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The semaphore solution for the Dining Philosophers problem is a bowl in the center of the table with two rocks in it. There is also a pile of four forks. (The fifth fork is useless.) When a philosopher wants to eat, he reaches in the bowl for a rock. If he finds one, then he can take two forks and proceed to eat spaghetti. When he is done eating, he first replaces the forks in the pile and then he replaces the rock in the bowl, after which he starts his thinking phase. If there is no rock in the bowl when he goes to get one, he must wait and try again. This is the blocked state.
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What is sem post and sem wait functions?
The sem_wait() function locks the semaphore referenced by sem by performing a semaphore lock operation on that semaphore. If the semaphore value is currently zero, then the calling thread will not return from the call to sem_wait()until it either locks the semaphore or the call is interrupted by a signal.Upon successful return, the state of the semaphore is locked and remains locked until the sem_post() function is executed and returns successfully. The sem_wait() function is interruptible by the delivery of a signal.The sem_post() function unlocks the semaphore referenced by sem by performing a semaphore unlock operation on that semaphore.If the semaphore value resulting from this operation is positive, then no threads were blocked waiting for the semaphore to become unlocked; the semaphore value is simply incremented.If the value of the semaphore resulting from this operation is zero, then one of the threads blocked waiting for the semaphore will be allowed to return successfully from its call to sem_wait(). If the symbol _POSIX_PRIORITY_SCHEDULING is defined, the thread to be unblocked will be chosen in a manner appropriate to the scheduling policies and parameters in effect for the blocked threads. In the case of the schedulers SCHED_FIFO and SCHED_RR, the highest priority waiting thread will be unblocked, and if there is more than one highest priority thread blocked waiting for the semaphore, then the highest priority thread that has been waiting the longest will be unblocked. If the symbol _POSIX_PRIORITY_SCHEDULING is not defined, the choice of a thread to unblock is unspecified.The sem_post() interface is reentrant with respect to signals and may be invoked from a signal-catching function.
Which is a high level abstraction over semaphore?
Monitor
What is the difference between MUTEX and Semaphore?
From wikipedia:"A mutex is a binary semaphore, usually including extra features like ownership or priority inversion protection. The differences between mutexes and semaphores are operating system dependent. Mutexes are meant to be used for mutual exclusion only and binary semaphores are meant to be used for event notification and mutual exclusion."They also have a good example as to the use of a semaphore:"A thread named A needs information from two databases before it can proceed. Access to these databases is controlled by two separate threads B, C. These two threads have a message-processing loop; anybody needing to use one of the databases posts a message into the corresponding thread's message queue. Thread A initializes a semaphore S with init(S,-1). A then posts a data request, including a pointer to the semaphore S, to both B and C. Then A calls P(S), which blocks. The other two threads meanwhile take their time obtaining the information; when each thread finishes obtaining the information, it calls V(S) on the passed semaphore. Only after both threads have completed will the semaphore's value be positive and A be able to continue. A semaphore used in this way is called a 'counting semaphore.'"Basically think of a semaphore as a lock that allows multiple threads to wait in line for the resource to be free. Usually they will block and the semaphore will wake them up when it is their turn.
How can semaphore be used to enforce mitual exclusion?
semaphore is a variable providing mutual exclusion in following manner- -It consist of two function called wait and signal, wait() { while(semav==0); semav--; } signal() { semav++; } where semav is semaphore. we apply wait() and signal() in following manner- while(true) { <entry section>; wait() <critical section> signal() <Exit section> } note that wait() always comes before signal(), a process is not allowed to execute critical section if the semaphore has a value 0 i.e. at most one process can execute critical section at a time.
Why was semaphore flags invented?
Samuel Garratt Burdett, Craig Burdett, Valarie Burdett, And John Burdett
How many semaphore flag methods are there?
there are 28 semaphore flag methods.
What is the initial value of semaphore?
The initial value of a semaphore is typically set by the programmer when the semaphore is initialized. This value determines the number of concurrent threads or processes that can access a shared resource protected by the semaphore at the same time.
When was Semaphore - album - created?
Semaphore - album - was created on 1998-03-16.
When did Semaphore railway line end?
Semaphore railway line ended in 1978.
When was Semaphore railway line created?
Semaphore railway line was created in 1882.
When was Smartlogic Semaphore Limited created?
Smartlogic Semaphore Limited was created in 2007.
How do you spell the philospher?
The Philosopher
How do you make semaphore in a sentence?
Since semaphore means some type of light then you could say, * Around the holidays many neighborhoods create a semaphore with their lights.
When was semaphore flags invented?
The original use of semaphore was invented around 1792 by Claude Chappe. The use of semaphore flags was brought about during the 1800's on ships at sea.
Who were philospher's?
Eat Mor Chikin'
What is difference between counting and binary semaphore in os?
Binary semaphore is a semaphore with the integer value ranges over 0 and 1 whereas the counting semaphore's integer value ranges over unrestricted domain. Binary semaphores are easier to implement comparing with the counting semaphore. Binary semaphore allows only one thread to access the resource at a time. But counting semaphore allows N accesses at a time. The 2 operations that are defined for binary semaphores are take and release. The 2 operations that are defined for counting semaphores are wait and signal
How do you use semaphore in a sentence?
Multiple threads must not initialize the same semaphore simultaneously.Upper Quadrant Signal A semaphore signal in which the arm was raised to indicate ' off ' or ' clear ' .If you don't have any proper semaphore flags, then you could make some.This view of Canterbury East signal box from the 1970s reveals both semaphore signal arms still in use.We have as yet no Semaphores on our coasts.
