March 5th, 2002



Pthreads are cool, as part of a homework problem I was to implement a monitor that would allow processes each with a unique priority to access a file concurrently so long as the sum of the priorities was less than n.

So I wrote it, tried again, rewrote it, etc. I probably had a solution in there somewhere, but didn't know it because it always ran the threads sequentially (due to some weirdness I'll explain later). Eventually I poked around on google and got a pthread monitor-like dummy implementation. I slapped that in, added the pieces I needed to protect the priority counter and ran it.

Unfortunately there is something odd about how the program reacted to being piped to a file or more. Whenever I did this it executed the threads sequentially, and more than one thread never executed in the monitor at once. When I used the console scrollback, I could see multiple threads executing in the monitor at once.

In all, my solution is fairly elegant. I pass in a function pointer that does the work in the monitor and gets called at the right time. This protects the programmer from their own mistakes and allows them to make a single call when attempting to do work.

Tomorrow I'll have to post a link to my solution.

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