Software updates typically require stopping and restarting anapplication, but many systems cannot afford to halt service, or wouldprefer not to. Dynamic software updating (DSU) addresses thisdifficulty by permitting programs to be updated while they run. DSUis appealing compared to other approaches for on-line upgrades becauseit is quite general and requires no redundant hardware. The challengeis in making DSU practical: it should be flexible, and yetsafe, efficient, and easy to use.In this paper, we present a DSU implementation for C that aims tomeet this challenge. We compile programs specially so that they canbe dynamically patched, and generate most of a dynamic patchautomatically. Our compiler performs a series of analyses that whencombined with some simple runtime support ensure that an update willnot violate type-safety while guaranteeing that data is keptup-to-date. We have used our system to construct and dynamicallyapply patches to three substantial open-source serverprograms - Very Secure FTP daemon, OpenSSH sshd daemon, andGNU Zebra. In total, we dynamically patched each program withthree years' worth of releases. Though the programs changedsubstantially, the majority of updates were easy to generate.Performance experiments show that all patches could be applied in lessthan 5 ms, and that the overhead on application throughput due toupdating support ranged from 0 to at most 32%.
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