Basic Concept

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The goal of this tutorial is explain how semaphores can be used to solved synchronization problems, which arise through cooperation between processes.

The tutorial will start with the basics on creating and setting-up semaphores, then tackle the most basic use of semaphores, to protect critical sections of code. Finally, the Bounded Buffer Problemdescribed in the tutorial on Cooperation binary semaphore wait and signal, will be tackled.

I assume you have read the tutorial on cooperating processes, and Interprocess Communication IPC facilities provided by the Operating System for process cooperation. The most synchronization problem confronting cooperating processes, is controling access to shared resource.

Binary semaphore wait and signal two processes share access to binary semaphore wait and signal file, or shared memory segment or when we discuss threads in a couple of weeks, we'll see threads share the same memory, so they must synchronize their actionsand at least one of these processes can modify the data in this shared area of memory.

That part of the code of each program, binary semaphore wait and signal one process is reading from or writing to a shared memory area, is a critical section of code, because we must ensure that only one process execute a critical section of code at a time.

The Critical Section Problem is to design a protocol that the processes can use to coordinate their activities when one wants to enter its critical section of code. Suppose we have two processes that share a memory segment of four bytes which stores an integer value. Let this value be named by the variable V. Process 1 P1 and Process 2 P2 have a section of code with the following lines: Here is an example of how the processes can interfere with each other: The protocol developed for solving the Critical Section Problem involved three steps: Before entering the critical section, a process must request permission to enter Critical section: After permission is granted, a process may execute the code in the critical section.

Other processes respect the request, and keep out of their critical sections. The process acknowledges it has left its critical section. The problem that remains is how to effectively implement this protocol. How can the processes communicate their requests and grant their permissions so that only one process at a time is in a critical section. The Dutch scientist E. Dijkstra showed how to solve the Critical Section Problem in the mid-sixties, and introduced the concept of a semaphore to control synchronization.

A semaphore is an integer variable which is accessed through through two special operations, called wait and signal.

Why we need special operations will be discussed shortly. Originally, Dijkstra called the two special operations P for proberenthe dutch word for test and V for verhogenthe dutch word to increment. When S is 1, P is allowed to enter its critical section While P is in its critical section, the value of S is zero, blocking other processes from entering their critical section. When P is finished and ready to leave its critical section, it executes signal resetting S to 1 and allowing another process to enter.

A semaphore which can take the value zero or one is called a binary semaphore, or mutexfor mut ually ex clusive. There is a glaring weakness with this implementation: For the semaphore to work, S must be shared, so it is tested and changed in waitand this requires synchronization between the processes; so how does this binary semaphore wait and signal the original problem?

The problem we have is that we have some lines of code which need to be executed without interference from other processes; this is no problem as long as a process has control of the CPU, but if it is taken-off the CPU by the Operating System before it can finish executing its section of code, another process may have an opportunity to trash its work. The code must be run atomically: So, for semaphores to solve the problem, the implementation of the two functions wait and signal must be atomic: Operating systems, like Unix, which have semaphores, guarantee that the operations wait and signal are run atomically.

How can they do this? So, the Operating System keeps track of the semaphore and its value: The Operating System then steps in and performs the operation, without interruption. Unix System V introduced semaphores to Unix in the mid-eighties. Since the Operating System maintains semaphores and operates on them, the actual code required is rather complicated.

To add to this confusion, a Unix semaphore is not really a single integer value, but an array of integer values. Binary semaphore wait and signal see later why we may want to use more than one semaphore. I will refer to Unix semaphores as semaphore structures, since they include possibly multiple semaphores. Semaphores are to be shared by processes which are unrelated to each other. This means that they need to have some means of coordinating their access to the same semaphore.

The way this is done is for all processes which will use the semaphore to exchange a shared key value. This key value is defined the same way in each process. The value of the key can be any integer, although if there already exists a semaphore structure with that keythat semaphore will be used.

One way to be sure that your key is unique is to see what semaphore structures the system already has set-up. Binary semaphore wait and signal ipcs -sem on the command line will list all semaphore structures. Unix only allows a few semaphore structures at a binary semaphore wait and signal a maximum of 10 in the whole systemso most any key you choose is likely to be unique.

There are five basic steps in allocating a semaphore structure: Request a semaphore structure, using the common key Initialize the semaphore structure by setting the values of each semaphore Define the basic operations we want to perform on the semaphore structure Use the basic operations in our program Remove the semaphore structure when we are done with it.

The first step is a request of a semaphore structure. Once you have the semaidyou initialize the structure, by setting the value of every semaphore in the set. The function for doing this is semctl which is a catch-all for alot of different functions. You will want to see the manpages: This union structure is not binary semaphore wait and signal defined in any of the header files, so you will have to define the structure in your program before you call main: We use a special structure type, struct sembufto define the basic operations we want to define on the semaphore set.

Here is the structure: This refers to the particular semaphore we are going to adjust in the semaphore structure. The number of semaphores, numsems was determined when the semaphores structure was created, using semget.

Counting starts at 0. The operation we will perform on the semaphore. If the binary semaphore wait and signal would be less than zero, the process will wait for the value to become large enough so that the sum is at least zero. Suppose your process decrements a semaphore, taking its value to zero, but terminates before it can reset the semaphore back to one.

Any process waiting for the semaphore to be one again would be stuck. The Operating System takes care of this if this flag is set. Lets look at how we would set-up the semaphore binary semaphore wait and signal wait and signal. Suppose we have created a semaphore structure having one semaphore, with semaphore ID semaidand this semaphore has been set to an initial value one.

Removing a semaphore structure can only be done by the user ID of the creator of the semaphore structure. You will want to refresh your memory Bounded Buffer Problem. We now add an additional feature to our Critical Section Problem: One possible solution to this problem is to treat use a single binary semaphore just as in the Critical Section Problem together with a variable COUNTERwhich is shared in common say as a shared memory segment.

Here is code for the Consumer: This presents one solution to the Bounded Buffer Problemand in many circumstances it may be perfectly fine. This creates what is called a spinlockwhere the process enters a useless cycle waiting for a resource. If the resource will be available quickly, this is not a problem; but if the binary semaphore wait and signal may take some time, this spinlock position wastes CPU time that could be more productively spent.

A solution to this problem is to use a semaphore to count as well. A semaphore which is used as a counter is called a counting semaphore. It is pretty clear how we can use a counting semaphore to restrict the Consumer's behavior: If the count goes to zero, Consumers are forced to wait.

Unfortunately, Unix System V semaphores effectively have no upper limit, so we cannot use just one counter to control both Consumers and Producers. Here is how we can implement a counter using semaphores. Binary semaphore wait and signal use two semaphores to implement our counter: Mainly to remove a semaphore structure from the system, and to set the binary semaphore wait and signal of each semaphore in the structure.

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Wikipedia has related information at Semaphores. A semaphore , in its most basic form, is a protected integer variable that can facilitate and restrict access to shared resources in a multi-processing environment.

The two most common kinds of semaphores are counting semaphores and binary semaphores. Counting semaphores represent multiple resources, while binary semaphores, as the name implies, represents two possible states generally 0 or 1; locked or unlocked. Semaphores were invented by the late Edsger Dijkstra. Semaphores can be looked at as a representation of a limited number of resources, like seating capacity at a restaurant.

If a restaurant has a capacity of 50 people and nobody is there, the semaphore would be initialized to As each person arrives at the restaurant, they cause the seating capacity to decrease, so the semaphore in turn is decremented.

When the maximum capacity is reached, the semaphore will be at zero, and nobody else will be able to enter the restaurant. Instead the hopeful restaurant goers must wait until someone is done with the resource, or in this analogy, done eating. When a patron leaves, the semaphore is incremented and the resource becomes available again.

A semaphore can only be accessed using the following operations: This would be equivalent to the arriving customer trying to get an open table. If there is an open table, or the semaphore is greater than zero, then he can take that resource and sit at the table.

If there is no open table and the semaphore is zero, that process must wait until it becomes available. The following is an implementation of this counting semaphore where the value can be greater than The standard Java library instead uses the name "acquire" for P and "release" for V. No other process can access the semaphore when P or V are executing. This is implemented with atomic hardware and code. An atomic operation is indivisible, that is, it can be considered to execute as a unit.

If there is only one count of a resource, a binary semaphore is used which can only have the values of 0 or 1. They are often used as mutex locks. Here is an implementation of mutual-exclusion using binary semaphores:. In this implementation, a process wanting to enter its critical section it has to acquire the binary semaphore which will then give it mutual exclusion until it signals that it is done.

For example, we have semaphore s, and two processes, P1 and P2 that want to enter their critical sections at the same time. P1 first calls wait s. The value of s is decremented to 0 and P1 enters its critical section. While P1 is in its critical section, P2 calls wait s , but because the value of s is zero, it must wait until P1 finishes its critical section and executes signal s. When P1 calls signal, the value of s is incremented to 1, and P2 can then proceed to execute in its critical section after decrementing the semaphore again.

Mutual exclusion is achieved because only one process can be in its critical section at any time. As shown in the examples above, processes waiting on a semaphore must constantly check to see if the semaphore is not zero. This continual looping is clearly a problem in a real multiprogramming system where often a single CPU is shared among multiple processes.

This is called busy waiting and it wastes CPU cycles. When a semaphore does this, it is called a spinlock. To avoid busy waiting, a semaphore may use an associated queue of processes that are waiting on the semaphore, allowing the semaphore to block the process and then wake it when the semaphore is incremented.

The state of the process is switched to the waiting state, and control is transferred to the CPU scheduler, which selects another process to execute. When another process increments the semaphore by calling signal and there are tasks on the queue, one is taken off of it and resumed.

In a slightly modified implementation, it would be possible for a semaphore's value to be less than zero. When a process executes wait , the semaphore count is automatically decremented. The magnitude of the negative value would determine how many processes were waiting on the semaphore:.

From Wikibooks, open books for an open world. There is 1 pending change awaiting review. Semaphores [ edit ] Wikipedia has related information at Semaphores A semaphore , in its most basic form, is a protected integer variable that can facilitate and restrict access to shared resources in a multi-processing environment.

The following is an implementation of this counting semaphore where the value can be greater than 1: Retrieved from " https: Views Read Latest draft Edit View history. Policies and guidelines Contact us.

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