Mastering C Structs: Definition, Variables, Access & Advanced Examples

Mastering C Structs

Learn how to define, create, and use C structs with clear examples and best practices.

What Is a C Struct?

A struct (short for structure) is a composite data type that groups variables of different types under a single name. It’s the foundation for creating complex data structures in C.

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Defining a Structure

Before you can create variables of a struct type, you must define the type. The struct keyword introduces the definition:

struct structureName {
  dataType member1;
  dataType member2;
  …
};

Example:

struct Person {
  char name[50];
  int citNo;
  float salary;
};

This creates a new type called struct Person. You can now declare variables of that type.

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Creating Structure Variables

Defining a struct type does not allocate memory. To work with it, declare variables:

struct Person {
  // members
};

int main() {
  struct Person person1, person2, p[20];
  return 0;
}

Alternatively, combine the definition and variable declaration:

struct Person {
  // members
} person1, person2, p[20];

• person1 and person2 are individual struct Person variables.
• p is an array of 20 struct Person instances.

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Accessing Struct Members

Two operators retrieve members:

1. . – Member access operator.
2. -> – Pointer-to-struct member operator (covered in a separate tutorial).

To read or modify the salary of person2:

person2.salary

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Example 1: Using a Struct in C

#include <stdio.h>
#include <string.h>

struct Person {
  char name[50];
  int citNo;
  float salary;
} person1;

int main() {
  strcpy(person1.name, "George Orwell");
  person1.citNo = 1984;
  person1.salary = 2500;

  printf("Name: %s\n", person1.name);
  printf("Citizenship No.: %d\n", person1.citNo);
  printf("Salary: %.2f", person1.salary);
  return 0;
}

Output

Name: George Orwell
Citizenship No.: 1984
Salary: 2500.00

Notice strcpy() copies the string into the fixed-size char array because the assignment operator cannot be used directly with arrays.

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Using typedef with Structs

The typedef keyword creates an alias for a type, simplifying declarations:

typedef struct Distance {
  int feet;
  float inch;
} distances;

int main() {
  distances d1, d2;
}

Without typedef, you’d write:

struct Distance d1, d2;

Example 2: Alias in Practice

#include <stdio.h>
#include <string.h>

typedef struct Person {
  char name[50];
  int citNo;
  float salary;
} person;

int main() {
  person p1;
  strcpy(p1.name, "George Orwell");
  p1.citNo = 1984;
  p1.salary = 2500;

  printf("Name: %s\n", p1.name);
  printf("Citizenship No.: %d\n", p1.citNo);
  printf("Salary: %.2f", p1.salary);
  return 0;
}

Output

Name: George Orwell
Citizenship No.: 1984
Salary: 2500.00

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Nested Structures

Structs can contain other structs, enabling hierarchical data modeling:

struct complex {
  int imag;
  float real;
};

struct number {
  struct complex comp;
  int integers;
} num1, num2;

To set imag of num2 to 11:

num2.comp.imag = 11;

Example 3: Nested Structs in Action

#include <stdio.h>

struct complex {
  int imag;
  float real;
};

struct number {
  struct complex comp;
  int integer;
} num1;

int main() {
  num1.comp.imag = 11;
  num1.comp.real = 5.25;
  num1.integer = 6;

  printf("Imaginary Part: %d\n", num1.comp.imag);
  printf("Real Part: %.2f\n", num1.comp.real);
  printf("Integer: %d", num1.integer);
  return 0;
}

Output

Imaginary Part: 11
Real Part: 5.25
Integer: 6

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Why Use Structs in C?

Suppose you need to store a person’s name, citizenship number, and salary. Without structs, you’d create separate variables for each field for every person, quickly leading to code duplication and confusion. Structs encapsulate related data into a single logical unit, promoting cleaner code and easier maintenance.

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What’s Next?

• Structs and pointers
• Passing structs to functions