  Links are an essential feature of the World Wide Web, and source code
repositories are no exception. However, despite their many undisputed benefits,
links can suffer from decay, insufficient versioning, and lack of bidirectional
traceability. In this paper, we investigate the role of links contained in
source code comments from these perspectives. We conducted a large-scale study
of around 9.6 million links to establish their prevalence, and we used a
mixed-methods approach to identify the links' targets, purposes, decay, and
evolutionary aspects. We found that links are prevalent in source code
repositories, that licenses, software homepages, and specifications are common
types of link targets, and that links are often included to provide metadata or
attribution. Links are rarely updated, but many link targets evolve. Almost 10%
of the links included in source code comments are dead. We then submitted a
batch of link-fixing pull requests to open source software repositories,
resulting in most of our fixes being merged successfully. Our findings indicate
that links in source code comments can indeed be fragile, and our work opens up
avenues for future work to address these problems.
