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<h2><span class="refentrytitle">Writing bindings for libvips</span></h2>
<p>Binding — How to write bindings for libvips</p>
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<div class="refsect3">
<a name="binding-goi"></a><h4>Binding and gobject-introspection</h4>
<p>
      The C source code 
      to libvips has been marked up with special comments describing the 
      interface in a standard way. These comments are read by 
      gobject-introspection
      when libvips is compiled and used to generate a 
      typelib, a description of how to call the library. Many languages have 
      gobject-introspection packages: all you need to do to call libvips
      from your favorite language is to start g-o-i, load the libvips typelib,
      and you should have the whole library available. For example, from 
      Python it's as simple as:

</p>
<pre class="programlisting">
from gi.repository import Vips
</pre>
<p>
    </p>
<p>
      libvips used in this way is likely to be rather bare-bones. For Python,
      we wrote a set of overrides which layer a more Pythonesque interface
      on top of the one provided for libvips by pygobject. These overrides
      are simply a set of Python classes.
    </p>
<p>
      To call a vips operation, you'll need to make a new operation with
      <a class="link" href="libvips-VipsOperation.html#vips-operation-new" title="vips_operation_new ()"><code class="function">vips_operation_new()</code></a> (all it does is look up the operation by name
      with <a class="link" href="libvips-VipsObject.html#vips-type-find" title="vips_type_find ()"><code class="function">vips_type_find()</code></a>, then call <code class="function">g_object_new()</code> for you), then
      use <a class="link" href="libvips-VipsObject.html#vips-argument-map" title="vips_argument_map ()"><code class="function">vips_argument_map()</code></a> and friends to loop over the operation's
      arguments setting them. Once you have set all arguments, use
      <a class="link" href="libvips-VipsOperation.html#vips-cache-operation-build" title="vips_cache_operation_build ()"><code class="function">vips_cache_operation_build()</code></a> to look up the operation in the cache
      and either build or dup it. If something goes wrong, you'll need
      to use <a class="link" href="libvips-VipsObject.html#vips-object-unref-outputs" title="vips_object_unref_outputs ()"><code class="function">vips_object_unref_outputs()</code></a> and <code class="function">g_object_unref()</code> to free the
      partially-built object.
      The Python binding uses this technique to implement a function which
      can call any vips operation, turning optional vips arguments into
      Python keyword arguments.
    </p>
<p>
      If your language does not have a gobject-introspection package, you'll 
      need to write something in C or C++ doing approximately the same thing. 
      The C++ API takes this route.
    </p>
<p>
      You can generate searchable docs from a <code class="code">.gir</code> (the thing that 
      is built from scanning libvips and which in turn turn the typelib is 
      made from) with <span class="command"><strong>g-ir-doc-tool</strong></span>, for example:

</p>
<pre class="programlisting">
$ g-ir-doc-tool --language=Python -o ~/mydocs Vips-8.0.gir
</pre>
<p>

      Then to view them, either:

</p>
<pre class="programlisting">
$ yelp ~/mydocs 
</pre>
<p>

      Or perhaps

</p>
<pre class="programlisting">
$ cd ~/mydocs 
$ yelp-build html .
</pre>
<p>

      To make HTML docs. This is an easy way to see what you can call in the 
      library. 
    </p>
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