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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.
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
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
When was Semaphore first used?
Semaphore communication was first used in the late 18th century by Claude Chappe in France, around the 1790s. Chappe developed a system of visual signals using large mechanical arms to communicate messages over long distances.
When was semaphore used?
Semaphore was invented in 1648 by Robert Hooke. The first use was by Claude Chappe in 1792 in France. The system conveyed messages from city to city by relay stations or towers.
What items are used to send messages in Semaphore code?
Semaphore is transmitted via flags. One is held in each hand. The positions of the arms determine the letter. Semaphore can also be transmitted via paddles or blades on towers.
Semaphore in operating system?
Semaphore data type used to maneuver access to multiple sources. The sources usually pertain to parallel programming, or multi user networks.
How many semaphore flag methods are there?
there are 28 semaphore flag methods.
Who when where and why was invented semaphore?
It is believed that semaphore was invented in the year 1684 by an English scientist Robert Hooke.By C pascoe 2008 from Bankstown grammar school B)* Added - The first use of semaphore signals for practical use was by Claude Chappe, in France, in 1792. Chappe is acknowledged as the inventor of the semaphore. He developed the system to convey messages from city to city within France by a series of relay stations similar to the way a telegraph was used n later years. Prior to this development, messages could take more than a day to go from point-to-point that could be reached in less than 2 minutes under his system.
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 use semaphore in an easy sentence?
The meaning of semaphore is to give information by using visual signals so when using it in a sentence, it must be used to replace the explanation of the signals. An example of a sentence is the airport grounds crew uses semaphores to guide the pilot of the plane.
What is Semaphore used for?
you use your hand to go up and down on the rod which can get slimmy
