using a resource without requesting it first (or after releasing it).Įven if all processes follow these rules, multi-resource deadlock may still occur when there are different resources managed by different semaphores and when processes need to use more than one resource at a time, as illustrated by the dining philosophers problem.Ĭounting semaphores are equipped with two operations, historically denoted as P and V (see § Operation names for alternative names).holding a resource for a long time without needing it.releasing a resource that was never requested.requesting a resource and forgetting to release it.Fairness and safety are likely to be compromised (which practically means a program may behave slowly, act erratically, hang or crash) if even a single process acts incorrectly. The success of the protocol requires applications to follow it correctly. The librarian above may turn the lights off in the study hall when there are no students remaining, or may place a sign that says the rooms are very busy when most of the rooms are occupied. The paradigm is especially powerful because the semaphore count may serve as a useful trigger for a number of different actions. Some other mechanism (possibly involving more semaphores) may be required to select a particular free resource. When used to control access to a pool of resources, a semaphore tracks only how many resources are free it does not keep track of which of the resources are free. And of course, a student needs to inform the clerk about releasing their room only after really leaving it, otherwise, there can be an awkward situation when such student is in the process of leaving the room (they are packing their textbooks, etc.) and another student enters the room before they leave it. If one of the rooms was released, but there are several students waiting, then any method can be used to select the one who will occupy the room (like FIFO or randomly picking one). If someone requests a room and the current value of the semaphore is 0, they are forced to wait until a room is freed (when the count is increased from 0). After the next student comes, it drops to 8, then 7 and so on. When a student requests a room, they are granted access, and the value of the semaphore is changed to 9. The value of the semaphore in this scenario is initially 10, with all rooms empty. In this scenario the front desk count-holder represents a counting semaphore, the rooms are the resource, and the students represent processes/threads. The room can be used for as long as desired, and so it is not possible to book rooms ahead of time. When a student releases a room, the clerk increases this number. When a student requests a room, the clerk decreases this number. In the simplest implementation, the clerk at the front desk knows only the number of free rooms available, which they only know correctly if all of the students actually use their room while they have signed up for them and return them when they're done. When a student has finished using a room, the student must return to the desk and indicate that one room has become free. If no rooms are free, students wait at the desk until someone relinquishes a room. Students must request a room from the front desk if they wish to use a study room. Suppose a physical library has 10 identical study rooms, to be used by one student at a time. That system eventually became known as THE multiprogramming system. The semaphore concept was invented by Dutch computer scientist Edsger Dijkstra in 1962 or 1963, when Dijkstra and his team were developing an operating system for the Electrologica X8. 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 and are used to implement locks. Semaphores are a useful tool in the prevention of race conditions however, their use is not a guarantee that a program is free from these problems. A trivial semaphore is a plain variable that is changed (for example, incremented or decremented, or toggled) depending on programmer-defined conditions.Ī useful way to think of a semaphore as used in a real-world system is as a record of how many units of a particular resource are available, coupled with operations to adjust that record safely (i.e., to avoid race conditions) as units are acquired or become free, and, if necessary, wait until a unit of the resource becomes available. Semaphores are a type of synchronization primitive. In computer science, a semaphore is a variable or abstract data type used to control access to a common resource by multiple threads and avoid critical section problems in a concurrent system such as a multitasking operating system.
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