1. Introduction

In C programming, structures and unions are both user-defined data types used to store multiple data types. However, they differ in how they manage memory and how they're used. A structure allocates enough space to store all its members, whereas a union allocates space equal to its largest member and shares this memory space among all its members.

2. Key Points

1. Structures allocate separate memory locations for each of their members.

2. Unions allocate a single memory location shared by all their members.

3. Changing one member in a union changes the values of all members.

4. Structures are used when you want to store different values for each member.

3. Differences

Structure	Union
Allocates separate memory for each member.	Allocates common shared memory for all members.
The size is the sum of the size of all members.	The size is the size of the largest member.
Each member has its own value.	Only one member can have a meaningful value at any given time.

4. Example

#include <stdio.h>

// Structure example
struct StructExample {
    int a;
    char b;
    float c;
};

// Union example
union UnionExample {
    int a;
    char b;
    float c;
};

int main() {
    struct StructExample se = {10, 'A', 3.14};
    union UnionExample ue;
    ue.a = 10;

    printf("Structure a: %d, b: %c, c: %f\n", se.a, se.b, se.c);
    printf("Union a: %d\n", ue.a);

    ue.b = 'B';
    printf("Union b: %c\n", ue.b);
    printf("Union a (after modifying b): %d\n", ue.a);

    return 0;
}

Output:

Structure a: 10, b: A, c: 3.140000
Union a: 10
Union b: B
Union a (after modifying b): 66

Explanation:

1. The structure example shows distinct values for each member.

2. In the union, changing b also alters the value of a because they share the same memory space. Hence, after b is set to 'B', which has an ASCII value of 66, a shows 66 instead of 10.