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Semaphore text, in my java. The take method sends a signal which is stored internally in the Semaphore. The release method waits for a signal.
When received the signal flag is cleared again, and the release method exited. Using a semaphore like this you can avoid missed signals.
You will call take instead of notify and release instead of wait. If the call to take happens before the call to release the thread calling release will still know that take was called, because the signal is stored internally in the signal variable.
This is not the case with wait and notify. The names take and release may seem a bit odd when using a semaphore for signaling.
The names origin from the use of semaphores as locks, as explained later in this text. In that case the names make more sense. Here is a simplified example of two threads signaling each other using a Semaphore:.
The Semaphore implementation in the previous section does not count the number of signals sent to it by take method calls. We can change the Semaphore to do so.
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 operations , i. However, the remainder of this section refers to semaphores with unary V and P operations, unless otherwise specified.
To avoid starvation , a semaphore has an associated queue of processes usually with FIFO semantics. 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 a 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 station 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 color , the train station cannot be accessed.
Whenever a user logs in, P is called, decrementing the semaphore S by 1. Whenever a user logs out, V is called, incrementing S by 1 representing a login slot that has become available.
Whenever S becomes greater than 0 login slots available , a login request is dequeued, and the user owning the request is allowed to log in. In the producer—consumer problem , one process the producer generates 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: To maintain integrity, emptyCount may be lower but never higher than the actual number of empty places in the queue, and fullCount may be lower but never higher than the actual number of items in the queue.
Empty places and items represent two kinds of resources, empty boxes and full boxes, and the semaphores emptyCount and fullCount maintain control over these resources.
The binary semaphore useQueue 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 queue simultaneously, thereby corrupting its internal state.
Alternatively a mutex could be used in place of the binary semaphore. Students must request a room from the front desk if they wish to use 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.
When a student requests a room, the clerk decreases this number. When a student releases a room, the clerk increases this number.
The room can be used for as long as desired, and so it is not possible to book rooms ahead of time. The value of the semaphore in this scenario is initially 10, with all rooms empty.
When a student requests a room, they are 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 current value of the semaphore is 0,  they are forced to wait until a room is freed when the count is increased from 0.
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 flipping a coin.
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.
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.
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. 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.
Even 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.
Operation V increments the semaphore S , and 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.