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Union in C

Last Updated on June 19, 2023 by Mayank Dham

In this blog, we’ll learn about union in C with a thorough introduction, the syntax of the union in C, an example of union followed by popper examples of union, accessing union in c member, advantages and disadvantages of union followed by some similarity and distinguishing features between union and structure, all with a thorough explanation from scratch.

What is a Union in C

In the C programming language, a union is a user-defined data type that allows different types of data to be stored in the same memory location. It provides a way to define a variable that may have different data types, but at any given time, only one member of the union can hold a value.
The memory allocated for a union is large enough to hold the largest member within it. All members of a union share the same memory space, which means that changing the value of one member will modify the other members as well. This feature is useful when you want to represent different types of data in a compact and efficient manner.

Syntax of the Union in C

We have to use a union keyword for declaring union followed by the name you wanna give to the container. Because enough bytes must be set aside to store the greatest-sized field, the union’s size is equal to the size of its largest field.

A collection of variable values and, optionally, a tag identifying the union are specified in a "union declaration." The "members" of the union, which are the variable values, might be of many sorts. In other languages, "variant records" are comparable to unions.

union phone
  int price;
  char name[50];

Above is the example of a general syntax of the union in c that you have to use the keyword named union after that the name of the container and in the container the values of its characteristics and properties.

Create Union Variables

We define a union in c outside the main function and to use a union in c inside the main function we have to create the user-defined variable here we will see the methods to create those user-defined methods as they are known as union variables.

union phone
  int price;
  char name[50];

int main()
  union phone phone1, phone2, *phone3;
  return 0;

In the above example we have created a union in c with the name of the phone which have two properties named price of the name and the name of the phone. In the main function, we have created the union variables by defining the anime of union first after that the name of the variables that we want to give to the variables we have given phone1 and phone 2 both ar union in c variables but phone 3 is union in c pointer.

We can create a union variable in an alternate way which is explained below:

union phone
  int price;
  char name[50];
}phone1, phone2, *phone3;

In the above example we have created union variables in an alternate way as here we have created them along the union in c declaration and not in the main function. Here we do not need to name the union and the name of the container again as we are declaring the union variable along the main union in the c declaration so there will be no need for a declaration. In this case also phone 1 and phone 2 are the union variables whereas phone 3 is a union in c pointer.

Example of Union in C

In this section, we will see the example of the union in c, in the example we will see the proper declaration of union in c with the initialization of union variables and the proper way of accessing union variables in the code.

Code Implementation:

#include <stdio.h>
#include <string.h>
union Example {
   int i;
   float f;
   char str[20];
int main( ) {

   union Example example;        

   example.i = 10;
   printf( "example.i : %d\n", example.i);
   example.f = 220.5;
   printf( "example.f : %f\n", example.f);
   strcpy( example.str, "C Programming");

   printf( "example.str : %s\n", example.str);

   return 0;


example.i : 10
example.f : 220.500000
example.str : C Programming

Explanation of the above code
In the above example we have created a union in c with the name Example and in that union in c we have an integer value, a float value, and a character array. Now in the main function, we have declared a union variable named example by following the syntax explained above and now we are initializing all the values of the union in c variable by just normal allocation of using the ‘.’ operator and you can see the output in the above image accordingly.

Advantages of Union in C

The union data type in C offers several advantages and use cases. Here are some of the main advantages of using unions:

  • Memory efficiency: Unions allow different data types to share the same memory space. This can be particularly useful when you have a scenario where only one member of the union is active at any given time. By sharing memory, unions can save memory compared to using separate variables for each data type.
  • Versatile representation: Unions provide a way to represent a single value that can take on different data types. This versatility allows you to handle scenarios where a value may need to be interpreted differently based on the context. For example, a union can be used to store a numeric value as either an integer or a floating-point number, depending on the situation.
  • Efficient data manipulation: Unions make it convenient to manipulate and interpret data in different ways. By accessing different members of a union, you can treat the same memory location as different data types, allowing for efficient data conversions and calculations.
  • Compact code: Using unions can result in more compact and readable code, especially in cases where you need to work with multiple data types that are closely related or interchangeable. Rather than declaring separate variables, you can define a single union variable, leading to cleaner and more concise code.
  • Compatibility with external data formats: Unions can be helpful when working with external data formats that have variable interpretations for certain fields. By using unions, you can easily map the different interpretations of a field without the need for extensive data type conversions.

Disadvantages of the Union in C

While unions in C offer advantages in terms of memory efficiency and versatility, they also come with some potential disadvantages. Here are a few of the disadvantages of using unions:

  • Memory interpretation issues: Unions can lead to memory interpretation issues if not used carefully. Since multiple members share the same memory space, accessing a member that is not currently active can result in undefined behavior. It is crucial to keep track of which member is valid at any given time to avoid incorrect interpretation of data.
  • Limited size: Unions can only allocate memory that is large enough to hold the largest member within them. This limitation can be problematic if you need to store data that exceeds the size of the largest member. In such cases, unions may not be suitable, and you might need to consider alternative approaches.
  • Lack of type checking: Unions do not provide built-in type checking mechanisms. It is the programmer’s responsibility to ensure that the correct member is accessed and that the data is interpreted correctly. Without proper handling, type mismatches can lead to unexpected results and bugs that can be challenging to identify and debug.
  • Potential for data loss: If different members of a union store data of different sizes, there is a risk of data loss or truncation. For example, if a union has a member that stores a 32-bit integer and another member that stores an 8-bit character, storing a value that exceeds the range of the character member may result in data loss.
  • Portability concerns: Unions might have portability issues across different platforms and compilers. The representation of data in memory can vary depending on the architecture and compiler implementation. Consequently, code using unions may behave differently on different systems, leading to non-portable code.
  • Code readability and maintainability: While unions can make code more compact, they can also make it less readable and maintainable, especially when used extensively or in complex scenarios. The implicit nature of unions can make it challenging for other developers to understand and modify the code, potentially leading to errors and difficulties in maintenance.

Similarities between Structure and Union

There are many similarities between union in c and structures and in this part we will see some of them.

  1. Both union and structure are used to store different types of data in a single unit and they are user-defined data types.
  2. Functions may accept a structure or union as a parameter and return it as a parameter. The type of the argument and the function parameter must match. Similar to a scalar variable, a structure or union is supplied by value as the equivalent parameter.
  3. Only the assignment = and sizeof operators are supported by unions and structures. The members and member types of the two structures or unions in the assignment must match.
  4. To access member variables inside of both user-defined datatypes, use the ‘.’ operator, which has one of the highest precedences.
  5. Any object, including other unions, structures, or arrays, may be one of their members. A bit field can also be a part of a member.

Difference between Union and Structure in C

Parameters Union Structure
keyword We use the keyword union to define union. We use the keyword structure to define the structure.
Memory Memory is shared by each individual Each member is allocated different memory.
Accessing Members In union in c we can only access one member at a time In this we can access individual members at a time
Size Allocates with the size of the largest variable Allocates with the aggregate of the size of all the variables.
Value Altering Altering the value of one member will affect the value of another variable Altering the value of one member will not affect the value of another variable.
Initialization of members We can only initialize the first member of the union in c We can initialize the value of many variables at the same time.

In conclusion, unions in the C programming language offer advantages in terms of memory efficiency, versatile data representation, and efficient data manipulation. They can help optimize memory usage by sharing memory space among different data types and provide flexibility when working with varying interpretations of data. However, unions also have disadvantages such as potential memory interpretation issues, lack of type checking, limited size, and potential for data loss. It is crucial to use unions carefully, ensuring proper handling of the active member and appropriate interpretation of data.

Here are a few frequently asked questions (FAQs) related to unions in C:

Q1. Can a union contain members of different sizes?
Yes, a union can contain members of different sizes. However, the union will allocate memory that can accommodate the largest member within it. Therefore, if a member requires more memory than the others, the union size will be based on that largest member.

Q2. Can unions be nested within structures or arrays?
Yes, unions can be nested within structures or arrays in C. This allows for more complex data structures where different parts may have varying interpretations or data types.

Q3. Can unions be used as function parameters or return types?
Yes, unions can be used as function parameters or return types in C. However, it’s essential to carefully manage the active member of the union to ensure correct interpretation and avoid undefined behavior.

Q4. Are there any restrictions on the types of data that can be stored in a union?
Unions can store any data type, including built-in types like int, float, char, as well as user-defined structures and enumerations. However, unions cannot contain members that are arrays or functions.

Q5. How can I determine which member of a union is currently active?
There is no built-in mechanism in unions to track the active member. It is the programmer’s responsibility to keep track of the active member or use additional variables or flags to indicate the currently valid member.

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