C arrays are just better pointers

July 3, 2017

452 words, a 3 minutes read

This post will be about how arrays work in C, at least to the point that I know of yet. It helped me understand things better on how they even work and I just wanna share this in case someone has the same problems I had :) As always, because C is so complex, I’ll tell you enough to be dangerous, but not much more. There probably is another layer of abstraction behind all this…

Ok, so you all know arrays. They’re taught to us as a series of variables of the same type. You gotta know beforehand how many you want (I know that there are other ways so that you don’t have to know the number beforehand, this is to keep it simple). Here’s an example of initializing an array of 5 ints.

int numbers[5];

Now you may have heard before, that C does not know how big your array really is. That is why it’s so easy to just keep writing past your array. If that something behind your array belongs outside of your program, you’re in luck - the OS is gonna shut your program down for accessing stuff outside its memory. If not, you overwrite something of your own stuff and you won’t even notice.

Well, why is that? I just told C that my array has a size of 5?!

You have to know here, that an array is just a better way of expressing and accessing pointers. We call that syntactic sugar.

Basically, numbers is a pointer of type int, pointing to the first element in that array. So


are the same. numbers is just the memory address of the first element.

Now how does this help? Well, these two are also the same:

*(numbers + x);

(The parentheses are just there for clarifying syntax, otherwise the result would be whatever’s at numbers plus the number 1.)

So how does this work with memory? The way I always understood it, is: let’s say numbers is at memory address 0x800. And let’s say, int is 4 bytes long (32 bit, remember this is C so it’s not set), which you can get by sizeof(int).

That way, numbers[x] will always lie at 0x800 + x * 4. So numbers[1] is at 0x804, numbers[2] is at 0x808, and numbers[4] is at 0x810 (remember, this is hexadecimal).

Or, more general (array and n are arbitrary)

TYPE array[n];

lies at memory address of array + sizeof(TYPE) * x. So array is the base address, and sizeof(TYPE) * x is the offset. If x happens to be 0, you end up with your first element. And I guess that’s also why arrays tend to start with 0.