Structures & Unions in C

A structure (struct) in C groups several related variables of different types under one name, so you can treat them as a single unit. A union looks similar but stores only one of its members at a time, sharing the same memory. Structures are everywhere in real C code — they model real-world things like a student, a point, or a date.

Defining a structure

struct Student {
    char name[30];
    int  roll;
    float marks;
};   /* don't forget the semicolon after the closing brace */

This defines a type called struct Student. Now you can create variables of that type and fill in each member:

#include <stdio.h>

struct Student {
    char name[30];
    int  roll;
    float marks;
};

int main(void) {
    struct Student s1 = {"Aarav", 12, 88.5f};   /* initialize members in order */

    /* Access members with the dot operator */
    printf("%s (roll %d) scored %.1f\n", s1.name, s1.roll, s1.marks);

    s1.marks = 91.0f;   /* update a single member */
    return 0;
}

The dot and arrow operators

• Use . (dot) when you have a struct variable: s1.roll.
• Use -> (arrow) when you have a pointer to a struct: ptr->roll. It's shorthand for (*ptr).roll.

struct Student s1 = {"Mira", 7, 76.0f};
struct Student *p = &s1;       /* pointer to the struct */

printf("%d\n", p->roll);       /* arrow: same as (*p).roll -> 7 */

This connects directly to pointers in C.

typedef for cleaner names

Writing struct Student everywhere is verbose. typedef gives the type a short alias:

typedef struct {
    int x;
    int y;
} Point;            /* now "Point" is the type name */

Point origin = {0, 0};   /* no "struct" keyword needed */

Unions: shared memory

A union has members like a struct, but they all share the same memory location, so it can hold only one member's value at a time. Its size is that of its largest member:

#include <stdio.h>

union Value {
    int   i;
    float f;
    char  c;
};   /* all three overlap in memory */

int main(void) {
    union Value v;
    v.i = 65;                 /* store an int */
    printf("%d\n", v.i);      /* 65 */

    v.f = 3.14f;              /* now the SAME memory holds a float */
    /* reading v.i now is meaningless -- it was overwritten */
    printf("%.2f\n", v.f);    /* 3.14 */
    return 0;
}

Use a union when a value can be one of several types at different times and you want to save memory. Use a struct when you need all the fields at once.

struct vs union at a glance

• struct: members each get their own memory; total size is the sum (plus padding). Holds all fields together.
• union: members share memory; total size is the largest member. Holds one field at a time.

Common mistakes

• Forgetting the semicolon after the closing } of a struct/union definition.
• Using . on a pointer (or -> on a plain variable): match the operator to whether you have a pointer.
• Reading a union member you didn't last write — you get garbage, because the memory was overwritten.
• Comparing structs with == — not allowed; compare members individually.
• Assuming a struct's size equals the sum of members — the compiler may add padding for alignment.

FAQ

Can a struct contain another struct? Yes — nesting is common, e.g. a Student containing a Date dob.

When would I use a union? Rarely as a beginner, but it's useful for memory-constrained code or type-tagged values.

Continue at the hub C Programming, then Pointers in C and Dynamic Memory in C.

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