ffi.sgml

<Sect1 id="sec-intro">
<Title>Introduction
</Title>

<Para>
The motivation behind this foreign function interface (FFI) specification is
to make it possible to describe in Haskell <Emphasis>source code</Emphasis>
the interface to foreign functionality in a Haskell system independent
manner. It builds on experiences made with the previous foreign function
interfaces provided by GHC and Hugs.  However, the FFI specified in this
document is not in the market of trying to completely bridge the gap between
the actual type of an external function, and what is a
<Emphasis>convenient</Emphasis> type for that function to the Haskell
programmer. That is the domain of tools like HaskellDirect or Green Card, both
of which are capable of generating Haskell code that uses this FFI.
</Para>

<Para>
Generally, the FFI consists of three parts:
<OrderedList>

<ListItem>
<Para>
extensions to the base language Haskell 98 (most notably <Literal>foreign
import</Literal> and <Literal>foreign export</Literal> declarations), which
are specified in the present document,
</Para>
</ListItem>

<ListItem>
<Para>
a low-level marshalling library, which is part of the
<Emphasis>Language</Emphasis> part of the <Emphasis>Haskell Extension
Library</Emphasis> (see <xref linkend="sec-Storable">), and a
</Para>
</ListItem>

<ListItem>
<Para>
a high-level marshalling library, which is still under development.
</Para>
</ListItem>

</OrderedList>
Before diving into the details of the language extension coming with the FFI,
let us briefly outline the two other components of the interface.
</Para>

<Para>
The low-level marshalling library consists of a portion that is independent of
the targeted foreign language and dedicated support for Haskell bindings to C
libraries (special support for other languages may be added in the future).
The language independent part is given by the module
<literal>Foreign</literal> module (see <xref linkend="sec-Foreign">).  It
provides support for handling references to foreign structures, for passing
references to Haskell structures out to foreign routines, and for storing
primitive data types in raw memory blocks in a portable manner.  The support
for C libraries essentially provides Haskell representations for all basic
types of C (see <xref linkend="sec-CTypes"> and <xref
linkend="sec-CTypesISO">).
</Para>

<Para>
The high-level library, of which the interface definition is not yet
finalised, provides routines for marshalling complex Haskell structures as
well as handling out and in-out parameters in a convenient, yet protable way.
</Para>

<Para>
In the following, we will discuss the language extensions of the FFI (ie, the
first point above).  They can be split up into two complementary halves; one
half that provides Haskell constructs for importing foreign functionality into
Haskell, the other which lets you expose Haskell functions to the outside
world. We start with the former, how to import external functionality into
Haskell.
</Para>

</Sect1>

<Sect1 id="sec-primitive">
<Title>Calling foreign functions
</Title>

<Para>
To bind a Haskell variable name and type to an external function, we
introduce a new construct: <Literal>foreign import</Literal>. It defines the type of a Haskell function together with the name of an external function that actually implements it. The syntax of <Literal>foreign import</Literal> construct is as follows:
</Para>

<Para>

<ProgramListing>
topdecl 
  : ...
  ..
  | 'foreign' 'import' [callconv] [ext_fun] ['unsafe'] varid '::' prim_type
</ProgramListing>

</Para>

<Para>
A <Literal>foreign import</Literal> declaration is only allowed as a toplevel
declaration. It consists of two parts, one giving the Haskell type
(<Literal>prim&lowbar;type</Literal>), Haskell name (<Literal>varid</Literal>) and a flag indicating whether the
primitive is unsafe, the other giving details of the name of the
external function (<Literal>ext&lowbar;fun</Literal>) and its calling interface
(<Literal>callconv</Literal>.)
</Para>

<Para>
Giving a Haskell name and type to an external entry point is clearly
an unsafe thing to do, as the external name will in most cases be
untyped. The onus is on the programmer using <Literal>foreign import</Literal> to
ensure that the Haskell type given correctly maps on to the
type of the external function. Section
<XRef LinkEnd="sec-mapping"> specifies the mapping from 
Haskell types to external types.
</Para>

<Sect2 id="sec-prim-name">
<Title>Giving the external function a Haskell name
</Title>

<Para>
The external function has to be given a Haskell name. The name
must be a Haskell <Literal>varid</Literal>, so the language rules regarding
variable names must be followed, i.e., it must start with a
lower case letter followed by a sequence of alphanumeric
(`in the Unicode sense') characters or '.

<Footnote>
<Para>
Notice that with Haskell 98, underscore ('&lowbar;') is included in
the character class <Literal>small</Literal>.
</Para>
</Footnote>

</Para>

<Para>
<ProgramListing>
varid : small ( small | large | udigit | ' )*
</ProgramListing>
</Para>

</Sect2>

<Sect2 id="sec-prim-ext-name">
<Title>Naming the external function
</Title>

<Para>
The name of the external function consists of two parts,
one specifying its location, the other its name:
</Para>

<Para>

<ProgramListing>
ext_fun  : ext_loc ext_name
         | ext_name

ext_name : string
ext_loc  : string
</ProgramListing>

</Para>

<Para>
For example,
</Para>

<Para>

<ProgramListing>
foreign import stdcall "Advapi32" "RegCloseKey" regCloseKey :: Addr -&#62; IO ()
</ProgramListing>

</Para>

<Para>
states that the external function named <Function>RegCloseKey</Function> at location
<Function>Advapi32</Function> should be bound to the Haskell name <Function>regCloseKey</Function>.
For a Win32 Haskell implementation that supports the loading of DLLs
on-the-fly, this declaration will most likely cause the run-time
system to load the <Filename>Advapi32.dll</Filename> DLL before looking up the 
function <Function>RegCloseKey()</Function> therein to get at the function pointer
to use when invoking <Function>regCloseKey</Function>. 
</Para>

<Para>
Compiled implementations may do something completely different, i.e.,
mangle "RegCloseKey" to convert it into an archive/import library
symbol, that's assumed to be in scope when linking. The details of
which are platform (and compiler command-line) dependent.
</Para>

<Para>
If the location part is left out, the name of the external function
specifies a symbol that is assumed to be in scope when linking.
</Para>

<Para>
The location part can either contain an absolute `address' (i.e.,
path) of the archive/DLL, or just its name, leaving it up to the
underlying system (system meaning both RTS/compiler and OS) to resolve
the name to its real location.
</Para>

<Para>
An implementation is <Emphasis>expected</Emphasis> to be able to intelligently
transform the <Literal>ext&lowbar;loc</Literal> location to fit platform-specific
practices for naming dynamic libraries. For instance, given the
declaration
</Para>

<Para>

<ProgramListing>
foreign import "Foo" "foo" foo :: Int -&#62; Int -&#62; IO ()
</ProgramListing>

</Para>

<Para>
an implementation should map <Filename>Foo</Filename> to <Filename>"Foo.dll"</Filename> on a Win32
platform, and <Filename>libFoo.so</Filename> on ELF platforms. If the lookup of the
dynamic library with this transformed location name should fail, the
implementation should then attempt to use the original name before
eventually giving up. As part of their documentation, implementations
of <Literal>foreign import</Literal> should specify the exact details of how
<Literal>ext&lowbar;loc</Literal>s are transformed and resolved, including the list of
directories searched (and the order in which they are.)
</Para>

<Para>
In the case the Haskell name of the imported function is identical to
the external name, the <Literal>ext&lowbar;fun</Literal> can be omitted. i.e.,
</Para>

<Para>

<ProgramListing>
foreign import sin :: Double -&#62; IO Double
</ProgramListing>

</Para>

<Para>
is identical to 
</Para>

<Para>

<ProgramListing>
foreign import "sin" sin :: Double -&#62; IO Double
</ProgramListing>

</Para>

</Sect2>

<Sect2 id="sec-cconv">
<Title>Calling conventions
</Title>

<Para>
The number of calling conventions supported is fixed:
</Para>

<Para>

<ProgramListing>
callconv : ccall | stdcall
</ProgramListing>

</Para>

<Para>
<VariableList>

<VarListEntry>
<Term><Literal>ccall</Literal></Term>
<ListItem>
<Para>
The 'default' calling convention on a platform, i.e., the one
used to do (C) function calls.
</Para>

<Para>
In the case of x86 platforms, the caller pushes function arguments
from right to left on the C stack before calling. The caller is
responsible for popping the arguments off of the C stack on return.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term><Literal>stdcall</Literal></Term>
<ListItem>
<Para>
A Win32 specific calling convention. The same as <Literal>ccall</Literal>, except
that the callee cleans up the C stack before returning.

<Footnote>
<Para>
The <Literal>stdcall</Literal> is a Microsoft Win32 specific wrinkle; it used
throughout the Win32 API, for instance. On platforms where
<Literal>stdcall</Literal> isn't meaningful, it should be treated as being equal
to <Literal>ccall</Literal>.
</Para>
</Footnote>

</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>

<Para>
<Emphasis remap="bf">Some remarks:</Emphasis>

<ItemizedList>
<ListItem>

<Para>
Interoperating well with external code is the name of the game here,
so the guiding principle when deciding on what calling conventions
to include in <Literal>callconv</Literal> is that there's a demonstrated need for
a particular calling convention. Should it emerge that the inclusion
of other calling conventions will generally improve the quality of
this Haskell FFI, they will be considered for future inclusion in
<Literal>callconv</Literal>.
</Para>
</ListItem>
<ListItem>

<Para>
Supporting <Literal>stdcall</Literal> (and perhaps other platform-specific calling
conventions) raises the issue of whether a Haskell FFI should allow
the user to write platform-specific Haskell code. The calling
convention is clearly an integral part of an external function's
interface, so if the one used differs from the standard one specified
by the platform's ABI <Emphasis>and</Emphasis> that convention is used by a
non-trivial amount of external functions, the view of the FFI authors
is that a Haskell FFI should support it.
</Para>
</ListItem>
<ListItem>

<Para>
For <Literal>foreign import</Literal> (and other <Literal>foreign</Literal> declarations),
supplying the calling convention is optional. If it isn't supplied,
it is treated as if <Literal>ccall</Literal> was specified. Users are encouraged
to leave out the specification of the calling convention, if possible.
</Para>
</ListItem>

</ItemizedList>

</Para>

</Sect2>

<Sect2 id="sec-prim-types">
<Title>External function types
</Title>

<Para>
The range of types that can be passed as arguments to an external
function is restricted (as are the range of results coming back):
</Para>

<Para>

<ProgramListing>
prim_type : IO prim_result
          | prim_result
          | prim_arg '-&#62;' prim_type
</ProgramListing>

</Para>

<Para>

<ItemizedList>
<ListItem>

<Para>
If you associate a non-IO type with an external function, you
have the same 'proof obligations' as when you make use of
<Function>IOExts.unsafePerformIO</Function> in your Haskell programs.
</Para>
</ListItem>
<ListItem>

<Para>
The external function is strict in all its arguments.
</Para>
</ListItem>
<ListItem>

<Para>
<Emphasis>GHC only:</Emphasis> The GHC FFI implementation provides one extension
to <Literal>prim&lowbar;type</Literal>:


<ProgramListing>
prim_type : ... 
          | unsafe_arr_ty '-&#62;' prim_type

unsafe_arr_ty : ByteArray a
              | MutableByteArray i s a
</ProgramListing>


GHC permits the passing of its byte array primitive types
to external functions. There's some restrictions on when
they can be used; see Section <XRef LinkEnd="sec-arguments">
for more details.
</Para>
</ListItem>

</ItemizedList>

</Para>

<Para>
Section <XRef LinkEnd="sec-results"> defines
<Literal>prim&lowbar;result</Literal>; Section <XRef LinkEnd="sec-arguments">
defines <Literal>prim&lowbar;arg</Literal>.
</Para>

<Sect3 id="sec-arguments">
<Title>Argument types
</Title>

<Para>
The external function expects zero or more arguments. The set of legal
argument types is restricted to the following set:
</Para>

<Para>

<ProgramListing>
prim_arg : ext_ty | new_ty | ForeignObj

new_ty : a Haskell newtype of a prim_arg.

ext_ty : int_ty   | word_ty | float_ty
       | Addr     | Char    | StablePtr a
       | Bool

int_ty       : Int   | Int8   | Int16   | Int32 | Int64
word_ty      : Word8 | Word16 | Word32  | Word64
float_ty     : Float | Double
</ProgramListing>

</Para>

<Para>

<ItemizedList>
<ListItem>

<Para>
<Literal>ext&lowbar;ty</Literal> represent the set of basic types supported by
C-like languages, although the numeric types are explicitly sized.

The <Emphasis>stable pointer</Emphasis> <Literal>StablePtr</Literal> type looks out of place in
this list of C-like types, but it has a well-defined and simple
C mapping, see Section <XRef LinkEnd="sec-mapping">
for details.

</Para>
</ListItem>
<ListItem>

<Para>
<Literal>prim&lowbar;arg</Literal> represent the set of permissible argument types. In
addition to <Literal>ext&lowbar;ty</Literal>, <Literal>ForeignObj</Literal> is also included.

The <Literal>ForeignObj</Literal> type represent values that are pointers to some
external entity/object. It differs from the <Literal>Addr</Literal> type in that
<Literal>ForeignObj</Literal>s are <Emphasis>finalized</Emphasis>, i.e., once the garbage collector
determines that a <Literal>ForeignObj</Literal> is unreachable, it will invoke a
finalising procedure attached to the <Literal>ForeignObj</Literal> to notify the
outside world that we're through with using it.

</Para>
</ListItem>
<ListItem>

<Para>
Haskell <Literal>newtype</Literal>s that wrap up a <Literal>prim&lowbar;arg</Literal> type can also
be passed to external functions. 
</Para>
</ListItem>
<ListItem>

<Para>
Haskell type synonyms for any of the above can also be used
in <Literal>foreign import</Literal> declarations. Qualified names likewise,
i.e. <Literal>Word.Word32</Literal> is legal.

</Para>
</ListItem>
<ListItem>

<Para>
<Literal>foreign import</Literal> does not support the binding to external
constants/variables. A <Literal>foreign import</Literal> declaration that takes no
arguments represent a binding to a function with no arguments.
</Para>
</ListItem>
<ListItem>

<Para>
<Emphasis>GHC only:</Emphasis> GHC's implementation of the FFI provides
two extensions:

<ItemizedList>
<ListItem>

<Para>
Support for passing heap allocated byte arrays to an external
function

<ProgramListing>
prim_type : ... 
          | prim_arg '-&#62;' prim_type
          | unsafe_arr_ty '-&#62;' prim_type

unsafe_arr_ty : ByteArray a
              | MutableByteArray i s a
</ProgramListing>


GHC's <Literal>ByteArray</Literal> and <Literal>MutableByteArray</Literal> primitive types are
(im)mutable chunks of memory allocated on the Haskell heap, and
pointers to these can be passed to <Literal>foreign import</Literal>ed external
functions provided they are marked as <Literal>unsafe</Literal>. Since it is
inherently unsafe to hand out references to objects in the Haskell
heap if the external call may cause a garbage collection to happen,
you have to annotate the <Literal>foreign import</Literal> declaration with
the attribute <Literal>unsafe</Literal>. By doing so, the user explicitly states
that the external function won't provoke a garbage collection,
so passing out heap references to the external function is allright.

</Para>
</ListItem>
<ListItem>

<Para>
Another GHC extension is the support for unboxed types:


<ProgramListing>
prim_arg : ...  | unboxed_h_ty
ext_ty   : .... | unboxed_ext_ty

unboxed_ext_ty : Int#   | Word#    | Char#
               | Float# | Double#  | Addr# 
	       | StablePtr# a
unboxed_h_ty : MutableByteArray# | ForeignObj#
             | ByteArray#
</ProgramListing>


Clearly, if you want to be portable across Haskell systems, using 
system-specific extensions such as this is not advisable; avoid
using them if you can. (Support for using unboxed types might
be withdrawn sometime in the future.)
</Para>
</ListItem>

</ItemizedList>

</Para>
</ListItem>

</ItemizedList>

</Para>

</Sect3>

<Sect3 id="sec-results">
<Title>Result type
</Title>

<Para>
An external function is permitted to return the following
range of types:
</Para>

<Para>

<ProgramListing>
prim_result : ext_ty | new_ext_ty | ()

new_ext_ty : a Haskell newtype of an ext_ty.
</ProgramListing>

</Para>

<Para>
where <Literal>()</Literal> represents <Literal>void</Literal> / no result. 
</Para>

<Para>

<ItemizedList>
<ListItem>

<Para>
External functions cannot raise exceptions (IO exceptions or non-IO ones.)
It is the responsibility of the <Literal>foreign import</Literal> user to layer
any error handling on top of an external function.
</Para>
</ListItem>
<ListItem>

<Para>
Only external types (<Literal>ext&lowbar;ty</Literal>) can be passed back, i.e., returning
<Literal>ForeignObj</Literal>s is not supported/allowed. 
</Para>
</ListItem>
<ListItem>

<Para>
Haskell newtypes that wrap up <Literal>ext&lowbar;ty</Literal> are also permitted.
</Para>
</ListItem>

</ItemizedList>

</Para>

</Sect3>

</Sect2>

<Sect2 id="sec-mapping">
<Title>Type mapping
</Title>

<Para>
For the FFI to be of any practical use, the properties and sizes of
the various types that can be communicated between the Haskell world
and the outside, needs to be precisely defined. We do this by
presenting a mapping to C, as it is commonly used and most other
languages define a mapping to it. Table
<XRef LinkEnd="sec-mapping-table">
defines the mapping between Haskell and C types.
</Para>

<Para>

<Table id="sec-mapping-table">
<Title>Mapping of Haskell types to C types</Title>

<TGroup Cols="4">
<ColSpec Align="Left" Colsep="0">
<ColSpec Align="Left" Colsep="0">
<ColSpec Align="Left" Colsep="0">
<ColSpec Align="Left" Colsep="0">
<TBody>
<Row RowSep="1">
<Entry>Haskell type </Entry>
<Entry> C type </Entry>
<Entry> requirement </Entry>
<Entry> range (9) </Entry>
<Entry> </Entry>
<Entry> </Entry>
</Row>
<Row>
<Entry>
<Literal>Char</Literal> </Entry>
<Entry> <Literal>HsChar</Literal> </Entry>
<Entry> unspec. integral type </Entry>
<Entry> <Literal>HS&lowbar;CHAR&lowbar;MIN</Literal> .. <Literal>HS&lowbar;CHAR&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Int</Literal> </Entry>
<Entry> <Literal>HsInt</Literal> </Entry>
<Entry> signed integral of unspec. size(4) </Entry>
<Entry> <Literal>HS&lowbar;INT&lowbar;MIN</Literal> ..
<Literal>HS&lowbar;INT&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Int8</Literal> (2) </Entry>
<Entry> <Literal>HsInt8</Literal> </Entry>
<Entry> 8 bit signed integral </Entry>
<Entry> <Literal>HS&lowbar;INT8&lowbar;MIN</Literal> 
..
<Literal>HS&lowbar;INT8&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Int16</Literal> (2) </Entry>
<Entry> <Literal>HsInt16</Literal> </Entry>
<Entry> 16 bit signed integral </Entry>
<Entry> <Literal>HS&lowbar;INT16&lowbar;MIN</Literal>
.. <Literal>HS&lowbar;INT16&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Int32</Literal> (2) </Entry>
<Entry> <Literal>HsInt32</Literal> </Entry>
<Entry> 32 bit signed integral </Entry>
<Entry> <Literal>HS&lowbar;INT32&lowbar;MIN</Literal> ..
<Literal>HS&lowbar;INT32&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Int64</Literal> (2,3) </Entry>
<Entry> <Literal>HsInt64</Literal> </Entry>
<Entry> 64 bit signed integral (3) </Entry>
<Entry> <Literal>HS&lowbar;INT64&lowbar;MIN</Literal> ..
<Literal>HS&lowbar;INT64&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Word8</Literal> (2) </Entry>
<Entry> <Literal>HsWord8</Literal> </Entry>
<Entry> 8 bit unsigned integral </Entry>
<Entry> <Literal>0</Literal> ..
<Literal>HS&lowbar;WORD8&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Word16</Literal> (2) </Entry>
<Entry> <Literal>HsWord16</Literal> </Entry>
<Entry> 16 bit unsigned integral </Entry>
<Entry> <Literal>0</Literal> ..
<Literal>HS&lowbar;WORD16&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Word32</Literal> (2) </Entry>
<Entry> <Literal>HsWord32</Literal> </Entry>
<Entry> 32 bit unsigned integral </Entry>
<Entry> <Literal>0</Literal> ..
<Literal>HS&lowbar;WORD32&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Word64</Literal> (2,3) </Entry>
<Entry> <Literal>HsWord64</Literal> </Entry>
<Entry> 64 bit unsigned integral (3) </Entry>
<Entry> <Literal>0</Literal> ..
<Literal>HS&lowbar;WORD64&lowbar;MAX</Literal></Entry>
</Row>
<Row>
<Entry>
<Literal>Float</Literal> </Entry>
<Entry> <Literal>HsFloat</Literal> </Entry>
<Entry> floating point of unspec. size (5) </Entry>
<Entry> (10) </Entry>
</Row>
<Row>
<Entry>
<Literal>Double</Literal> </Entry>
<Entry> <Literal>HsDouble</Literal> </Entry>
<Entry> floating point of unspec. size (5) </Entry>
<Entry> (10) </Entry>
</Row>
<Row>
<Entry>
<Literal>Bool</Literal> </Entry>
<Entry> <Literal>HsBool</Literal> </Entry>
<Entry> unspec. integral type </Entry>
<Entry> (11) </Entry>
</Row>
<Row>
<Entry>
<Literal>Addr</Literal> </Entry>
<Entry> <Literal>HsAddr</Literal> </Entry>
<Entry> void* (6) </Entry>
<Entry> </Entry>
</Row>
<Row>
<Entry>
<Literal>ForeignObj</Literal> </Entry>
<Entry> <Literal>HsForeignObj</Literal> </Entry>
<Entry> void* (7) </Entry>
<Entry> </Entry>
</Row>
<Row>
<Entry>
<Literal>StablePtr</Literal> </Entry>
<Entry> <Literal>HsStablePtr</Literal> </Entry>
<Entry> void* (8) </Entry>
<Entry> </Entry>
</Row>
</TBody>

</TGroup>

</Table>

</Para>

<Para>
<Emphasis remap="bf">Some remarks:</Emphasis>

<OrderedList>
<ListItem>

<Para>
A Haskell system that implements the FFI will supply a header file
<Filename>HsFFI.h</Filename> that includes target platform specific definitions
for the above types and values.
</Para>
</ListItem>
<ListItem>

<Para>
The sized numeric types <Literal>Hs&lcub;Int,Word&rcub;&lcub;8,16,32,64&rcub;</Literal> have
a 1-1 mapping to ISO C 99's <Literal>&lcub;,u&rcub;int&lcub;8,16,32,64&rcub;&lowbar;t</Literal>. For systems
that doesn't support this revision of ISO C, a best-fit mapping
onto the supported C types is provided.
</Para>
</ListItem>
<ListItem>

<Para>
An implementation which does not support 64 bit integral types
on the C side should implement <Literal>Hs&lcub;Int,Word&rcub;64</Literal> as a struct. In
this case the bounds <Constant>HS&lowbar;INT64&lowbar;&lcub;MIN,MAX&rcub;</Constant> and <Constant>HS&lowbar;WORD64&lowbar;MAX</Constant>
are undefined.
</Para>
</ListItem>
<ListItem>

<Para>
A valid Haskell representation of <Literal>Int</Literal> has to be equal to or
wider than 30 bits. The <Literal>HsInt</Literal> synonym is guaranteed to map
onto a C type that satisifies Haskell's requirement for <Literal>Int</Literal>.
</Para>
</ListItem>
<ListItem>

<Para>
It is guaranteed that <Literal>Hs&lcub;Float,Double&rcub;</Literal> are one of C's
floating-point types <Literal>float</Literal>/<Literal>double</Literal>/<Literal>long double</Literal>.
</Para>
</ListItem>
<ListItem>

<Para>
It is guaranteed that <Literal>HsAddr</Literal> is of the same size as <Literal>void*</Literal>, so
any other pointer type can be converted to and from HsAddr without any
loss of information (K&amp;R, Appendix A6.8).
</Para>
</ListItem>
<ListItem>

<Para>
Foreign objects are handled like <Literal>Addr</Literal> by the FFI, so there
is again the guarantee that <Literal>HsForeignObj</Literal> is the same as
<Literal>void*</Literal>. The separate name is meant as a reminder that there is
a finalizer attached to the object pointed to.
</Para>
</ListItem>
<ListItem>

<Para>
Stable pointers are passed as addresses by the FFI, but this is
only because a <Literal>void*</Literal> is used as a generic container in most
APIs, not because they are real addresses. To make this special
case clear, a separate C type is used here. 
</Para>
</ListItem>
<ListItem>

<Para>
The bounds are preprocessor macros, so they can be used in
<Literal>&num;if</Literal> and for array bounds.
</Para>
</ListItem>
<ListItem>

<Para>
Floating-point limits are a little bit more complicated, so
preprocessor macros mirroring ISO C's <Filename>float.h</Filename> are provided:

<ProgramListing>
HS_{FLOAT,DOUBLE}_RADIX
HS_{FLOAT,DOUBLE}_ROUNDS
HS_{FLOAT,DOUBLE}_EPSILON
HS_{FLOAT,DOUBLE}_DIG
HS_{FLOAT,DOUBLE}_MANT_DIG
HS_{FLOAT,DOUBLE}_MIN
HS_{FLOAT,DOUBLE}_MIN_EXP
HS_{FLOAT,DOUBLE}_MIN_10_EXP
HS_{FLOAT,DOUBLE}_MAX
HS_{FLOAT,DOUBLE}_MAX_EXP
HS_{FLOAT,DOUBLE}_MAX_10_EXP
</ProgramListing>

</Para>
</ListItem>
<ListItem>

<Para>
It is guaranteed that Haskell's <Literal>False</Literal>/<Literal>True</Literal> map to
C's <Literal>0</Literal>/<Literal>1</Literal>, respectively, and vice versa. The mapping of
any other integral value to <Literal>Bool</Literal> is left unspecified.
</Para>
</ListItem>
<ListItem>

<Para>
To avoid name clashes, identifiers starting with <Literal>Hs</Literal> and
macros starting with <Literal>HS&lowbar;</Literal> are reserved for the FFI.
</Para>
</ListItem>
<ListItem>

<Para>
<Emphasis>GHC only:</Emphasis> The GHC specific types <Literal>ByteArray</Literal> and
<Literal>MutableByteArray</Literal> both map to <Literal>char*</Literal>.
</Para>
</ListItem>

</OrderedList>

</Para>

</Sect2>

<Sect2 id="sec-prim-remarks">
<Title>Some <Literal>foreign import</Literal> wrinkles
</Title>

<Para>

<ItemizedList>
<ListItem>

<Para>
By default, a <Literal>foreign import</Literal> function is <Emphasis>safe</Emphasis>. A safe
external function may cause a Haskell garbage collection as a result
of being called. This will typically happen when the imported
function end up calling Haskell functions that reside in the same
'Haskell world' (i.e., shares the same storage manager heap) -- see
Section <XRef LinkEnd="sec-entry"> for
details of how the FFI let's you call Haskell functions from the outside.

If the programmer can guarantee that the imported function won't
call back into Haskell, the <Literal>foreign import</Literal> can be marked as
'unsafe' (see Section <XRef LinkEnd="sec-primitive"> for details of
how to do this.)

Unsafe calls are cheaper than safe ones, so distinguishing the two
classes of external calls may be worth your while if you're extra
conscious about performance.

</Para>
</ListItem>
<ListItem>

<Para>
A <Literal>foreign import</Literal>ed function should clearly not need to know that
it is being called from Haskell. One consequence of this is that the
lifetimes of the arguments that are passed from Haskell <Emphasis>must</Emphasis>
equal that of a normal C call. For instance, for the following decl,


<ProgramListing>
foreign import "mumble" mumble :: ForeignObj -&#62; IO ()

f :: Addr -&#62; IO ()
f ptr = do
  fo &#60;- newForeignObj ptr myFinalizer
  mumble fo
</ProgramListing>


The <Literal>ForeignObj</Literal> must live across the call to <Function>mumble</Function> even if
it is not subsequently used/reachable. Why the insistence on this?
Consider what happens if <Function>mumble</Function> calls a function which calls back
into the Haskell world to execute a function, behind our back as it
were. This evaluation may possibly cause a garbage collection, with
the result that <Literal>fo</Literal> may end up being finalised.

By guaranteeing that <Literal>fo</Literal> will be considered live across the call
to <Function>mumble</Function>, the unfortunate situation where <Literal>fo</Literal> is finalised
(and hence the reference passed to <Function>mumble</Function> is suddenly no longer
valid) is avoided.


</Para>
</ListItem>

</ItemizedList>

</Para>