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In computer science, dynamic software updating (or ''DSU'') is a field of research pertaining to upgrading programs while they are running. DSU is not currently widely used in industry. However, researchers have developed a wide variety of systems and techniques for implementing DSU. These systems are commonly tested on real-world programs. Current operating systems and programming languages are typically not designed with DSU in mind. As such, DSU implementations commonly either utilize existing tools, or implement specialty compilers. These compilers preserve the semantics of the original program, but instrument either the source code or object code to produce a dynamically updateable program. Researchers compare DSU-capable variants of programs to the original program to assess safety and performance overhead. == Introduction == Any running program can be thought of a tuple , where is the current program state and is the current program code. Dynamic software updating systems transform a running program to a new version . In order to do this, the state must be transformed into the representation expects. This requires a ''state transformer'' function. Thus, DSU transforms a program to . An update is considered valid if and only if the running program can be reduced to a point tuple that is reachable from the starting point of the new version of the program, . The location in a program where a dynamic update occurs is referred to as an update point. Existing DSU implementations vary widely in their treatment of update points. In some systems, such as UpStare and PoLUS, an update can occur at any time during execution. Ginseng's compiler will attempt to infer good locations for update points, but can also use programmer-specified update points. Kitsune and Ekiden require developers to manually specify and name all update points. Updating systems differ in the types of program changes that they support. For example, Ksplice only supports code changes in functions, and does not support changes to state representation. This is because Ksplice primarily targets security changes, rather than general updates. In contrast, Ekiden can update a program to any other program capable of being executed, even one written in a different programming language. Systems designers can extract valuable performance or safety assurances by limiting the scope of updates. For example, any update safety check limits the scope of updates to updates which pass that safety check. The mechanism used to transform code and state influences what kinds of updates a system will support. DSU systems, as tools, can also be evaluated on their ease-of-use and clarity to developers. Many DSU systems, such as Ginseng, require programs to pass various static analyses. While these analyses prove properties of programs that are valuable for DSU, they are by nature sophisticated and difficult to understand. DSU systems that do not use a static analysis might require use of a specialized compiler. Some DSU systems require neither static analysis nor specialty compilers. Programs that are updated by a DSU system are referred to as target programs. Academic publications of DSU systems commonly include several target programs as case studies. vsftpd, OpenSSH, PostgreSQL, Tor, Apache, GNU Zebra, memcached, and Redis are all dynamic updating targets for various systems. Since few programs are written with support for dynamic updating in mind, retrofitting existing programs is a valuable means of evaluating a DSU system for practical use. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dynamic software updating」の詳細全文を読む スポンサード リンク
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