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<title>Arrays Unions Enums (libffi: the portable foreign function interface library)</title>
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<div class="subsection" id="Arrays-Unions-Enums">
<div class="header">
<p>
Next: <a href="Type-Example.html" accesskey="n" rel="next">Type Example</a>, Previous: <a href="Size-and-Alignment.html" accesskey="p" rel="prev">Size and Alignment</a>, Up: <a href="Types.html" accesskey="u" rel="up">Types</a> [<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<span id="Arrays_002c-Unions_002c-and-Enumerations"></span><h4 class="subsection">2.3.4 Arrays, Unions, and Enumerations</h4>
<ul class="section-toc">
<li><a href="#Arrays" accesskey="1">Arrays</a></li>
<li><a href="#Unions" accesskey="2">Unions</a></li>
<li><a href="#Enumerations" accesskey="3">Enumerations</a></li>
</ul>
<div class="subsubsection" id="Arrays">
<h4 class="subsubsection">2.3.4.1 Arrays</h4>
<p><code>libffi</code> does not have direct support for arrays or unions.
However, they can be emulated using structures.
</p>
<p>To emulate an array, simply create an <code>ffi_type</code> using
<code>FFI_TYPE_STRUCT</code> with as many members as there are elements in
the array.
</p>
<div class="example">
<pre class="example">ffi_type array_type;
ffi_type **elements
int i;
elements = malloc ((n + 1) * sizeof (ffi_type *));
for (i = 0; i < n; ++i)
elements[i] = array_element_type;
elements[n] = NULL;
array_type.size = array_type.alignment = 0;
array_type.type = FFI_TYPE_STRUCT;
array_type.elements = elements;
</pre></div>
<p>Note that arrays cannot be passed or returned by value in C –
structure types created like this should only be used to refer to
members of real <code>FFI_TYPE_STRUCT</code> objects.
</p>
<p>However, a phony array type like this will not cause any errors from
<code>libffi</code> if you use it as an argument or return type. This may
be confusing.
</p>
</div>
<div class="subsubsection" id="Unions">
<h4 class="subsubsection">2.3.4.2 Unions</h4>
<p>A union can also be emulated using <code>FFI_TYPE_STRUCT</code>. In this
case, however, you must make sure that the size and alignment match
the real requirements of the union.
</p>
<p>One simple way to do this is to ensue that each element type is laid
out. Then, give the new structure type a single element; the size of
the largest element; and the largest alignment seen as well.
</p>
<p>This example uses the <code>ffi_prep_cif</code> trick to ensure that each
element type is laid out.
</p>
<div class="example">
<pre class="example">ffi_abi desired_abi;
ffi_type union_type;
ffi_type **union_elements;
int i;
ffi_type element_types[2];
element_types[1] = NULL;
union_type.size = union_type.alignment = 0;
union_type.type = FFI_TYPE_STRUCT;
union_type.elements = element_types;
for (i = 0; union_elements[i]; ++i)
{
ffi_cif cif;
if (ffi_prep_cif (&cif, desired_abi, 0, union_elements[i], NULL) == FFI_OK)
{
if (union_elements[i]->size > union_type.size)
{
union_type.size = union_elements[i];
size = union_elements[i]->size;
}
if (union_elements[i]->alignment > union_type.alignment)
union_type.alignment = union_elements[i]->alignment;
}
}
</pre></div>
</div>
<div class="subsubsection" id="Enumerations">
<h4 class="subsubsection">2.3.4.3 Enumerations</h4>
<p><code>libffi</code> does not have any special support for C <code>enum</code>s.
Although any given <code>enum</code> is implemented using a specific
underlying integral type, exactly which type will be used cannot be
determined by <code>libffi</code> – it may depend on the values in the
enumeration or on compiler flags such as <samp>-fshort-enums</samp>.
See <a data-manual="gcc" href="https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation">(gcc)Structures unions enumerations and bit-fields implementation</a>,
for more information about how GCC handles enumerations.
</p>
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