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<h4 class="subsection" id="Size-and-Alignment-1"><span>2.3.3 Size and Alignment<a class="copiable-link" href="#Size-and-Alignment-1"> &para;</a></span></h4>

<p><code class="code">libffi</code> will set the <code class="code">size</code> and <code class="code">alignment</code> fields of
an <code class="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 class="code">libffi</code>.  However, there are some caveats.
</p>
<ul class="itemize mark-bullet">
<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 class="code">libffi</code> until it has been passed to <code class="code">ffi_prep_cif</code> or
<code class="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 class="code">ffi_type</code> object to <code class="code">ffi_prep_cif</code> or
<code class="code">ffi_get_struct_offsets</code> first.  This function will do all the
needed setup.
</p>
<div class="example">
<pre class="example-preformatted">ffi_type *desired_type;
ffi_abi desired_abi;
...
ffi_cif cif;
if (ffi_prep_cif (&amp;cif, desired_abi, 0, desired_type, NULL) == FFI_OK)
  {
    size_t size = desired_type-&gt;size;
    unsigned short alignment = desired_type-&gt;alignment;
  }
</pre></div>

<p><code class="code">libffi</code> also provides a way to get the offsets of the members of
a structure.
</p>
<a class="index-entry-id" id="index-ffi_005fget_005fstruct_005foffsets"></a>
<dl class="first-deffn first-defun-alias-first-deffn">
<dt class="deffn defun-alias-deffn" id="index-ffi_005fstatus-2"><span class="category-def">Function: </span><span><strong class="def-name">ffi_status</strong> <var class="def-var-arguments">ffi_get_struct_offsets (ffi_abi abi, ffi_type *struct_type, size_t *offsets)</var><a class="copiable-link" href="#index-ffi_005fstatus-2"> &para;</a></span></dt>
<dd><p>Compute the offset of each element of the given structure type.
<var class="var">abi</var> is the ABI to use; this is needed because in some cases the
layout depends on the ABI.
</p>
<p><var class="var">offsets</var> is an out parameter.  The caller is responsible for
providing enough space for all the results to be written &ndash; one
element per element type in <var class="var">struct_type</var>.  If <var class="var">offsets</var> is
<code class="code">NULL</code>, then the type will be laid out but not otherwise
modified.  This can be useful for accessing the type&rsquo;s size or layout,
as mentioned above.
</p>
<p>This function returns <code class="code">FFI_OK</code> on success; <code class="code">FFI_BAD_ABI</code> if
<var class="var">abi</var> is invalid; or <code class="code">FFI_BAD_TYPEDEF</code> if <var class="var">struct_type</var>
is invalid in some way.  Note that only <code class="code">FFI_STRUCT</code> types are
valid here.
</p></dd></dl>

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