Understanding Pointers

These are my notes on pointers in C, mostly from reading Beej's Guide to C Programming.

Okay so when you initialize a variable, prefixing it with * designates it as a pointer to a variable of a certain type.

E.g. int *p would be a variable p, which holds a pointer to an int variable. As it holds a pointer variable, it needs to store a valid memory address. For example, if I have a variable i with the value 10, I can't just put that in p. I have to use the & operator which gets the memory address of i.

So therefore int *p = &i basically stores the memory address of i into the variable p.

The * can also be used to dereference variables - in other words, get the value stored at a certain memory address. This memory address is a virtual memory address - not a physical location in RAM. The virtual address is then mapped to an actual physical location at the OS level (from what I understand).

So let's see this in an example.

printf("The memory address of p is %p", p);
printf("The value stored at the memory address is %d", *p);

So the first print statement, we use the %p format replacement and pass in p directly, which gives us the memory address. Instead when we prefix it with * it "dereferences the pointer", in other words it gets the value stored at memory address in p. As this is an int, we change our format replacement thing with %d.

Okay so what's the point of pointers?

Let's take a traditional function scenario.

Functions in C cannot modify values outside of it. If you pass in a variable a to a function, it actually passes a copy of a. Therefore if you do a = a + 1, it will return a brand new value. It will not affect a outside of the function.

So, what about pointers?

Well if you pass a pointer to a function, similarly it will pass a copy of a pointer.

However unlike us just copying the value, we are actually getting the exact memory address for the data stored in a.

This means we can use this address to dereference it and perform adjustments to it inside of a function, and affect it outside of the function.

When I manipulate a value inside of this function, I am manipulating it with *. So this means I am actually getting the value stored at the address and directly manipulating it.

Null pointers are basically pointers which contain NULL as a value. Null like in most other programming languages represents the absence of a value. In C, it seems like this acts like a special value. For example doing int *p = NULL; isn't really assigning any value - NULL is not a memory address pointing to a value of type int as it suggests, so this is what makes it a special kind of value. It is used to create a pointer which points to "nothing".