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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
Why was semaphore first used?
Semaphore was first used as a means of long-distance communication, allowing messages to be transmitted quickly over land and sea. Developed in the late 18th century, it utilized visual signals, such as flags or mechanical arms, to convey information between stations. This system was particularly useful for military and naval operations, where rapid communication was essential for coordinating movements and strategies. Its effectiveness in overcoming geographical barriers made it a significant advancement in communication technology at the time.
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.
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.
What are semaphore flags?
YELLOW and RED
What do you call the code in which you have to wave flags?
Semaphore
What colours are Semaphore flags?
YELLOW and RED
What is the name of the comminication system that uses flags?
semaphore
Why was semaphore invented?
Semaphore was invented as a visual signaling system to relay messages over long distances. It was developed to communicate with ships at sea or between different points on land where other methods of communication like smoke signals or flags were not practical. Semaphore systems used different flag positions to represent different letters or words and were widely used before the invention of the telegraph.
Is Naval semaphore which uses flags to communicate a type of telegraph?
True
What is the name given to the process of signaling with two flags?
Semaphore is one such system.
Where was the semaphore invented?
It was invented in France. It was popular in the late 18thC
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.
What do you call the flag used in Morse code in the Philippines?
Morse code is an audio code (or blinking light code), it has nothing to do with flags.When flags are used to send messages, this is called semaphore, so they would be "semaphore flags".See the related links below.
Why did Robert Hooke invented semaphore?
The year 1967.
