ the need to wait for blocked threads when a standalone
+ executable exits. Since the program and all its threads are about to
+ terminate at the same time that the code is removed from memory, it
+ isn't necessary to ensure that the threads have exited first.
+ (Unofficially, if you want to use this fast and loose version of
+ <literal>hs_exit()</literal>, then call
+ <literal>shutdownHaskellAndExit()</literal> instead).</para>
+ </sect3>
+ </sect2>
+
+ <sect2 id="ffi-floating-point">
+ <title>Floating point and the FFI</title>
+
+ <para>
+ The standard C99 <literal>fenv.h</literal> header
+ provides operations for inspecting and modifying the state of
+ the floating point unit. In particular, the rounding mode
+ used by floating point operations can be changed, and the
+ exception flags can be tested.
+ </para>
+
+ <para>
+ In Haskell, floating-point operations have pure types, and the
+ evaluation order is unspecified. So strictly speaking, since
+ the <literal>fenv.h</literal> functions let you change the
+ results of, or observe the effects of floating point
+ operations, use of <literal>fenv.h</literal> renders the
+ behaviour of floating-point operations anywhere in the program
+ undefined.
+ </para>
+
+ <para>
+ Having said that, we <emphasis>can</emphasis> document exactly
+ what GHC does with respect to the floating point state, so
+ that if you really need to use <literal>fenv.h</literal> then
+ you can do so with full knowledge of the pitfalls:
+ <itemizedlist>
+ <listitem>
+ <para>
+ GHC completely ignores the floating-point
+ environment, the runtime neither modifies nor reads it.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The floating-point environment is not saved over a
+ normal thread context-switch. So if you modify the
+ floating-point state in one thread, those changes may be
+ visible in other threads. Furthermore, testing the
+ exception state is not reliable, because a context
+ switch may change it. If you need to modify or test the
+ floating point state and use threads, then you must use
+ bound threads
+ (<literal>Control.Concurrent.forkOS</literal>), because
+ a bound thread has its own OS thread, and OS threads do
+ save and restore the floating-point state.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ It is safe to modify the floating-point unit state
+ temporarily during a foreign call, because foreign calls
+ are never pre-empted by GHC.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect2>
</sect1>
</chapter>
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