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Basics of Memory Addresses in C

| Monday, September 3, 2012
Memory Addresses

It is helpful to think of everything in C in terms of computer memory. Let’s think of computer memory as an array of bytes where each address in memory holds 1 byte. If our computer has 4K of memory for example, it would have 4096 elements in the memory array. When we talk about pointers storing addresses, we are talking about a pointer storing an index to an element in the memory array. Dereferencing a pointer would be getting the value at that index in the array. All of this is of course a lie. How operating systems handle memory is much more complex than this. Memory is not necessarily contiguous and it is not necessarily handed out sequentially. But the analogy provides an easy way to think about memory in C to get started.
Confused about pointers, addresses and dereferencing? Take a look at this 5-Minute Guide to Pointers.
Say our computer has 4K of memory and the next open address is index 2048. We declare a new char variable i = ‘a’. When the variable gets declared memory is set aside for its value and the variable name is linked to that location in memory. Our char i has a value ‘a’ stored at the address 2048. Our char is a single byte so it only takes up index 2048. If we use the address-of operator (&) on our variable i it would return the address 2048. If the variable was a different type, int for instance, it would take up 4 bytes and use up elements 2048-2051 in the array. Using the address-of operator would still return 2048 though because the int starts at that index even though it takes up 4 bytes. Let’s look at an example.

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