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.

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

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

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

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.