A pointer to a pointer to a what?

Pointers can be quite challenging at times for anyone starting out in C programming. But once you master it, there’s quite a lot you can do with them and quite a lot go wrong. Even after years of programming in C, I stumble upon bugs that take forever to find and fix. Most of these bugs are due to passing an incorrect pointer or accessing some location that doesn’t exist anymore. Now if pointers can be a problem, what about double and triple pointers which make things even more difficult?

Now, in the process of finding this bug, I start out my drawing a bunch of diagrams showing the stack frames and little boxes representing dynamically created memory (ones that are malloced). Eventually, the entire page is full of boxes and rectangles and I lose track of what I am trying to do.

I stuck to this way of drawing diagrams until I stumbled upon a great little tool called Python Tutor. that made my life much easier and helped me visualize computer memory and how it is handled. The name might be a bit misleading as it not only helps visualize programs in python but a few other languages. A quick view of their home page shows you its capabilities. You can see how they show the different objects and functions are shown visually.

As you can see in the image above, I created a pointer variable that points to a character that points a globally initialized string. The main function does nothing but print out the address of that string. Once you type out the code and hit the “Visualize Execution” button under the editor, you see the code visualized. Now, what can we infer from the visualization? The memory for the string is created in the heap and the address is assigned to the pointer variable. We can also see the ‘\0’ at the end of the string which is a string terminator in C. This is how we know where the string ends. The little arrow indicates where the pointer is pointing to. In this case, it is pointing to the beginning of the string. The editor also has a neat feature where you can step through a code like a debugger. You can hit the “Prev” and “Next” buttons to execute your code line by line and see each statement visualized on the right. The output that you print out (the string passed to printf) goes into the box on the right as well.

Now that we know how to use the tool, let’s move on to a bit more complex program where we have control over creating memory. The code here shows a function that creates a function “allocateArray” that creates an array dynamically based on the size and fills it the value provided via the arguments. Pretty simple, right? We pass in the pointer to the array of integers we created in main and the function does the rest. As you can see below, we create a pointer to an integer called vector and pass it to the function which creates the memory and fills it with the value and on the right we see the same happening. Everything seems to be fine until we exit the function.

The moment we return from the function, the array variable which has the pointer to the newly created memory region disappears and that pointer gets lost as we store that in a local variable. This is shown in the image below.

How do we fix this? This is where double pointers come in. A double pointer is basically a pointer to a pointer to something. The image below shows the same c ode as before but with a minor modification. We pass a a pointer to a pointer to the vector variable.

Everything here looks similar to what we saw before. But when we return from the function, we keep the address of the newly created array. The same is shown in the image below where we can see that we still have access to the array we created dynamically.

It can be quite difficult to understand pointers and their use cases but with the right learning resources, it shouldn’t be that difficult. This tool over a period of time has helped me understand and master pointers and hopefully, can be of help to many others who faced similar issues. If you are just getting started with pointers, then I would recommend the book Understanding and using C Pointers by Richard Reese. The example shown above was taken from that book. This book had helped me a lot when I was getting started with my career in programming.