Demistifying C pointer - A Mental Model
C is fun to learn! The C pointer is a concept in the language that deals with memory allocation. Dynamic allocation on the heap is impossible without pointers.
As I led a section for my Operating Systems course, I realized that students often don’t have a good mental model of C pointers. Rightfully so, lots of CS students were not taught how to think abstractly about the problem or how to build a good mental model when working with new concepts. So in this blog post, I’m going to go over a mental model for thinking about the C pointer. But first, what is a mental model?
Wiki definition of a Mental model: A mental model is an explanation of someone’s thought process about how something works in the real world.
My definition of a Mental model: A way to think about the concepts without concerning too much with the nitty details.
“C pointer is just a memory address. THAT’S IT. Repeat it 25 times and you’ll get it” - my OS professor.
The statement is true. A C pointer is an address in memory. In another word, it’s a variable whose value is a memory address. That’s a mouthful. So let’s break it down by building up our mental model. I will show you mine.
Let’s look at an example:
int a = 1;
int* b = &a;
int** c= &b;
““ denotes the pointer type in C. “&” means “get me the memory address of the variable to the right of &”. ”**” can be confusing. Sometimes, you would see it used as an operator, called dereference operator. Here’s my mental model for the example above:
Code Stack (on x86-64)
int a = 1; -> a [ 1 ] 0x108
int* b = &a; -> b [ 0x108 ] 0x116
int** c = &b; -> c [ 0x116 ] 0x124
As you see, each variable declaration is a box with its name, value, and memory address drawn on a stack. The first declaration is specifying that a is stored on the stack with value 1. Then b will have the address of a, which is 0x108. Subsequently, c will have the address of b, which is 0x116.
As we dereference these pointers, we just need to follow the box that it points to. I think pointers get a lot of bad reps. But it’s not a mystery at all. Rather, it’s a very intuitive concept!