Foundations Updated 2026-09 View as Markdown

Structures

C structures are special, large variables which contain several named variables inside. Structures are the basic foundation for objects and classes in C.

C structures are special, large variables which contain several named variables inside. Structures are the basic foundation for objects and classes in C. Structures are used for:

• Serialization of data

• Passing multiple arguments in and out of functions through a single argument

• Data structures such as linked lists, binary trees, and more

The most basic example of structures are points, which are a single entity that contains two variables - x and y. Let’s define a point:

struct point {
    int x;
    int y;
};

Now, let’s define a new point, and use it. Assume the function draw receives a point and draws it on a screen. Without structs, using it would require two arguments - each for every coordinate:

/* draws a point at 10, 5 */
int x = 10;
int y = 5;
draw(x, y);

Using structs, we can pass a point argument:

/* draws a point at 10, 5 */
struct point p;
p.x = 10;
p.y = 5;
draw(p);

To access the point’s variables, we use the dot . operator.

Typedefs#

Typedefs allow us to define types with a different name - which can come in handy when dealing with structs and pointers. In this case, we’d want to get rid of the long definition of a point structure. We can use the following syntax to remove the struct keyword from each time we want to define a new point:

typedef struct {
    int x;
    int y;
} point;

This will allow us to define a new point like this:

point p;

Structures can also hold pointers - which allows them to hold strings, or pointers to other structures as well - which is their real power. For example, we can define a vehicle structure in the following manner:

typedef struct {
    char * brand;
    int model;
} vehicle;

Since brand is a char pointer, the vehicle type can contain a string (which, in this case, indicates the brand of the vehicle).

vehicle mycar;
mycar.brand = "Ford";
mycar.model = 2007;

John is 27 years old.

#include <stdio.h>

typedef struct {
    char * name;
    int age;
} person;

int main() {
    person john;

    /* testing code */
    john.name = "John";
    john.age = 27;
    printf("%s is %d years old.", john.name, john.age);
}