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This demonstrates use of shared memory and semaphores via two applications
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named after Mrs. Premise and Mrs. Conclusion of the Monty Python sketch.
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http://www.youtube.com/watch?v=crIJvcWkVcs
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Like those two characters, these programs chat back and forth and the result
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is a lot of nonsense. In this case, what the programs are saying isn't the
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interesting part. What's interesting is how they're doing it.
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Mrs. Premise and Mrs. Conclusion (the programs, not the sketch characters)
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communicate through POSIX shared memory with a semaphore to control access
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Mrs. Premise starts things off by creating the shared memory and semaphore
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and writing a random string (the current time) to the memory. She then sits
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in a loop reading the memory. If it holds the same message she wrote, she
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does nothing. If it is a new message, it must be from Mrs. Conclusion.
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Meanwhile, Mrs. Conclusion is doing exactly the same thing, except that she
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assumes Mrs. Premise will write the first message.
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When either of these programs reads a new message, they write back an md5
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hash of that message. This serves two purposes. First, it ensures that
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subsequent messages are very different so that if a message somehow gets
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corrupted (say by being partially overwritten by the next message), it will
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not escape notice. Second, it ensures that corruption can be detected if
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it happens, because Mrs. Premise and Mrs. Conclusion can calculate what the
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other's response to their message should be.
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Since they use a semaphore to control access to the shared memory, Mrs.
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Premise and Mrs. Conclusion won't ever find their messages corrupted no
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matter how many messages they exchange. You can experiment with this by
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setting ITERATIONS in params.txt to a very large value. You can change
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LIVE_DANGEROUSLY (also in params.txt) to a non-zero value to tell Mrs.
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Premise and Mrs. Conclusion to run without using the semaphore. The shared
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memory will probably get corrupted in fewer than 1000 iterations.
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To run the demo, start Mrs. Premise first in one window and then run
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Mrs. Conclusion in another.
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If you want to get fancy, you can play with C versions of Mrs. Premise and
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Mrs. Conclusion. The script make_all.sh will compile them for you. (Linux
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users will need to edit the script and uncomment the line for the
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Linux-specific linker option.)
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The resulting executables are called premise and conclusion and work exactly
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the same as their Python counterparts. You can have the two C programs talk
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to one another, or you can have premise.py talk to the C version of
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conclusion...the possibilities are endless. (Actually, there are only four
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possible combinations but "endless" sounds better.)
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demo/ReadMe.txt
