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Unit-12.md

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Memory Address

When a variable is created in C, a memory address is assigned to the variable. The memory address is the location of where the variable is stored on the computer. When we assign a value to the variable, it is stored in this memory address. To access it, use the reference operator (&), and the result represents where the variable is stored:

int myAge = 43;
printf("%p", &myAge); // Outputs 0x7ffe5367e044

We should also note that &myAge is often called a "pointer". A pointer basically stores the memory address of a variable as its value. To print pointer values, we use the %p format specifier.

Pointers

A pointer is a variable that stores the memory address of another variable as its value.  A pointer variable points to a data type (like int) of the same type, and is created with the * operator. 

int myAge = 43;     // An int variable
int* ptr = &myAge;  // A pointer variable, with the name ptr, that stores the address of myAge

// Output the value of myAge (43)
printf("%d\n", myAge);

// Output the memory address of myAge (0x7ffe5367e044)
printf("%p\n", &myAge);

// Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

Example explained

Create a pointer variable with the name ptr, that points to an int variable (myAge). Note that the type of the pointer has to match the type of the variable you're working with (int in our example).

Use the & operator to store the memory address of the myAge variable, and assign it to the pointer.

Now, ptr holds the value of myAge's memory address.

Dereference

We can also get the value of the variable the pointer points to, by using the * operator (the dereference operator):

int myAge = 43;     // Variable declaration
int* ptr = &myAge;  // Pointer declaration

// Reference: Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

// Dereference: Output the value of myAge with the pointer (43)
printf("%d\n", *ptr);

Note that the * sign can be confusing here, as it does two different things in our code:

  • When used in declaration (int* ptr), it creates a pointer variable.
  • When not used in declaration, it act as a dereference operator.