Saturday, September 18, 2010

Operating Systems -- A shared memory project

Well I decided to start hacking away at my homework for my operating systems class, in the course of about 3 hours, I managed to finish the project and make breakfast....All in all I would say it was a productive morning.

I am not going to share my source code, for reasons I'm debating about writing a post about, to make a long story short, is it ethical to post about your homework? I do think that sharing knowledge in this day and age is what will enable us (humanity) to become smarter as a whole (I think), however there is a line, explaining about shared memory for example and how it works, with examples is valid, literally posting my code up for people to simply copy and paste into their HW not so awesome.

Lets get into it, as part of the assignment we were asked to record our observations and the likes.

First off, the first problem wasn't really all that hard, initially I started to do it recursively, but decided not to, as I was just not thinking right this morning, and writing a simple for loop to do this was WAY easier.

Lets get into the meat of the project, the two main goals were forking and shared memory, the forking was relatively easy, simply creating a variable to hold your pid and then checking whether it's 0 or some other number tells you whether you are the parent or not. Then simply doing something different based on whether you are a parent or not (or if there was a pid error) was all that was required.

Shared memory was a little trickier, and I think I finally get it. Here is the deal, first we create a numerical identifier for the shared memory. Now we can begin to fork, and in the child process we create an attachment to the memory in each child process, THEN in the parent we do the same (This way we KNOW we aren't using the shared variable, it's actually memory, because we're using two different variables and I didn't assign them as pointers).

Now when you're done in the children we just detach from the processes.

In the parent we will detach then deallocate the memory. I was thinking about it, and I decided in my mind that if i deallocate that memory, then still try to point to it with my variable, I should get a segfault, guess what... I did, so that's how my program ends, with a seg fault, but it's on purpose, and I actually know why, and it just plain makes sense. Heres the output of each of the parts of my program.

Part 1
Fibbonacci with only forking
Part 2
Fibbonacci 2 with shared memory