Pointers in C

March 21, 2025

The C programming language is quite (in)famous for its concept of pointers. They’re widely considered somewhat difficult to grasp, but the idea behind them is actually quite simple.

Take the following int variable as a starting point:

int legal_age = 18;

With that simple declaration, I’ve bound the integer value 18 to some in-memory location. Now, I can access that value in my program via its variable name, legal_age.

printf("The legal age to vote is %d.", legal_age);

But, what if I want to know where that variable has been stored? That’s where the “address-of” (&) operator comes into play. You can also use the %p format specifier to insert an address into a formatted string.

printf("legal_age is stored at %p.", &legal_age);

That’s fine and all, but what if you want to store that address in a variable? Let’s give that a go.

int legal_age_address = &legal_age;

error: incompatible pointer to integer conversion initializing 'int' with an expression of type 'int *'; remove &

Hmm, not good…

But that error message (thrown by the Clang compiler) did provide us with a valuable clue. As suggested, let’s try changing that int keyword to int *. Or, as the compiler says, converting an “integer” to a “pointer”.

int *legal_age_address = &legal_age;
printf("legal_age is stored at %p.", legal_age_address);

Success! The address has now been stored in a variable. And in the process, we’ve discovered what pointers are: they’re variables that store an address to another value.

So, to recap:

// This is a regular variable declaration.
int some_number = 42;

// This gets a variable's address using the address-of operator.
&some_number;

// This stores a variable's address in a pointer.
int *some_number_address = &some_number;

Also importantly, the * character has a double meaning in C:

When used before the assignment operator, it creates a pointer.
When used after the assignment operator, it dereferences a pointer.

The term “deferencing” is just a fancy way of saying that it unpacks its value.

So, this:

printf("The legal age to vote is %d.", legal_age);

Yields the same output as this:

printf("The legal age to vote is %d.", *some_number_address);

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