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What are semaphores?

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βˆ™ 2006-07-08 22:19:50

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Click on the link to your right for the answer. Signaling devices, typically to give permission to go ahead or to stop, but can also have other meanings. On a rail line, a lifted green flag can be the semaphore that means to go ahead. Traffic lights are a kind of semaphore. In computer RS232 communication lines the RTS and CTS lines are sometimes called semaphores.

2006-07-08 22:19:50
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Related Questions

What are Linux semaphores?

Search Google with: "Linux semaphores" and have a look through the websites on the subject.

How many semaphores are used in the producer and consumer problem?

As the use of semaphores in the past has caused more problems than created solutions therefore semaphores are not use in the producer and consumer problem.

How do semaphores work?

Like condoms.

What makes semaphores unusual?

Their efficiency...

What is the definition of the word 'Semaphores'?

There are a number of ways the word semaphores might be used in a sentence. The definition of this word is that it is a system of sending messages using a series of flags in certain positions, signifying different messages with several meanings.

What do you call the system of signaling with flags?

The use of semaphores

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.

Discuss the concept and importance of semaphores?

n programming, especially in Unix systems, semaphores are a technique for coordinating or synchronizing activities in which multiple processes compete for the same operating system resources. A semaphore is a value in a designated place in operating system (or kernel) storage that each process can check and then change. Depending on the value that is found, the process can use the resource or will find that it is already in use and must wait for some period before trying again. Semaphores can be binary (0 or 1) or can have additional values. Typically, a process using semaphores checks the value and then, if it using the resource, changes the value to reflect this so that subsequent semaphore users will know to wait.Semaphores are commonly use for two purposes: to share a common memory space and to share access to files. Semaphores are one of the techniques for inter-process communication (IPC). The C programming language provides a set of interfaces or "functions" for managing semaphores.

What were traffic lights installed in 1868 in England called?

They were called Semaphores.

What is the difference between binary and general semaphores?

the differents is the bathroom time

What are the types of semaphores?

Three types of semaphores: 1.General/Counting semaphores: (can take any non-negative value) These are used when you might have multiple devices (like 3 printers or multiple memory buffers). 2.Binary semaphores: (can either be 0 or 1) These are used to gain exclusive access to a single resource (like the serial port, a non-reentrant library routine, or a hard disk drive). A counting semaphore that has a maximum value of 1 is equivalent to a binary semaphore (because the semaphore's value can only be 0 or 1). 3.Mutex semaphores: These are optimized for use in controlling mutually exclusive access to a resource. There are several implementations of this type of semaphore.

What is spinlock?

Semaphores are a useful tool for mutual exclusion, but they are not the only such tool provided by the kernel. Instead, most locking is implemented with a mechanism called a spinlock. Unlike semaphores, spinlocks may be used in code that cannot sleep, such as interrupt handlers. When properly used, spinlocks offer higher performance than semaphores in general. They do, however, bring a different set of constraints on their use.

What is semaphores?

Semaphores are any means of communication when flags or lights are employed to communicate . For example , semaphore flags are used to convey messages from one ship to another and traffic lights are a means in which to communicate a signal to traffic . See related link below .

Why were railroad semaphores always very high?

They weren't = some were on the ground. The height was for long distance visibility.

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Because they are completely unrelated things? Synchronization can be implemented with semaphores or mutexes.

Is everything in Linux is treated as a file?

No, of course not. For example processes, memory-pages, environment variables and semaphores are not treated as files.

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 is semaphores and perimeter same?

They are not. Semaphore is a method of sending signals by using flags or lights whereas perimeter is a geometric property of closed shapes!

What is the similarity between semaphore and monitor?

:) :)The wait and signal operations on condition variables in a monitor are similar to P and Voperations on counting semaphores. A wait statement can block a process's execution, while a signal statement can cause another process to be unblocked.Both provide mutual exclusion.Monitors and Semaphores, both are used to solve the synchronization problems

How do the ARPANet differ from the traditional computer networks before it?

ARPA stands for Ask and Respond Public Announce transmitted through a network of military semaphores.

What does ipcs command do in unix?

It lists the contents of several ipc queues, including semaphores, shared memory segments, etc. It can also remove items from these queues.

What are the features of xenix?

The features of Xenix include its AT&T Unix base, supporting libraries, and text editor. Other features of Xenix are semaphores and file locking.

What is the strength and weakness of semaphore and monitor?

Monitors provide an easier programming technique to provide mutal exclusion, whereas the use of semaphores are tricker and more prone to programmer error.

Advantages and disadvantages of semaphore?

In semaphores there is no spinning, hence no waste of resources due to no busy waiting. That is because threads intending to access the critical section are queued. And could access the priority section when the are de-queued, which is done by the semaphore implementation itself, hence, unnecessary CPU time is not spent on checking if a condition is satisfied to allow the thread to access the critical section.Semaphores permit more than one thread to access the critical section, in contrast to alternative solution of synchronization like monitors, which follow the mutual exclusion principle strictly. Hence, semaphores allow flexible resource management.Finally, semaphores are machine independent, as they are implemented in the machine independent code of the microkernel services.DisadvantagesProblem 1: Programming using Semaphores makes life harder as utmost care must be taken to ensure Ps and Vs are inserted correspondingly and in the correct order so that mutual exclusion and deadlocks are prevented. In addition, it is difficult to produce a structured layout for a program as the Ps and Vs are scattered all over the place. So the modularity is lost. Semaphores are quite impractical when it comes to large scale use.Problem 2: Semaphores involve a queue in its implementation. For a FIFO queue, there is a high probability for a priority inversion to take place wherein a high priority process which came a bit later might just have to wait when a low priority one is in the critical section. For example, consider a case when a new smoker joins and is desperate to smoke. What if the agent who handles the distribution of the ingredients follows a FIFO queue (wherein the desperate smoker is last according to FIFO) and chooses the ingredients apt for another smoker who would rather wait some more time for a next puff?

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.