User-Defined Types: Structs & Unions

Structures and unions allow you to bundle variables of different data types together under a single custom type name.

1. Structures (struct)

In a structure, every member is allocated its own unique location in memory. All members can hold values simultaneously.

#include <stdio.h>
#include <string.h>

struct Product {
    char name[30];
    int id;
    double price;
};

int main() {
    struct Product p1;
    strcpy(p1.name, "Notebook");
    p1.id = 4501;
    p1.price = 2.99;

    printf("Item: %s (ID: %d), Price: $%.2lf\n", p1.name, p1.id, p1.price);
    return 0;
}

2. Pointer to Structs (-> operator)

When accessing struct members through a pointer, use the arrow operator (->) as a shorthand for dereferencing and accessing:

struct Product *ptr = &p1;
ptr->price = 3.49; // Equivalent to (*ptr).price = 3.49

3. Memory Padding & Alignment

The size of a struct is not always the sum of its member sizes. Compilers add invisible bytes (padding) to align member variables to CPU word boundaries for faster access:

struct Sample {
    char c;   // 1 byte
    // 3 bytes of padding inserted here
    int i;    // 4 bytes
}; // sizeof(Sample) evaluates to 8 bytes, not 5!

4. Unions

Unlike structures, all members in a union share the same memory space. The size of a union is equal to the size of its largest member:

union Measurement {
    int meters;
    float precision_value;
}; // Shares 4 bytes of memory. Writing to 'meters' overwrites 'precision_value'.