What is a Structure (struct) in C?

A structure (struct) is a user-defined composite data type that groups variables of different types under a single named entity. Unlike arrays (which hold elements of the same type), structures represent complex domain records such as students, coordinates, or database rows.

Declaring and Using Structs

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

struct Student {
    int id;
    char name[50];
    float gpa;
};

int main() {
    struct Student s1;
    s1.id = 101;
    strcpy(s1.name, "Alice Johnson");
    s1.gpa = 3.85f;

    printf("Student ID: %d\n", s1.id);
    printf("Name: %s\n", s1.name);
    printf("GPA: %.2f\n", s1.gpa);

    return 0;
}

Using typedef for Cleaner Declarations

The typedef keyword creates an alias for structured types, avoiding the repeated use of the struct keyword:

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

typedef struct {
    int x;
    int y;
} Point;

int main() {
    Point p1 = {15, 30};
    printf("Point Coordinates: (%d, %d)\n", p1.x, p1.y);
    return 0;
}

Struct Pointers and Arrow Operator (->)

When accessing structure fields via a pointer, use the arrow operator (ptr->member), which is shorthand for (*ptr).member:

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

typedef struct {
    int width;
    int height;
} Rectangle;

void printRectangle(const Rectangle *rect) {
    int area = rect->width * rect->height;
    printf("Rectangle [%d x %d] | Area: %d\n", rect->width, rect->height, area);
}

int main() {
    Rectangle box = {10, 20};
    printRectangle(&box);
    return 0;
}

Structs vs. Unions

• struct: Allocates distinct memory for every member. Total size is at least the sum of all members (plus padding).
• union: All members share the exact same memory location. The size of the union equals the size of its largest member.

Related Tutorials

• Learn how to persist structs to disk in File Handling in C.
• Review memory addresses and dereferencing in Pointers in C Guide.
• Master homogeneous collections in Arrays in C Programming.