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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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