Dave Beckett's blog

Flickcurl - C API to Flickr

2007-08-03 22:59

In January 2007 just for fun I started writing a C API to Flickr using the Flickr web services called Flickcurl. The name was because it was originally built using Flickr via libCurl to do the HTTP work ... although right now it contains more use of libxml than of libcurl.

I started this for a bunch of reasons, including to learn more about "web 2.0" web APIs, see how RESTy the Flickr API really is (Answer: not much, it's very much an RPC model) and the issues with developing a Web API. It's clear this is an evolved and evolving one since now and then I discover undocumented returned attributes in the XML and cases where it is not clear why attributes were used instead of elements. It's very suited towards dynamic scripting languages where it is easy to pass around dictionaries / hashes / associative arrays of parameters that can grow. So in some sense, making something feel like a natural API in a static language like C is rather going against the grain and rather slow work.

There are, however, things available to help. There are method reflection APIs so I wrote a code generating program that can nicely automate writing many of the simpler calls that return no value or just a single one. I also used a lot of similar patterns so that parsing tags xml is quite similar to parsing comments xml. The XML is primarily read via XPath and a little DOM.

One other nice thing about this is that this a piece of work with a fixed size, albeit growing slowly. The Flickr API currently has 104 calls - depending on how you measure them - so it's easy to check progress, and that's how I've been doing it. I built tools to read the docu-comments (javadoc, gnome-doc, kernel-doc style) and mark the Flickcurl coverage release by release.

The news today is that I have reached the half way point: 50% of the APi with the release of Flickcurl 0.11 at least until they add something more! I have also done most of what I think are the trickier parts - the uploading, searching and getting info about photos. The remaining API parts are more regular, so I feel like I'm coding downhill now.

Now there's something else it does - and this won't be a surprise to most given my interests. Flickcurl generates RDF descriptions from Flickr photos with a flickrdf utility, including reading Machine Tags. The namespaces are either well known ones, or invented by me, pointing at the machinetags.org wiki - you can create your own definition.

flickrdf uses Raptor to do nicer serializing when it is available. So this means I can turn jellyfish into Turtles. W00t! (*)

$ ./flickrdf -o turtle http://www.flickr.com/photos/dajobe/196308964/
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix foaf: <http://xmlns.com/foaf/0.1/#> .
@prefix geo: <http://www.w3.org/2003/01/geo/wgs84_pos#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

    dc:creator [
        a foaf:Person;
        foaf:maker <http://www.flickr.com/photos/dajobe/196308964/>;
        foaf:name "Dave Beckett";
        foaf:nick "dajobe"
    dc:dateSubmitted "2006-07-23T18:16:13Z"^^xsd:dateTime;
    dc:rights <http://creativecommons.org/licenses/by-nc-sa/2.0/>;
    dc:modified "2007-02-25T07:45:46Z"^^xsd:dateTime;
    dc:issued "2006-07-23T18:16:13Z"^^xsd:dateTime;
    dc:created "2006-07-23T05:28:50Z"^^xsd:dateTime;
    geo:lat "36.620487";
    geo:long "-121.904468";
    dc:title "Jellyfish at Monterey Aquarium";
    dc:subject "jellyfish" .

After that bad joke (and it could have been worse if I had a picture of a Turtle) here's what you need to know. Get it at

flickcurl-0.11.tar.gz (md5sum eea351e4d35e8d1c63b124cd8ee257ba, sha1sum d220f6371c0c5334c824a51ba848d9358d73e533) or the latest in the Flickcurl Subversion It's licensed under the GPL2 / LGPL2 / Apache 2.0 or any newer versions of any of them.

Note: I work for Yahoo! and although Flickr is part of Yahoo! this project is my own personal work.

(*) Actually I'm slightly cheating with this example, there's a couple of bug fixes in SVN after the release which are needed to get this output.