Difference Between Structure and Union in C

Structures and unions are user-defined data types in C that allow grouping of different data types. While they look similar, they differ mainly in memory allocation and usage behavior.

What is a Structure?

A structure is a collection of variables of different data types, where each member has its own separate memory location.

// Structure example
#include <stdio.h>

struct Student {
    int id;
    float marks;
};

int main() {
    struct Student s1 = {1, 95.5};
    printf("%d %.1f", s1.id, s1.marks);
    return 0;
}

What is a Union?

A union is a collection of variables where all members share the same memory location. Only one member can hold a value at a time.

// Union example
#include <stdio.h>

union Data {
    int i;
    float f;
};

int main() {
    union Data d;
    d.i = 10;
    printf("%d", d.i);
    return 0;
}

Key Differences Between Structure and Union

• Structure allocates separate memory for each member, union shares memory
• All structure members can be used simultaneously, only one union member at a time
• Structure size is sum of all members, union size is size of largest member
• Structures are used for complex data records, unions for memory optimization
• Changing one union member affects others

Comparison Table

Memory Example

// Size comparison
printf("%lu", sizeof(struct Student));
printf("%lu", sizeof(union Data));

When to Use Structure?

• When all data members are needed
• For storing records (student, employee)
• When clarity is important
• For complex data representation

When to Use Union?

• When memory optimization is required
• When only one value is needed at a time
• In embedded systems
• For variant data types

Real-World Applications

• Structures in databases
• Unions in hardware programming
• Structures in file systems
• Unions in memory-constrained systems
• Both used in system-level programming

Common Mistakes to Avoid

• Accessing inactive union member
• Misunderstanding memory sharing
• Incorrect size assumptions
• Improper initialization
• Confusing struct and union usage

Advanced Concepts

• Nested structures and unions
• Bit fields
• Padding and alignment
• Anonymous unions
• Tagged unions

Practice Exercises

• Create struct for employee record
• Compare struct and union sizes
• Use union for memory optimization
• Implement tagged union
• Explore alignment issues

Conclusion

Structures and unions serve different purposes in C programming. Structures are best for storing complete data sets, while unions are useful when memory efficiency is critical.