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<title>The Closure API (libffi: the portable foreign function interface library)</title>

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<h3 class="section" id="The-Closure-API-1"><span>2.5 The Closure API<a class="copiable-link" href="#The-Closure-API-1"> &para;</a></span></h3>

<p><code class="code">libffi</code> also provides a way to write a generic function &ndash; a
function that can accept and decode any combination of arguments.
This can be useful when writing an interpreter, or to provide wrappers
for arbitrary functions.
</p>
<p>This facility is called the <em class="dfn">closure API</em>.  Closures are not
supported on all platforms; you can check the <code class="code">FFI_CLOSURES</code>
define to determine whether they are supported on the current
platform.
<a class="index-entry-id" id="index-closures"></a>
<a class="index-entry-id" id="index-closure-API"></a>
<a class="index-entry-id" id="index-FFI_005fCLOSURES"></a>
</p>
<p>Because closures work by assembling a tiny function at runtime, they
require special allocation on platforms that have a non-executable
heap.  Memory management for closures is handled by a pair of
functions:
</p>
<a class="index-entry-id" id="index-ffi_005fclosure_005falloc"></a>
<dl class="first-deffn first-defun-alias-first-deffn">
<dt class="deffn defun-alias-deffn" id="index-void-1"><span class="category-def">Function: </span><span><strong class="def-name">void</strong> <var class="def-var-arguments">*ffi_closure_alloc (size_t <var class="var">size</var>, void **<var class="var">code</var>)</var><a class="copiable-link" href="#index-void-1"> &para;</a></span></dt>
<dd><p>Allocate a chunk of memory holding <var class="var">size</var> bytes.  This returns a
pointer to the writable address, and sets *<var class="var">code</var> to the
corresponding executable address.
</p>
<p><var class="var">size</var> should be sufficient to hold a <code class="code">ffi_closure</code> object.
</p></dd></dl>

<a class="index-entry-id" id="index-ffi_005fclosure_005ffree"></a>
<dl class="first-deffn first-defun-alias-first-deffn">
<dt class="deffn defun-alias-deffn" id="index-void-2"><span class="category-def">Function: </span><span><strong class="def-name">void</strong> <var class="def-var-arguments">ffi_closure_free (void *<var class="var">writable</var>)</var><a class="copiable-link" href="#index-void-2"> &para;</a></span></dt>
<dd><p>Free memory allocated using <code class="code">ffi_closure_alloc</code>.  The argument is
the writable address that was returned.
</p></dd></dl>

<p>Once you have allocated the memory for a closure, you must construct a
<code class="code">ffi_cif</code> describing the function call.  Finally you can prepare
the closure function:
</p>
<a class="index-entry-id" id="index-ffi_005fprep_005fclosure_005floc"></a>
<dl class="first-deffn first-defun-alias-first-deffn">
<dt class="deffn defun-alias-deffn" id="index-ffi_005fstatus-3"><span class="category-def">Function: </span><span><strong class="def-name">ffi_status</strong> <var class="def-var-arguments">ffi_prep_closure_loc (ffi_closure *<var class="var">closure</var>, ffi_cif *<var class="var">cif</var>, void (*<var class="var">fun</var>) (ffi_cif *<var class="var">cif</var>, void *<var class="var">ret</var>, void **<var class="var">args</var>, void *<var class="var">user_data</var>), void *<var class="var">user_data</var>, void *<var class="var">codeloc</var>)</var><a class="copiable-link" href="#index-ffi_005fstatus-3"> &para;</a></span></dt>
<dd><p>Prepare a closure function.  The arguments to
<code class="code">ffi_prep_closure_loc</code> are:
</p>
<dl class="table">
<dt><var class="var">closure</var></dt>
<dd><p>The address of a <code class="code">ffi_closure</code> object; this is the writable
address returned by <code class="code">ffi_closure_alloc</code>.
</p>
</dd>
<dt><var class="var">cif</var></dt>
<dd><p>The <code class="code">ffi_cif</code> describing the function parameters.  Note that this
object, and the types to which it refers, must be kept alive until the
closure itself is freed.
</p>
</dd>
<dt><var class="var">user_data</var></dt>
<dd><p>An arbitrary datum that is passed, uninterpreted, to your closure
function.
</p>
</dd>
<dt><var class="var">codeloc</var></dt>
<dd><p>The executable address returned by <code class="code">ffi_closure_alloc</code>.
</p>
</dd>
<dt><var class="var">fun</var></dt>
<dd><p>The function which will be called when the closure is invoked.  It is
called with the arguments:
</p>
<dl class="table">
<dt><var class="var">cif</var></dt>
<dd><p>The <code class="code">ffi_cif</code> passed to <code class="code">ffi_prep_closure_loc</code>.
</p>
</dd>
<dt><var class="var">ret</var></dt>
<dd><p>A pointer to the memory used for the function&rsquo;s return value.
</p>
<p>If the function is declared as returning <code class="code">void</code>, then this value
is garbage and should not be used.
</p>
<p>Otherwise, <var class="var">fun</var> must fill the object to which this points,
following the same special promotion behavior as <code class="code">ffi_call</code>.
That is, in most cases, <var class="var">ret</var> points to an object of exactly the
size of the type specified when <var class="var">cif</var> was constructed.  However,
integral types narrower than the system register size are widened.  In
these cases your program may assume that <var class="var">ret</var> points to an
<code class="code">ffi_arg</code> object.
</p>
</dd>
<dt><var class="var">args</var></dt>
<dd><p>A vector of pointers to memory holding the arguments to the function.
</p>
</dd>
<dt><var class="var">user_data</var></dt>
<dd><p>The same <var class="var">user_data</var> that was passed to
<code class="code">ffi_prep_closure_loc</code>.
</p></dd>
</dl>
</dd>
</dl>

<p><code class="code">ffi_prep_closure_loc</code> will return <code class="code">FFI_OK</code> if everything
went ok, and one of the other <code class="code">ffi_status</code> values on error.
</p>
<p>After calling <code class="code">ffi_prep_closure_loc</code>, you can cast <var class="var">codeloc</var>
to the appropriate pointer-to-function type.
</p></dd></dl>

<p>You may see old code referring to <code class="code">ffi_prep_closure</code>.  This
function is deprecated, as it cannot handle the need for separate
writable and executable addresses.
</p>
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