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<title>Size and Alignment (libffi: the portable foreign function interface library)</title>
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<div class="subsection" id="Size-and-Alignment">
<div class="header">
<p>
Next: <a href="Arrays-Unions-Enums.html" accesskey="n" rel="next">Arrays, Unions, and Enumerations</a>, Previous: <a href="Structures.html" accesskey="p" rel="prev">Structures</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="Size-and-Alignment-1"></span><h4 class="subsection">2.3.3 Size and Alignment</h4>
<p><code>libffi</code> will set the <code>size</code> and <code>alignment</code> fields of
an <code>ffi_type</code> object for you. It does so using its knowledge of
the ABI.
</p>
<p>You might expect that you can simply read these fields for a type that
has been laid out by <code>libffi</code>. However, there are some caveats.
</p>
<ul>
<li> The size or alignment of some of the built-in types may vary depending
on the chosen ABI.
</li><li> The size and alignment of a new structure type will not be set by
<code>libffi</code> until it has been passed to <code>ffi_prep_cif</code> or
<code>ffi_get_struct_offsets</code>.
</li><li> A structure type cannot be shared across ABIs. Instead each ABI needs
its own copy of the structure type.
</li></ul>
<p>So, before examining these fields, it is safest to pass the
<code>ffi_type</code> object to <code>ffi_prep_cif</code> or
<code>ffi_get_struct_offsets</code> first. This function will do all the
needed setup.
</p>
<div class="example">
<pre class="example">ffi_type *desired_type;
ffi_abi desired_abi;
…
ffi_cif cif;
if (ffi_prep_cif (&cif, desired_abi, 0, desired_type, NULL) == FFI_OK)
{
size_t size = desired_type->size;
unsigned short alignment = desired_type->alignment;
}
</pre></div>
<p><code>libffi</code> also provides a way to get the offsets of the members of
a structure.
</p>
<span id="index-ffi_005fget_005fstruct_005foffsets"></span>
<dl class="def">
<dt id="index-ffi_005fstatus-2"><span class="category">Function: </span><span><strong>ffi_status</strong> <em>ffi_get_struct_offsets (ffi_abi abi, ffi_type *struct_type, size_t *offsets)</em><a href='#index-ffi_005fstatus-2' class='copiable-anchor'> ¶</a></span></dt>
<dd><p>Compute the offset of each element of the given structure type.
<var>abi</var> is the ABI to use; this is needed because in some cases the
layout depends on the ABI.
</p>
<p><var>offsets</var> is an out parameter. The caller is responsible for
providing enough space for all the results to be written – one
element per element type in <var>struct_type</var>. If <var>offsets</var> is
<code>NULL</code>, then the type will be laid out but not otherwise
modified. This can be useful for accessing the type’s size or layout,
as mentioned above.
</p>
<p>This function returns <code>FFI_OK</code> on success; <code>FFI_BAD_ABI</code> if
<var>abi</var> is invalid; or <code>FFI_BAD_TYPEDEF</code> if <var>struct_type</var>
is invalid in some way. Note that only <code>FFI_STRUCT</code> types are
valid here.
</p></dd></dl>
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