{"id":2630,"date":"2023-12-23T18:18:49","date_gmt":"2023-12-23T17:18:49","guid":{"rendered":"https:\/\/oopm.org\/?page_id=2630"},"modified":"2024-11-26T14:36:01","modified_gmt":"2024-11-26T13:36:01","slug":"11-3-implementing-a-monitor-system","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=2630","title":{"rendered":"16.3 Implementing a Monitor System"},"content":{"rendered":"
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Here we will show how to implement a Monitor<\/code> system using preemptive coroutines and Semaphore.<\/p>\n\n\n\n

Below we show our first version of class Monitor<\/code>:<\/p>\n\n\n\n

class<\/strong> Monitor:\n   entry:\n      mutex.wait\n      inner(entry)\n      mutex.signal\n   %private\n   mutex: obj<\/strong> Semaphore(1)<\/code><\/pre>\n\n\n\n
A Monitor<\/code>-object has a Semaphore<\/code> attribute and a local method pattern, entry<\/code>, which is to be used as a supermethod for methods in subclasses of Monitor<\/code>. As can be seen, entry<\/code> has an inner(entry)<\/code>; mutex.wait<\/code> is executed before inner(entry) <\/code>and mutex.signal<\/code> is executed after inner(entry)<\/code>.<\/p>\n\n\n\n
The Account<\/code> class may be described as a subclass of Monitor<\/code> in the following way:<\/p>\n\n\n\n
class<\/strong> Account: Monitor\n   deposit(amount: var<\/strong> float): entry\n      balance := balance + amount\n   withdraw(amount: var<\/strong> float): entry\n      balance := balance - amount\n   %private\n   balance: var<\/strong> float<\/code><\/pre>\n\n\n\n
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If anAccount<\/code> refers to an Account<\/code>-object, then execution of anAccount.deposit(200)<\/code> has the effect  that the supermethod entry<\/code> of deposit<\/code> works like a wrapper around the statement balance := balance + amount<\/code>. This ensures that mutex.signal<\/code> is executed before the statement and mutex.signal<\/code> is executed afterwards. The same is the case for execution of a anAccount.withDraw(300)<\/code>. All in all, using entry<\/code> as a supermethod ensures that at most one deposit<\/code> or withdraw<\/code> may be executed at the same time, which guarantees exclusive acces to data-items within the Monitor<\/code> object.<\/p>\n\n\n\n
In section