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Coursework 1 : Semaphore Implementation

Coursework 1 : Semaphore Implementation Task : To write a package of functions or methods which implement P and V semaphore operations and to emulate their behaviour in an emulated multi-tasking environment. Background Semaphores are to be identified by a string of characters e.g. “mutex”.

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Coursework 1 : Semaphore Implementation

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  1. Operating Systems : Coursework 1 Coursework 1 : Semaphore Implementation Task : To write a package of functions or methods which implement P and V semaphore operations and to emulate their behaviour in an emulated multi-tasking environment. Background • Semaphores are to be identified by a string of characters e.g. “mutex”. • Processes are to be identified by an ID number within some range e.g. 0-255. Specifications The following functions should be implemented : create_semaphore (string semaphore); P (string semaphore, int process); V (string semaphore); present_status (); In this emulation, no run queue or process scheduling is required. Instead of branching to a dispatcher, messages are printed out monitoring the effect of the operation just performed. The P operation tests the value of the semaphore. If it is free, the process is allocated the semaphore and a message to this effect is printed out. If the semaphore is not free, the process must be queued in some appropriate data structure and a message printed out to that effect. The V operation also tests the semaphore. If there are any processes queued waiting for it, the next one is notionally dispatched and a message is printed out saying this process now has the semaphore. For example, the function calls : create_semaphore (“mutex”); P(“mutex”, 1); P(“mutex”, 2); V(“mutex”); V(“mutex”); should produce output of the form semaphore “mutex” created P(“mutex”, 1) : “mutex” allocated to process 1 P(“mutex”, 2) : process 2 queued on “mutex” V(“mutex”) : “mutex” released by process 1 “mutex” allocated to process 2 V(“mutex”) : “mutex” released by process 2 “mutex” free

  2. Operating Systems : Coursework 1 • The present_status function should print out the current status of all the semaphore queues. For example, after the calls : • P (“fred”, 1); • P(“jim”, 1); • P(“fred”, 2); • P(“fred”, 3); • the status printed should be something like : • “fred” : allocated to process 1 • awaited by process 2 • awaited by process 3 • “jim” : allocated to process 1 • This function should also detect deadlocks. For example, after : • P (“mike”, 1); • P(“roland”, 2); • P(“roland”, 1); • P(“mike”, 2); • the status printed out should be : • “mike” : allocated to process 1 • awaited by process 2 • “roland” : allocated to process 2 • awaited by process 1 • deadlock detected : processes 1, 2 • Notes • Semaphores should initially be free • In practice, any one process cannot be queued awaiting more than one semaphore. A test should be incorporated to preclude this arising by mistake in the emulation requested. • Multiple disjoint deadlock cycles, each of any length, should be detected. • First-Come-First-Served order of queuing for semaphores should be preserved. • Any implementation language desired may be used. • Efficiency may be taken into account when grading submissions. • Hand-in • An annotated listing of your program • Representative tests and the resulting output • Deadline : 5pm, Friday 16th November, to the ITO.

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