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<a name="The-Closure-API"></a>
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<a name="The-Closure-API-1"></a>
<h3 class="section">2.5 The Closure API</h3>

<p><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>closure API</em>.  Closures are not
supported on all platforms; you can check the <code>FFI_CLOSURES</code>
define to determine whether they are supported on the current
platform.
<a name="index-closures"></a>
<a name="index-closure-API"></a>
<a name="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 name="index-ffi_005fclosure_005falloc"></a>
<dl>
<dt><a name="index-void-1"></a>Function: <strong>void</strong> <em>*ffi_closure_alloc (size_t <var>size</var>, void **<var>code</var>)</em></dt>
<dd><p>Allocate a chunk of memory holding <var>size</var> bytes.  This returns a
pointer to the writable address, and sets *<var>code</var> to the
corresponding executable address.
</p>
<p><var>size</var> should be sufficient to hold a <code>ffi_closure</code> object.
</p></dd></dl>

<a name="index-ffi_005fclosure_005ffree"></a>
<dl>
<dt><a name="index-void-2"></a>Function: <strong>void</strong> <em>ffi_closure_free (void *<var>writable</var>)</em></dt>
<dd><p>Free memory allocated using <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>ffi_cif</code> describing the function call.  Finally you can prepare
the closure function:
</p>
<a name="index-ffi_005fprep_005fclosure_005floc"></a>
<dl>
<dt><a name="index-ffi_005fstatus-2"></a>Function: <strong>ffi_status</strong> <em>ffi_prep_closure_loc (ffi_closure *<var>closure</var>, ffi_cif *<var>cif</var>, void (*<var>fun</var>) (ffi_cif *<var>cif</var>, void *<var>ret</var>, void **<var>args</var>, void *<var>user_data</var>), void *<var>user_data</var>, void *<var>codeloc</var>)</em></dt>
<dd><p>Prepare a closure function.
</p>
<p><var>closure</var> is the address of a <code>ffi_closure</code> object; this is
the writable address returned by <code>ffi_closure_alloc</code>.
</p>
<p><var>cif</var> is the <code>ffi_cif</code> describing the function parameters.
</p>
<p><var>user_data</var> is an arbitrary datum that is passed, uninterpreted,
to your closure function.
</p>
<p><var>codeloc</var> is the executable address returned by
<code>ffi_closure_alloc</code>.
</p>
<p><var>fun</var> is the function which will be called when the closure is
invoked.  It is called with the arguments:
</p><dl compact="compact">
<dt><var>cif</var></dt>
<dd><p>The <code>ffi_cif</code> passed to <code>ffi_prep_closure_loc</code>.
</p>
</dd>
<dt><var>ret</var></dt>
<dd><p>A pointer to the memory used for the function&rsquo;s return value.
<var>fun</var> must fill this, unless the function is declared as returning
<code>void</code>.
</p>
</dd>
<dt><var>args</var></dt>
<dd><p>A vector of pointers to memory holding the arguments to the function.
</p>
</dd>
<dt><var>user_data</var></dt>
<dd><p>The same <var>user_data</var> that was passed to
<code>ffi_prep_closure_loc</code>.
</p></dd>
</dl>

<p><code>ffi_prep_closure_loc</code> will return <code>FFI_OK</code> if everything
went ok, and something else on error.
</p>
<p>After calling <code>ffi_prep_closure_loc</code>, you can cast <var>codeloc</var>
to the appropriate pointer-to-function type.
</p></dd></dl>

<p>You may see old code referring to <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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