Structs in C

March 19, 2025

In the C programming language, there is no such thing as “classes” and “objects”.

It’s a procedural language by design, and you’re expected to write procedural code; no object-oriented shenanigans, and no functional fanciness.

But sometimes, you have to declare a bunch of variables just to define a single “entity”. These values only make sense when they’re bundled together.

// These three variables define a single entity
char *name = "John Doe";
int age = 42;

What if you have a function that’s expected to return such an entity? That’s problematic since, unlike in Python, you can’t return multiple values in C (thankfully).

// This is NOT valid C code
char *, int create_person() {
  char *name = "John Doe";
  int age = 42;

  return *name, age;
}

For that very purpose, C has structs (short for “structures”).

Simply put, a struct is a “collection of fields”, and every instance of a struct has a value assigned to each field. Unlike classes and objects though, a struct doesn’t have any behavior, only data.

// Defining a new struct
struct Person {
  char *name;
  int age;
}

Instantiating a struct can be done in various ways, but the most failsafe technique is to use the so-called “designated initializer”.

// Instantiating a struct
struct Person person = {
  .name = "John Doe",
  .age = 42,
}

And now, to get back to our function return example, you’d be able to return this single struct.

// This IS valid C code
struct Person {
  char *name;
  int age;
}

struct Person create_person() {
  struct Person person = {
    .name = "John Doe",
    .age = 42,
  }

  return person;
}

Notice how vebose it is to spell out the entire struct Voter when specifying the type? For that reason, it’s generally a good idea to alias your struct names by using the typedef keyword. Also, tt’s conventional to end your alias names with a "_t".

// "struct Person" can now be aliased to "person_t"
typedef struct Person {
  char *name;
  int age;
} person_t;

person_t person = {
  // ...
}

Once a struct has been instantiated, you can access or modify values stored in a specific field by using the dot (.) notation.

// Getting a value
printf("Person's name: %s\n", person.name); // Voter's name: "John Doe"

// Setting a value
voter.age = 43;
printf("Person's age: %d\n", person.age); // Voter's age: 43

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