Data Structures In C

Design clean, efficient data models. Understand memory alignment, padding, unions, and enumerations to represent complex real-world entities.

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Struct Basics

Grouping related variables into a single unit.

student.c

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

int main() {
    struct Student s1;
    s1.id = 101;
    s1.gpa = 3.8;
}

Cleaner syntax without 'struct' keyword.

typedef struct {
    int x, y;
} Point;

Point p1; // No 'struct' needed

Pass by value copies data. Pass by reference is faster.

void print(Point p);   // Copy
void modify(Point *p); // Reference

Padding and Alignment. Why structs are larger than you think.

padding.c

struct Example {
    char c;      // 1 byte
    int i;       // 4 bytes
};
// Size is NOT 5 bytes! It is 8 bytes.
// [c] [padding:3] [i:4]

                Optimization Tip: Order members from largest to smallest to minimize padding wastage.
                
double > int > short > char

Shared memory space. Only one member active at a time.

union Data {
    int i;
    float f;
    char str[20];
};

union Data data;
data.i = 10;
data.f = 220.5; // Overwrites i!

Named integer constants for readability.

enum Status {
    PENDING = 0,
    RUNNING = 1,
    DONE = 2
};

enum Status s = RUNNING;