C Notes

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Unions in C – Shared Memory Data Types

Complete guide to unions in C programming — definition, syntax, shared memory concept, difference between struct and union, practical examples, type punning, tagged unions, and interview questions.

A union looks almost identical to a structure in syntax, but its behavior is fundamentally different. While a struct allocates separate memory for each member, a union stores all members in the same memory location. Only one member can hold a valid value at any time.

What is a Union?

Example

union Data { int i; // 4 bytes float f; // 4 bytes char str[20]; // 20 bytes }; // Size of union = size of LARGEST member = 20 bytes

All members share the same starting address. Writing to one member overwrites the others.

Memory Comparison: Struct vs Union

struct Example { union Example { int i; // offset 0 int i; // offset 0 float f; // offset 4 float f; // offset 0 (SAME!) char c; // offset 8 char c; // offset 0 (SAME!) }; }; Size: 12 bytes (sum) Size: 4 bytes (max member) STRUCT memory: UNION memory: ┌─────┬─────┬─────┐ ┌─────────────┐ │ i │ f │ c │ │ i / f / c │ ← All share this space └─────┴─────┴─────┘ └─────────────┘ 4B 4B 4B = 12B 4 bytes total

Basic Union Example

Example

#include <stdio.h> union Data { int i; float f; char str[20]; }; int main() { union Data data; printf("Size of union: %zu bytes\n\n", sizeof(union Data)); data.i = 42; printf("data.i = %d\n", data.i); data.f = 3.14; printf("data.f = %.2f\n", data.f); printf("data.i = %d (corrupted!)\n\n", data.i); // Garbage now strcpy(data.str, "Hello"); printf("data.str = %s\n", data.str); printf("data.i = %d (corrupted!)\n", data.i); // Garbage now printf("data.f = %f (corrupted!)\n", data.f); // Garbage now return 0; }

Output

Size of union: 20 bytes

data.i = 42
data.f = 3.14
data.i = 1078523331 (corrupted!)

data.str = Hello
data.i = 1819043144 (corrupted!)
data.f = 1143139122437582505939828736.000000 (corrupted!)

Struct vs Union — Complete Comparison

Feature	struct	union
Memory	Sum of all members (+ padding)	Size of largest member
Members active	All simultaneously	Only one at a time
Access	Any member anytime	Only last-written member
Use case	Group related data	Variant/alternative types
Syntax	`struct Name {}`	`union Name {}`

Practical Use: Tagged Union (Variant Type)

The most common real-world use of unions is a tagged union — a union paired with a tag indicating which member is currently valid:

Example

#include <stdio.h> #include <string.h> enum ValueType { INT_VAL, FLOAT_VAL, STRING_VAL }; struct Value { enum ValueType type; // The "tag" union { int i; float f; char str[50]; } data; // The union }; void print_value(struct Value v) { switch (v.type) { case INT_VAL: printf("Integer: %d\n", v.data.i); break; case FLOAT_VAL: printf("Float: %.2f\n", v.data.f); break; case STRING_VAL: printf("String: \"%s\"\n", v.data.str); break; } } int main() { struct Value values[3]; values[0].type = INT_VAL; values[0].data.i = 42; values[1].type = FLOAT_VAL; values[1].data.f = 3.14; values[2].type = STRING_VAL; strcpy(values[2].data.str, "Hello World"); for (int i = 0; i < 3; i++) { printf("Value %d → ", i); print_value(values[i]); } return 0; }

Output

Value 0 → Integer: 42
Value 1 → Float: 3.14
Value 2 → String: "Hello World"

Type Punning with Unions

Unions allow you to reinterpret the same bits as different types:

c example

#include <stdio.h>

union FloatBits {
    float f;
    unsigned int bits;
};

int main() {
    union FloatBits fb;
    fb.f = 3.14f;

    printf("Float: %f\n", fb.f);
    printf("Bits:  0x%08X\n", fb.bits);
    printf("Binary representation of 3.14f in IEEE 754\n");

    // Detect sign, exponent, mantissa
    int sign = (fb.bits >> 31) & 1;
    int exp = (fb.bits >> 23) & 0xFF;
    int mantissa = fb.bits & 0x7FFFFF;

    printf("Sign: %d, Exponent: %d, Mantissa: 0x%06X\n", sign, exp, mantissa);

    return 0;
}

Output

Float: 3.140000
Bits:  0x4048F5C3
Binary representation of 3.14f in IEEE 754
Sign: 0, Exponent: 128, Mantissa: 0x48F5C3

Union for Memory-Efficient Storage

c example

#include <stdio.h>

struct Event {
    int type;  // 1 = keyboard, 2 = mouse, 3 = timer
    union {
        struct { int key_code; int modifiers; } keyboard;
        struct { int x; int y; int button; } mouse;
        struct { int timer_id; float elapsed; } timer;
    } data;
};

int main() {
    struct Event e;

    // Mouse click event
    e.type = 2;
    e.data.mouse.x = 150;
    e.data.mouse.y = 300;
    e.data.mouse.button = 1;

    printf("Event type: Mouse\n");
    printf("Position: (%d, %d), Button: %d\n",
           e.data.mouse.x, e.data.mouse.y, e.data.mouse.button);

    printf("\nSize of Event: %zu bytes\n", sizeof(struct Event));
    printf("(Without union it would be much larger)\n");

    return 0;
}

Output

Event type: Mouse
Position: (150, 300), Button: 1

Size of Event: 16 bytes
(Without union it would be much larger)

Anonymous Unions (C11)

Example

#include <stdio.h> struct Token { int type; union { // Anonymous — access directly int int_val; float float_val; char str_val[32]; }; }; int main() { struct Token t; t.type = 1; t.int_val = 42; // Access directly without .data prefix printf("Token value: %d\n", t.int_val); return 0; }

Output

Token value: 42

Interview Questions

Q1: What is the size of a union?

The size of a union equals the size of its largest member (plus any padding needed for alignment). All members share the same memory, so the union must be large enough to hold the biggest one.

Q2: Can you access all union members at the same time?

No. Only the last-written member contains a valid value. Reading any other member after writing a different one is technically undefined behavior (though type punning through unions is widely supported by compilers as an extension).

Q3: What is a tagged union and why is it useful?

A tagged union pairs a union with an enum/int "tag" that records which member is currently valid. This implements variant/sum types safely — the tag prevents you from reading the wrong member. It's used in interpreters, event systems, and protocol parsers.

Q4: When would you use a union instead of a struct?

When you have multiple possible representations for a piece of data, but only one is active at a time. Examples: a JSON value (int OR float OR string), a network packet (TCP OR UDP header), a hardware register with bitfield and integer interpretations.

Q5: Is it safe to use unions for type punning?

In standard C (C99+), reading a union member other than the last one written is implementation-defined (not strictly undefined). Most compilers (GCC, Clang) explicitly support this as an extension. It's commonly used for inspecting float bits, network byte manipulation, etc.

Summary

Unions store all members at the same memory address — only one is valid at a time
Union size = size of the largest member
Use tagged unions (union + enum tag) for type-safe variants
Useful for memory-efficient event systems, protocol parsing, and hardware registers
Type punning through unions lets you reinterpret bits (widely supported)
Anonymous unions (C11) allow direct member access without naming the union

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Unions in C – Shared Memory Data Types.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, structures, and, unions, unions in c – shared memory data types

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Structures And UnionsArrays of Structures in C – Managing CollectionsComplete guide to arrays of structures in C — declaring and initializing struct arrays, student database example, sorting, searching, passing to functions, practical examples with output.Structures And UnionsEnums in C – Enumerated Data TypesComplete guide to enums in C programming — definition, syntax, automatic and custom values, enum vs #define, use in switch statements, bit flags, practical examples with output.Structures And UnionsNested Structures in C – Struct Within StructComplete guide to nested structures in C — defining structures inside structures, accessing nested members, practical examples like address within employee, date within record, with output.Structures And UnionsPointers to Structures in C – Arrow OperatorComplete guide to pointers to structures in C — arrow operator (->), dynamic struct allocation, passing structs by pointer, linked list basics, practical examples with output.Structures And UnionsStructures in C – User-Defined Data TypesComplete guide to structures in C programming — definition, declaration, initialization, accessing members with dot operator, struct memory layout, size and padding, practical examples with output.Structures And Unionstypedef in C – Creating Type AliasesComplete guide to typedef in C programming — creating type aliases, typedef with structures, typedef with pointers and arrays, function pointer typedefs, improving code readability, practical examples.

Previoustypedef in C – Creating Type Aliases

C Notes

Unions in C – Shared Memory Data Types

Complete guide to unions in C programming — definition, syntax, shared memory concept, difference between struct and union, practical examples, type punning, tagged unions, and interview questions.

What is a Union?

Example

union Data { int i; // 4 bytes float f; // 4 bytes char str[20]; // 20 bytes }; // Size of union = size of LARGEST member = 20 bytes

All members share the same starting address. Writing to one member overwrites the others.

Memory Comparison: Struct vs Union

Basic Union Example

Example

Output

Size of union: 20 bytes

data.i = 42
data.f = 3.14
data.i = 1078523331 (corrupted!)

data.str = Hello
data.i = 1819043144 (corrupted!)
data.f = 1143139122437582505939828736.000000 (corrupted!)

Struct vs Union — Complete Comparison

Feature	struct	union
Memory	Sum of all members (+ padding)	Size of largest member
Members active	All simultaneously	Only one at a time
Access	Any member anytime	Only last-written member
Use case	Group related data	Variant/alternative types
Syntax	`struct Name {}`	`union Name {}`

Practical Use: Tagged Union (Variant Type)

The most common real-world use of unions is a tagged union — a union paired with a tag indicating which member is currently valid:

Example

Output

Value 0 → Integer: 42
Value 1 → Float: 3.14
Value 2 → String: "Hello World"

Type Punning with Unions

Unions allow you to reinterpret the same bits as different types:

c example

#include <stdio.h>

union FloatBits {
    float f;
    unsigned int bits;
};

int main() {
    union FloatBits fb;
    fb.f = 3.14f;

    printf("Float: %f\n", fb.f);
    printf("Bits:  0x%08X\n", fb.bits);
    printf("Binary representation of 3.14f in IEEE 754\n");

    // Detect sign, exponent, mantissa
    int sign = (fb.bits >> 31) & 1;
    int exp = (fb.bits >> 23) & 0xFF;
    int mantissa = fb.bits & 0x7FFFFF;

    printf("Sign: %d, Exponent: %d, Mantissa: 0x%06X\n", sign, exp, mantissa);

    return 0;
}

Output

Float: 3.140000
Bits:  0x4048F5C3
Binary representation of 3.14f in IEEE 754
Sign: 0, Exponent: 128, Mantissa: 0x48F5C3

Union for Memory-Efficient Storage

c example

#include <stdio.h>

struct Event {
    int type;  // 1 = keyboard, 2 = mouse, 3 = timer
    union {
        struct { int key_code; int modifiers; } keyboard;
        struct { int x; int y; int button; } mouse;
        struct { int timer_id; float elapsed; } timer;
    } data;
};

int main() {
    struct Event e;

    // Mouse click event
    e.type = 2;
    e.data.mouse.x = 150;
    e.data.mouse.y = 300;
    e.data.mouse.button = 1;

    printf("Event type: Mouse\n");
    printf("Position: (%d, %d), Button: %d\n",
           e.data.mouse.x, e.data.mouse.y, e.data.mouse.button);

    printf("\nSize of Event: %zu bytes\n", sizeof(struct Event));
    printf("(Without union it would be much larger)\n");

    return 0;
}

Output

Event type: Mouse
Position: (150, 300), Button: 1

Size of Event: 16 bytes
(Without union it would be much larger)

Anonymous Unions (C11)

Example

Output

Token value: 42

Interview Questions

Q1: What is the size of a union?

The size of a union equals the size of its largest member (plus any padding needed for alignment). All members share the same memory, so the union must be large enough to hold the biggest one.

Q2: Can you access all union members at the same time?

Q3: What is a tagged union and why is it useful?

Q4: When would you use a union instead of a struct?

Q5: Is it safe to use unions for type punning?

Summary

Unions store all members at the same memory address — only one is valid at a time
Union size = size of the largest member
Use tagged unions (union + enum tag) for type-safe variants
Useful for memory-efficient event systems, protocol parsing, and hardware registers
Type punning through unions lets you reinterpret bits (widely supported)
Anonymous unions (C11) allow direct member access without naming the union

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Unions in C – Shared Memory Data Types.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, structures, and, unions, unions in c – shared memory data types

Continue learning this concept

Previoustypedef in C – Creating Type Aliases