C Unions

1. What is a C Unions?

C Unions is similar to a structure (struct) in C, but all members share the same memory location.
Only one member can store a value at a time.
Useful for memory-efficient programs when you need alternative data types in the same location.

Syntax:

2. Example: Basic Union

#include <stdio.h>

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

int main() {
union Data data;

data.i = 10;
printf("data.i = %d\n", data.i);

data.f = 220.5;
printf("data.f = %.2f\n", data.f);

// Only the last assigned value is stored
strcpy(data.str, "Hello");
printf("data.str = %s\n", data.str);

// Previous values are overwritten
printf("data.i = %d\n", data.i); // may show garbage
printf("data.f = %.2f\n", data.f); // may show garbage

return 0;
}

#include <stdio.h>

union Data {

int i;

float f;

char str[20];

};

int main() {

union Data data;

data.i = 10;

printf("data.i = %d\n", data.i);

data.f = 220.5;

printf("data.f = %.2f\n", data.f);

// Only the last assigned value is stored

strcpy(data.str, "Hello");

printf("data.str = %s\n", data.str);

// Previous values are overwritten

printf("data.i = %d\n", data.i); // may show garbage

printf("data.f = %.2f\n", data.f); // may show garbage

return 0;

}

Output:

Only one member’s value is valid at a time.

3. Size of a Union

The size of a union is equal to the size of its largest member.

Output:

str is the largest member → union size = 20 bytes.

4. Union with Struct Example

#include <stdio.h>
#include <string.h>union Data {
int i;
float f;
char str[20];
};

struct Info {
char name[50];
union Data data;
};

int main() {
struct Info info;

strcpy(info.name, "Alice");
info.data.i = 100;

printf("Name: %s\n", info.name);
printf("Integer: %d\n", info.data.i);

info.data.f = 220.5; // overwrites previous value
printf("Float: %.2f\n", info.data.f);

return 0;
}

#include <stdio.h>

#include <string.h>union Data {

int i;

float f;

char str[20];

};

struct Info {

char name[50];

union Data data;

};

int main() {

struct Info info;

strcpy(info.name, "Alice");

info.data.i = 100;

printf("Name: %s\n", info.name);

printf("Integer: %d\n", info.data.i);

info.data.f = 220.5; // overwrites previous value

printf("Float: %.2f\n", info.data.f);

return 0;

}

Output:

5. Key Points About Unions

All members share the same memory location.
Only one member can hold a value at a time.
Memory size = size of largest member.
Useful for memory-efficient storage and type alternatives.
Access union members with dot . (normal) or arrow -> (pointer).
Often used in embedded systems, protocol implementations, and type conversion.

C Unions

1. What is a C Unions?

2. Example: Basic Union

3. Size of a Union

4. Union with Struct Example

5. Key Points About Unions

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