In the C programming language, structures and unions are key ideas that let programmers define and work with complicated data types.

It is simpler to organize and handle data in a computer by using these structures, which give a means to group relevant variables together.

The ideas of structures and unions in C, their syntax, usage, and certain key concerns will all be covered in this article.

Overview of Structures and Unions

Structures and unions are user-defined data types in the C programming language that let programmers assemble many sorts of variables into a single entity.

Unions are a means to store several forms of data in the same memory area, whereas structures are used to combine related variables of various sorts.

Structure Definition and Declaration

In C, we use the struct keyword, followed by the name of the structure and a list of member variables surrounded in curly braces. There is a comma between the name and data type of each member variable. For example:

struct Person {
    char name[50];
    int age;
    float height;

Similar to declaring variables of other data types, when declaring a variable of the structure type, we only use the structure name followed by the variable name.

struct Person person1;

Accessing Members of a Structure

The dot operator (.) allows us to access a structure’s individual members. For instance, we may do the following to give the name member of the person1 structure a value:

strcpy(, "John");
Similarly, to access the age member, we can use:
person1.age = 25;

Initializing Structures

Structures can be initialized at the time of declaration by utilizing curly brackets.

struct Person person2 = {"Alice", 30, 1.65};

Alternatively, we can initialize individual members using the dot operator.

struct Person person3; = "Bob";
person3.age = 35;
person3.height = 1.75;

Arrays of Structures

We can build arrays of structures in C. When declaring an array of structures, we must first define the kind of structure, then the array name and size.

struct Person people[10];

Each element of the array can be accessed using the array index. For instance, to access the third element’s name:

strcpy(people[2].name, "Carol");

Nested Structures

In C, we can also have nested structures, which are structures inside other structures. This enables us to build complex data structures. Here’s an example:

struct Address {
    char street[50];
    char city[30];

struct Employee {
    char name[50];
    int age;
    struct Address address;

To access nested structure members, we use the dot operator multiple times.

strcpy(, "New York");

Pointers to Structures

In C, pointers are also compatible with structures. A structure’s name followed by an asterisk * can be used to declare a pointer to it.

struct Person *ptr;

The arrow operator -> is used to access members of a structure through a pointer. 

ptr->age = 40;

Overview of Unions

Unions in C let various data types be stored in the same memory address, in contrast to structures. A union is given a memory that is the same size as its largest member. When memory efficiency is a problem, unions are helpful.

Declaring and Using Unions

In C, we use the union keyword, the union name, and a list of member variables surrounded in curly brackets to declare a union. The data type of each member variable is distinct.

union Data {
    int i;
    float f;
    char str[20];

Declaring a variable of the union type can be done as follows:

union Data data;

Members of a union can have values assigned to them, and we can access them similarly to how we do with structures.

Differences between Unions and Structures

Structures allocate memory for each member variable, whereas unions only allocate memory for one member at a time.

Structures and Unions in C Language

This is the primary distinction between structures and unions. Unions become more memory-efficient as a result, but their use is constrained when several members must be accessed at once.

Tips for Using Unions and Structures

  • Utilize structures to combine related variables of various types.
  • Use unions when storing various types of data in the same memory area or when memory efficiency is an issue.
  • To avoid utilizing too much memory or having alignment problems, carefully plan your data structures.
  • To improve code readability, give structure and union members meaningful names.
  • To prevent undefined behavior, correctly initialize structures and unions.

Examples of Unions and Structures in C Programming

Here are some instances of real-world programs that make use of structures and unions:

  • preserving student data (name, age, grade) in a system.
  • creating a framework with real and imaginary components for complex number representation.
  • saving memory in embedded systems by storing different data types in a union.

Common Mistakes and Advice

  • When using structures, take care of memory alignment and padding.
  • Avoid changing union members that have not been properly synchronized or type checked.
  • Reduce the size of unions and structures as needed to avoid consuming too much RAM.


In summary, the C language’s fundamental principles of structures and unions give programmers the tools they need to efficiently organize data and generate complex data types.

Unions enable the storage of many types of data in the same memory space, whereas structures are used to group related variables.

Developers can design more productive and well-organized C programs by comprehending and successfully employing these constructs.

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