Semaphores/Sockets

Semaphores/Sockets

1. Concurrency = Processes executed simultaneously ie. several apps running on one machine

2. Issue? - Uncontrolled access to shared data ie. this may allow two people to book the same tickets at a concert

3. To combat this? - Only one process allowed in the critical section at any one time

4. Processes compete for the same, shared, resources - Without synchronisation unpredictable and incorrect executions can result

   • Synchronisation = Mutual exclusion of any process not in the critical section
     • Synchronisation is implimented by the use of Semaphores
       • Semaphores = An object with methods Called by processes sharing a resource
         • There are two types of semaphores Binary semaphore, either 0 or 1 that is essentially a flag to signal when using critical region OR Counting semaphore, > 1 Basic implementation every process issues a wait on entering critical section and Signal finished on exiting
           • See producer/consumer problem

5. A socket is an endpoint for communication

6. Two communicating processes over a network each use a socket to identify them

7. Socket defined by port number and ip address

8. How does this work? - Client-server architecture A server listens for client requests When a request is received the server uses the socket details to set up a connection.

9. More... - Well-known services: telnet:23, ftp:21, http:80 --- All ports below 1024 are considered well-known --- The client will allocate the port number to the process initiating communication --- Random port greater than 1024.

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