Difference Between Structure and Union in C (With Memory Layout)

In low-level memory modeling, knowing the difference between structure and union layouts is critical for hardware registers and network packet parsers. The difference between the two structures: structure and union in C is that structure stores all the elements in different memory location sequentially (along with the alignment padding) and union stores all the elements in a same memory location in which the largest sized member can be stored. Writing to one union member overwrites the shared bytes, so unions are perfect for type-safe tagged variants and for protocols which have to be kept in memory-constrained embedded systems.

Difference Between Structure and Union in C: sizeof, Padding & Shared Memory — Technical Overview

#include <stdio.h>

struct PacketStruct {
    int id;
    double payload;
};

union PacketUnion {
    int id;
    double payload;
};

int main(void) {
    printf("sizeof(struct PacketStruct) = %zu bytes\n", sizeof(struct PacketStruct));
    printf("sizeof(union PacketUnion)   = %zu bytes\n", sizeof(union PacketUnion));
    return 0;
}

Runnable C example

#include <stdio.h>

struct PacketStruct {
    int id;
    double payload;
};

union PacketUnion {
    int id;
    double payload;
};

int main(void) {
    printf("sizeof(struct PacketStruct) = %zu bytes\n", sizeof(struct PacketStruct));
    printf("sizeof(union PacketUnion)   = %zu bytes\n", sizeof(union PacketUnion));
    return 0;
}

Open example in compiler

Expected Output

sizeof(struct PacketStruct) = 16 bytes
sizeof(union PacketUnion)   = 8 bytes