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<title>Streams and File I/O: Cairo: A Vector Graphics Library</title>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bindings-streams"></a>Streams and File I/O</h2></div></div></div>
<p>
      Various places in the cairo API deal with reading and writing
      data, whether from and to files, or to other sources and
      destinations. In these cases, what is typically provided in the
      C API is a simple version that just takes a filename, and a
      complex version that takes a callback function.
      An example is the PNG handling functions:
    </p>
<pre class="programlisting">
cairo_surface_t *
cairo_image_surface_create_from_png (const char	*filename);

cairo_surface_t *
cairo_image_surface_create_from_png_stream (cairo_read_func_t read_func,
					    void             *closure);

cairo_status_t
cairo_surface_write_to_png (cairo_surface_t	*surface,
			    const char		*filename);

cairo_status_t
cairo_surface_write_to_png_stream (cairo_surface_t	*surface,
				   cairo_write_func_t	write_func,
				   void			*closure);</pre>
<p>
      The expectation is that the filename version will be mapped
      literally in the language binding, but the callback version
      will be mapped to a version that takes a language stream
      object. For example, in Java, the four functions above
      might be mapped to:
    </p>
<pre class="programlisting">
static public ImageSurface createFromPNG (String filename) throws IOException;
static public ImageSurface createFromPNG (InputStream stream) throws IOException;
public void writeToPNG (String filename) throws IOException;
public void writeToPNG (OutputStream stream) throws IOException;
</pre>
<p>
      In many cases, it will be better to
      implement the filename version internally
      using the stream version, rather than building it on top of the
      filename version in C. The reason for this is that will
      naturally give a more standard handling of file errors for
      the language, as seen in the above Java example, where
      <code class="methodname">createFromPNG()</code> is marked as raising
      an exception. Propagating exceptions from inside the callback
      function to the caller will pose a challenge to the language
      binding implementor, since an exception must not propagate
      through the Cairo code. A technique that will be useful in
      some cases is to catch the exception in the callback,
      store the exception object inside a structure pointed to by
      <em class="parameter"><code>closure</code></em>, and then rethrow it once
      the function returns.
    </p>
<p class="remark"><em><span class="remark">
      I'm not sure how to handle this for
      <a class="link" href="cairo-PDF-Surfaces.html#cairo-pdf-surface-create-for-stream" title="cairo_pdf_surface_create_for_stream ()"><code class="function">cairo_pdf_surface_create_for_stream()</code></a>.
      Other than keep a “exception to rethrow” thread-specific
      variable
      that is checked after <span class="emphasis"><em>every</em></span> call to a Cairo
      function.
    </span></em></p>
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