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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.
What is weak semaphore and strong semaphore?
strong semaphores specify the order in which processes are removed from the queue, which guarantees avoiding starvation. Weak semaphores do not specify the order in which processes are removed from the queue.
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.
Advantages and disadvantages of semaphore in os?
Advantage of semaphore is simplicity. Disadvantage of semaphore is more prone to programmer error. It does not guarantee that if programmer misplace the P and V then it will also work correctly. It may occur deadlock or violation of mutual exclusion due to programmer error.
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.
What is the role of semaphore in critical section problem?
janina
Is semaphore synchronize critical resources to prevent deadlock?
yes
What is mutually exclusive?
It is one of thecharacteristics of deadlock. When semaphores are used or mutual exclusion, the semaphore has an initial value of 1, and P() is called before the critical section, and V() is called after the critical section as shown below : semaphore-> P(); critical section semaphore-> V(); remainder section let us suppose that one process A is already executing its critical section then it implies that semaphore value at that time is zero. If process B now tries to enter this critical section , it cannot enter the critical section because it will have to wait before semaphore becomes greater than zero. This is possible only when process A executes its signal operation; after executing its critical section.
What are semaphores in Unix?
A semaphore is like a signal that can indicate the start and end of something called a critical section. The semaphore ensures that only 1 thread of code is allowed at a time in the code.
What is Semaphore and state its implementation?
Semaphores are used to provide mutual exclusion and condition synchronization. Locks provide mutual exclusion and have special properties that make them useful in object-oriented programs Implementation: public interface semaphore { //creating the semaphore interface public void semWait(); //method signature public void semSignal(); //method signature } Semaphore lock = new semaphoreImplementation(1); //implementing the interface //non-critical instructions lock.semWait(); //method calling to enter critical region //access data // accessing shared data lock.semSignal(); // method calling to exit critical region //non-critical instructions
What are the things that a programmer must know in order to solve a problem?
a programmer must be resourceful and must have critical thinking
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.
What is weak semaphore and strong semaphore?
strong semaphores specify the order in which processes are removed from the queue, which guarantees avoiding starvation. Weak semaphores do not specify the order in which processes are removed from the queue.
How many semaphore flag methods are there?
there are 28 semaphore flag methods.
When was Semaphore - album - created?
Semaphore - album - was created on 1998-03-16.
