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<title>The Closure API - libffi</title>
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<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>This facility is called the <dfn>closure API</dfn>.  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-36"></a><a name="index-closure-API-37"></a><a name="index-FFI_005fCLOSURES-38"></a>
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-39"></a>

<div class="defun">
&mdash; Function: <b>void</b><var> *ffi_closure_alloc </var>(<var>size_t size, void **code</var>)<var><a name="index-void-40"></a></var><br>
<blockquote><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><var>size</var> should be sufficient to hold a <code>ffi_closure</code> object. 
</p></blockquote></div>

   <p><a name="index-ffi_005fclosure_005ffree-41"></a>

<div class="defun">
&mdash; Function: <b>void</b><var> ffi_closure_free </var>(<var>void *writable</var>)<var><a name="index-void-42"></a></var><br>
<blockquote><p>Free memory allocated using <code>ffi_closure_alloc</code>.  The argument is
the writable address that was returned. 
</p></blockquote></div>

   <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-43"></a>

<div class="defun">
&mdash; Function: <b>ffi_status</b><var> ffi_prep_closure_loc </var>(<var>ffi_closure *closure, ffi_cif *cif, void </var>(<var>*fun</var>) (<var>ffi_cif *cif, void *ret, void **args, void *user_data</var>)<var>, void *user_data, void *codeloc</var>)<var><a name="index-ffi_005fstatus-44"></a></var><br>
<blockquote><p>Prepare a closure function.

        <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><var>cif</var> is the <code>ffi_cif</code> describing the function parameters.

        <p><var>user_data</var> is an arbitrary datum that is passed, uninterpreted,
to your closure function.

        <p><var>codeloc</var> is the executable address returned by
<code>ffi_closure_alloc</code>.

        <p><var>fun</var> is the function which will be called when the closure is
invoked.  It is called with the arguments:
          <dl>
<dt><var>cif</var><dd>The <code>ffi_cif</code> passed to <code>ffi_prep_closure_loc</code>.

          <br><dt><var>ret</var><dd>A pointer to the memory used for the function's return value. 
<var>fun</var> must fill this, unless the function is declared as returning
<code>void</code>. 
<!-- FIXME: is this NULL for void-returning functions? -->

          <br><dt><var>args</var><dd>A vector of pointers to memory holding the arguments to the function.

          <br><dt><var>user_data</var><dd>The same <var>user_data</var> that was passed to
<code>ffi_prep_closure_loc</code>. 
</dl>

        <p><code>ffi_prep_closure_loc</code> will return <code>FFI_OK</code> if everything
went ok, and something else on error. 
<!-- FIXME: what? -->

        <p>After calling <code>ffi_prep_closure_loc</code>, you can cast <var>codeloc</var>
to the appropriate pointer-to-function type. 
</p></blockquote></div>

   <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.

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