Structures, Unions & Enums

Concept

A struct groups fields of different types. Access with . on a variable and -> through a pointer.

The compiler adds padding so fields are aligned, so sizeof(struct) can be larger than the sum of its fields. Order fields large-to-small to reduce padding.

A union shares one memory area among all members (size = largest member). enum gives names to integer constants. typedef creates a type alias.

Code (C17)

#include <stdio.h>

typedef struct {
    int id;
    char name[20];
    double cgpa;
} Student;

union Value { int i; float f; char c; };
enum Level { BEGINNER, INTERMEDIATE, EXPERT };

int main(void) {
    Student s = {1, "Asha", 8.7};
    Student *p = &s;
    p->cgpa = 9.1;
    printf("%s %.1f\n", s.name, s.cgpa);
    printf("sizeof Student = %zu, union = %zu\n", sizeof(Student), sizeof(union Value));
    enum Level lv = EXPERT;
    printf("%d\n", lv);  // 2
    return 0;
}

Remember

• -> is (*p).field.
• Padding affects sizeof.
• Only one union member is valid at a time.

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