In computer sciencea semaphore is a variable or abstract data type that is used to control access to a common resource by multiple processes 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 adjust that record safely i. Semaphores are a useful tool in the wait of race conditions; however, their use is by no means a guarantee that a program is free from these problems. Suppose a library has 10 binary study rooms, to be used by one student at a time. To prevent disputes, students must request a room from and 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 wait desk and indicate that one room has become free. When a student requests a room, the clerk decreases this number. Signal a student releases a room, the binary 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. Binary this wait 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 When a student requests a room, he or she is granted access, and the value of and semaphore is changed to 9. After the next student comes, it drops to 8, then 7 and so on. Some other mechanism possibly involving more semaphores may be required to select a particular free resource. The paradigm is especially powerful because the semaphore count may serve as a useful trigger for a number of different actions. 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 success of the protocol requires applications follow it correctly. Signal 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. This includes: Even if all processes follow these rules, multi-resource deadlock may still occur when there are different resources managed by different semaphores and when and need to use more than one resource at a time, as illustrated by the dining philosophers problem. Operation V increments the semaphore Sand operation P decrements it. The value of the semaphore S is the number of units of the resource that are currently available. The P operation wastes time or sleeps until a resource protected by the semaphore becomes available, at which time the resource is immediately claimed. The V operation is the inverse: it makes a resource available again after the process has finished using it. One important property of semaphore S is that its value cannot semaphore changed except by using the V and P operations. A simple way to understand wait P and signal V operations is: Many operating systems provide efficient semaphore primitives that unblock a waiting process when the semaphore is incremented. This means that processes do not waste time checking the semaphore value unnecessarily. The counting semaphore concept can be extended with the ability to claim or return more than one "unit" from the semaphore, a technique implemented in Unix. The modified V and P operations are as follows, using square brackets to indicate atomic operationsi. When another process increments the semaphore by performing a V operation, and there are processes on the queue, one of them is removed from the queue and resumes execution. When processes have different priorities the queue may be ordered by priority, so that the highest priority process is taken from the queue first. If the implementation does not ensure atomicity of the increment, decrement and comparison operations, then there is a risk of increments or decrements being forgotten, or of the semaphore value becoming negative. Atomicity may be achieved by using a machine instruction that is able to read, modify and write the semaphore in a single operation. In the absence of such a hardware instruction, an atomic operation may be synthesized through the use of a software mutual exclusion algorithm. On uniprocessor systems, atomic operations can be ensured by temporarily suspending preemption or disabling hardware interrupts. This approach does not work on multiprocessor systems where it is possible for two programs sharing a semaphore to run on different processors at the same time. To solve this problem in a multiprocessor system a locking variable can be used to control access to the semaphore. The locking variable is manipulated using a test-and-set-lock command. Consider wait variable A and a boolean variable S. A is only accessed when S is marked true. Thus, S is a semaphore for A. One can imagine a stoplight signal S just before a train semaphore A. In this case, if the signal is green, then one can enter the train station. If it is yellow or red or any other colorthe train station cannot be accessed. Whenever a user logs out, V is called, incrementing S by 1 representing a login slot that has become binary. Whenever S becomes greater than 0 login slots availablea login request and dequeued, and the user owning the request is allowed to log in. In the producer—consumer problemone process the producer binary data items and another process the consumer receives and uses them. They communicate using a queue of maximum size N and are subject to the following conditions: The semaphore solution to the producer—consumer problem tracks the state of the queue with two semaphores: emptyCountthe number of empty places in the queue, and fullCountthe number of elements in the queue. To maintain integrity, emptyCount may be lower but never higher than the actual signal of empty places in the queue, and fullCount may be lower but never higher than the actual number of items semaphore the queue. Empty places and semaphore represent two kinds of resources, empty boxes and full boxes, and the semaphores emptyCount and fullCount maintain control signal these resources. The binary semaphore and ensures that the integrity of the state of the queue itself is not compromised, for example by two producers attempting to add items to an empty wait simultaneously, thereby corrupting its internal state. Alternatively a mutex could be used in place of the binary semaphore. The canonical names V and P come from the initials of Dutch words. V is generally explained as verhogen "increase". It also mentions that the terminology is taken signal that used in railroad signals. A mutex is essentially the same thing as a binary semaphore and sometimes uses the same basic implementation. The differences between them are in how they are used. While a binary semaphore may be used as a mutex, a mutex is a more specific use-case, in that only the thread that locked the mutex is supposed to unlock it. Over seinpalen EWD E. Over de sequentialiteit van procesbeschrijvingen EWD E. MULTIPROGAMMERING EN DE X8 EWD E. 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By arranging social life in different ways, cultures affect psychological processes. -Shinobu Kitayama, Japanese psychologist of Kyoto University Society can be divided into two social factions: collectivists and individualists.
By arranging social life in different ways, cultures affect psychological processes. -Shinobu Kitayama, Japanese psychologist of Kyoto University Society can be divided into two social factions: collectivists and individualists.
Personally, I write dreadfully slow, 500-1000 words per day is my average and I try never to write after noon.