Homework #5 Solution

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This program will mimic the safe interaction of shared resources utilizing the

“dirty/clean fork” algorithm found with the dining philosopher problem.

There will be a given number of programs executing concurrently (the number

will be specified at runtime and could change with each run). We will refer to

each process as Phil (five executing concurrently would be Phil 0, Phil 1, …

P3, P4). The purpose of each Phil is to generate some text and write it to a

file. Because Phils are overly skeptical, they will only write their data if

they can simultaneously write it out to TWO separate (but specific!) files

(which ensures the data will be safe). Phil 0 will write to File 0 and File 1.

Phil 1 will write to File 1 and File 2. … Phil N (the last Phil) will write

to File N and File 0. Clearly there is a possibility Phil 0 and Phil 1 could

attempt to write to File 1 at the same time (this is the critical resource

whose safe access we need to ensure). Each Phil will be coded with a sequence

of “generating” (simulating creation of content), followed by the desire to

“write” (use the shared resource to output the generation into the files).

Each Phil needs to keep track of their file’s “status” (i.e., which of the

Phils can write to that file) and can only write when they have access to

*BOTH* of their associated files. To simplify things, each file will be

assigned to a Phil (I’ll call it “available” to that Phil) – with all

files initially assigned to the highest-numbered Phil (so Phil N has File

0 and File N “available”, Phil 0 has File 0 and File 1 both “unavailable”,


Additionally, each file’s status will contain a “used” indicator which will

help prevent starvation – all initially set to “used”. After a Phil has

“generated” something to output, it checks to see if its two files are

“available” and will write out when that is the case. If file(s) are

unavailable, messages will be sent requesting the unavailable file(s) and

Phil will have to wait until it receives (a) message(s) back stating the

file(s) are available. When a file’s “availability” changes from one Phil

to another, the receiving Phil will update their local status to “unused”.

When a Phil writes out to the files, the files’ status will be updated to

“used” (indicating it has had a chance to “use” the files it was requesting).

When a message request is received by Phil x, if the file is available and

used, Phil x will mark the file as unavailable and alert the sender the file

is free (available for the sender to write to). If the file is currently marked

as “unused” Phil will wait until their next file write has finished and then

alert the sender the file is free.