Computer Systems Organization : Recitation 02

Some of these exercises have multiple ways to solve them.

Basic Unix usage

    mkdir unix/backup
    cp unix/foo.txt unix/backup/

    mv unix/foo.txt unix/answers.txt

    mkdir -p multiple/directories

    touch multiple/directories/test.txt

    echo "Hello!" > multiple/directories/test.txt

    cat multiple/directories/test.txt

    rm -r multiple/

    wget http://nginx.org/download/nginx-1.9.2.tar.gz

    tar -xvf nginx-1.9.2.tar.gz

    grep -r "main(" nginx-1.9.2 (or just grep -r "main(" assuming you are in nginx-1.9.2)

    OR

    find . -name '*.c' | xargs grep "main("

And by inspection, the file is nginx-1.9.2/src/core/nginx.c

    wc -l nginx-1.9.2/src/core/nginx.c 

And there are 1362 lines.

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C Programming

Part 1

    int findMax(int a, int b)
    {
        if (a > b)
            return a;
        else
            return b;    
    } 

Part 2

    int fib(int n) 
    {
        if (n <= 1)
            return n;
        else {
            return fib(n - 1) + fib(n - 2);
        }
    }

Part 3

    int sum = 0;
    for (i = 0 ; i < size ; i++) {
        sum += a[i];
    }

---

Debugging

• After the foo binary is compiled with debugging enabled, run gdb with gdb foo

• To start running the foo binary, type run.

• To set a breakpoint, type b file.c:5 where file.c is a source code file, and 5 is the line to break at.

• To view a snippet of the source code, type l.

• To continue execution until the next breakpoint, type c.

• To continue execution until the next line of code, type n.

• With this information, we see that the program continuously executes lines 6 and 7, which are the lines for the condition of a while loop, and its body. We can example the contents of sum and i per iteration by doing print i or print sum. We notice that every iteration, sum is incremented by 1, and i stays constant. With this knowledge, we see that the variable i is never incremented, so our loop executes forever.

• To fix this bug, we simply add the line i++;.

• gcc -g hello_world.c -o hello_world

• For graders: Students will have made the fix to foo. If they have, it is correct.