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:

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);

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:

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;

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 createFromPNG() 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 closure, and then rethrow it once the function returns.

I'm not sure how to handle this for cairo_pdf_surface_create_for_callback(). Other than keep a “exception to rethrow” thread-specific variable that is checked after every call to a Cairo function.