  Stack Overflow hosts valuable programming-related knowledge with 11,926,354
links that reference to the third-party websites. The links that reference to
the resources hosted outside the Stack Overflow websites extend the Stack
Overflow knowledge base substantially. However, with the rapid development of
programming-related knowledge, many resources hosted on the Internet are not
available anymore. Based on our analysis of the Stack Overflow data that was
released on Jun. 2, 2019, 14.2% of the links on Stack Overflow are broken
links. The broken links on Stack Overflow can obstruct viewers from obtaining
desired programming-related knowledge, and potentially damage the reputation of
the Stack Overflow as viewers might regard the posts with broken links as
obsolete. In this paper, we characterize the broken links on Stack Overflow.
65% of the broken links in our sampled questions are used to show examples,
e.g., code examples. 70% of the broken links in our sampled answers are used to
provide supporting information, e.g., explaining a certain concept and
describing a step to solve a problem. Only 1.67% of the posts with broken links
are highlighted as such by viewers in the posts' comments. Only 5.8% of the
posts with broken links removed the broken links. Viewers cannot fully rely on
the vote scores to detect broken links, as broken links are common across posts
with different vote scores. The websites that host resources that can be
maintained by their users are referenced by broken links the most on Stack
Overflow -- a prominent example of such websites is GitHub. The posts and
comments related to the web technologies, i.e., JavaScript, HTML, CSS, and
jQuery, are associated with more broken links. Based on our findings, we shed
lights for future directions and provide recommendations for practitioners and
researchers.
