Key Differences: struct vs union

• struct: Every member has its own distinct memory location. The total size is at least the sum of sizes of all members (plus padding for alignment).
• union: All members share the same starting memory location. The total size is equal to the size of the largest member.

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Complete C Source Code

c (ISO Standard)
Run in Compiler.cCopy
#include <stdio.h>

struct StructData {
    int integer;
    float decimal;
    char character;
};

union UnionData {
    int integer;
    float decimal;
    char character;
};

int main() {
    struct StructData s;
    union UnionData u;

    printf("Memory Size Comparison:\n");
    printf("  sizeof(struct StructData) = %zu bytes\n", sizeof(struct StructData));
    printf("  sizeof(union UnionData)   = %zu bytes\n\n", sizeof(union UnionData));

    /* Using struct: all fields retain distinct values */
    s.integer = 42;
    s.decimal = 3.14f;
    s.character = 'Z';
    printf("Struct Members (Independent):\n");
    printf("  int: %d | float: %.2f | char: %c\n\n", s.integer, s.decimal, s.character);

    /* Using union: fields share the same memory location */
    u.integer = 100;
    printf("Union after setting integer:\n");
    printf("  u.integer = %d\n", u.integer);

    u.character = 'A';
    printf("Union after setting character:\n");
    printf("  u.character = %c | u.integer (corrupted) = %d\n", u.character, u.integer);

    return 0;
}

Sample Output

text (ISO Standard)
Copy
Memory Size Comparison:
  sizeof(struct StructData) = 12 bytes
  sizeof(union UnionData)   = 4 bytes

Struct Members (Independent):
  int: 42 | float: 3.14 | char: Z

Union after setting integer:
  u.integer = 100
Union after setting character:
  u.character = A | u.integer (corrupted) = 65

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Complexity Analysis

• Access Time: O(1) direct offset access for both.
• Memory Conservation: Unions save memory in embedded and low-level network packet header decoders.

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Related Guides

• Comprehensive struct guide in Structures in C.
• Pointer access via arrow operator (->) in Pointers in C.