How does a C program get information from an array internally?

Question

I'm relatively new to programming, so when someone suggested that building an array of structs (each containing n attributes of a particular "item") was faster than building n arrays of attributes, I found that I didn't know enough about arrays to argue one way or the other.

I read this:

how do arrays work internally in c/c++

and

Basic Arrays Tutorial

But I still don't really understand how a C program retrieves a particular value from an array by index.

It seems pretty clear that data elements of the array are stored adjacent in memory, and that the array name points to the first element.

Are C programs smart enough to do the arithmetic based on data-type and index to figure out the exact memory address of the target data, or does the program have to somehow iterate over every intermediary piece of data before it gets there (as in the linkedlist data structure)?

More fundamentally, if a program asks for a piece of information by its memory address, how does the machine find it?

Answer 1

Let's take a simpler example. Let's say you have a array int test[10] which is stored like this at address 1000:

1|2|3|4|5|6|7|8|9|10

The complier knows that, for example, an int is 4 bytes. The array access formula is this:

baseaddr + sizeof(type) * index

The size of a struct is just the sum of the sizes of its elements plus any padding added by the compiler. So the size of this struct:

struct test {
  int i;
  char c;
}

Might be 5. It also might not be, because of padding.

As for your last question, very shortly (this is very complicated) the MMU uses the page table to translate the virtual address to a physical address, which is then requested, which if it's in cache, is returned, and otherwise is fetched from main memory.