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In computer science, a semaphore is a variable or abstract data type that is used for controlling access, by multiple processes, to a common resource in a concurrent system such as a multiprogramming operating system. A trivial semaphore is a plain variable that is changed (for example, incremented or decremented, or toggled) depending on programmer-defined conditions. The variable is then used as a condition to control access to some system resource. A useful way to think of a semaphore as used in the real-world systems is as a record of how many units of a particular resource are available, coupled with operations to ''safely'' (i.e., without race conditions) adjust that record as units are required or become free, and, if necessary, wait until a unit of the resource becomes available. Semaphores are a useful tool in the prevention of race conditions; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores. The semaphore concept was invented by Dutch computer scientist Edsger Dijkstra in 1962 or 1963,〔 (undated, 1962 or 1963)〕 and has found widespread use in a variety of operating systems. It has also been used as the control mechanism for I/O controllers, for example in the Electrologica X8 computer. ==Library analogy== Suppose a library has 10 identical study rooms, to be used by one student at a time. To prevent disputes, students must request a room from the front desk if they wish to make use of a study room. If no rooms are free, students wait at the desk until someone relinquishes a room. When a student has finished using a room, the student must return to the desk and indicate that one room has become free. In the simplest implementation, the clerk at the front desk does not need to keep track of which rooms are occupied or who is using them, nor does she know if any given room is actually being used, only the number of free rooms available, which she only knows correctly if all of the students actually use their room while they've signed up for them and return them when they're done. When a student requests a room, the clerk decreases this number. When a student releases a room, the clerk increases this number. Once access to a room is granted, the room can be used for as long as desired, and so it is not possible to book rooms ahead of time. In this scenario the front desk count-holder represents a counting semaphore, the rooms are the resources, and the students represent processes. The value of the semaphore in this scenario is initially 10. When a student requests a room she is granted access and the value of the semaphore is changed to 9. After the next student comes, it drops to 8, then 7 and so on. If someone requests a room and the resulting value of the semaphore would be negative,〔(''The Little Book of Semaphores'' ) Allen B. Downey〕 they are forced to wait until a room is freed (when the count is increased from 0). 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「Semaphore (programming)」の詳細全文を読む スポンサード リンク
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