Structures in C

01What is a Structure?

A structure (struct) is a user-defined data type that allows you to group different data types together under one name. Think of it as creating your own custom data type.

WHY Do We Need Structures?

The Problem: Without Structs

// Managing a student needs 4 separate variables!

char name1[50] = "Alice";

int id1 = 101;

float gpa1 = 3.85;

int age1 = 20;

// What about 100 students? 400 variables!

✓The Solution: With Structs

// Everything in one neat package!

struct Student alice = {

"Alice", 101, 3.85, 20

};

// 100 students? Just an array!

Think of It Like a Blueprint

struct Student

• name (text)

• id (number)

• gpa (decimal)

Blueprint (Template)

No memory yet!

→

student1

Alice

101

3.85

Actual Variable

student2

Bob

102

3.92

Actual Variable

🆚 Arrays vs Structures

Array

• All elements same type
• Access by index [0], [1], [2]...
• Example: 5 integers

All same type (int)

Structure

• Different types together
• Access by name .age, .name
• Example: Person data

Alice

3.85

Mixed types (string, int, float)

Real-World Examples

‍Student

name, ID, GPA

Book

title, author, price

Point

x, y coordinates

D Date

day, month, year

02Defining a Structure

structure_basic.c

1#include <stdio.h>
2
3// Define a structure (blueprint)
4struct Student {
5    int id;              // Member 1: integer
6    char name[50];       // Member 2: string (char array)
7    float gpa;           // Member 3: float
8    int age;             // Member 4: integer
9};  // Don't forget the semicolon!
10
11int main() {
12    // Create a structure variable
13    struct Student student1;
14    
15    // Assign values to members using dot (.) operator
16    student1.id = 101;
17    student1.gpa = 3.85;
18    student1.age = 20;
19    // For strings, use strcpy
20    // strcpy(student1.name, "Alice");
21    
22    // Or initialize at declaration
23    struct Student student2 = {102, "Bob", 3.92, 21};
24    
25    // Print values
26    printf("Student ID: %d\n", student2.id);
27    printf("Name: %s\n", student2.name);
28    printf("GPA: %.2f\n", student2.gpa);
29    printf("Age: %d\n", student2.age);
30    
31    return 0;
32}

Output:

Student ID: 102
Name: Bob
GPA: 3.92
Age: 21

How This Program Works

struct Student {...} — Defines a blueprint (template) for Student data. No memory allocated yet.

struct Student student1; — Creates an actual variable using the blueprint. Memory is now allocated.

student1.id = 101 — The . operator accesses member "id" and assigns value 101 to it.

{102, "Bob", 3.92, 21} — Initialize all members at once in order they were declared.

student2.name — Access the name member. Since it's a char array, use %s to print.

Key Points

• struct keyword defines the structure
• Members are listed inside curly braces {}
• Semicolon ; required after closing brace
• Use . (dot) operator to access members

03Structure Memory Layout

Structure members are stored in contiguous memory in the order they are declared. However, the compiler may add padding bytes for alignment.

Memory Layout: struct Point { int x; int y; }

x (int)

4 bytes

y (int)

4 bytes

Total: 8 bytes (contiguous memory)

Memory Padding & Alignment

The compiler adds padding bytes to align data on memory boundaries for faster access:

struct Example { char a; int b; char c; }

pad

b (int)

4 bytes

pad

Total: 12 bytes (not 6!) due to padding

struct_padding.c

1#include <stdio.h>
2
3struct WithPadding {
4    char a;     // 1 byte + 3 padding
5    int b;      // 4 bytes
6    char c;     // 1 byte + 3 padding
7};  // Total: 12 bytes
8
9struct Optimized {
10    int b;      // 4 bytes
11    char a;     // 1 byte
12    char c;     // 1 byte + 2 padding
13};  // Total: 8 bytes (better!)
14
15int main() {
16    printf("WithPadding size: %zu bytes\n", sizeof(struct WithPadding));   // 12
17    printf("Optimized size: %zu bytes\n", sizeof(struct Optimized));       // 8
18    
19    return 0;
20}

Optimization Tip

Order structure members from largest to smallest to minimize padding and save memory.

04typedef with Structures

typedef creates an alias (shorter name) for a type, so you don't have to write struct every time.

typedef_struct.c

1#include <stdio.h>
2
3// Without typedef - must use "struct Student" everywhere
4struct Student {
5    int id;
6    char name[50];
7};
8
9// With typedef - can use just "Person"
10typedef struct {
11    int id;
12    char name[50];
13    int age;
14} Person;  // "Person" is now the type name
15
16int main() {
17    // Without typedef
18    struct Student s1 = {101, "Alice"};
19    
20    // With typedef (cleaner!)
21    Person p1 = {201, "Bob", 25};
22    Person p2 = {202, "Carol", 30};
23    
24    printf("Student: %s\n", s1.name);
25    printf("Person 1: %s, Age: %d\n", p1.name, p1.age);
26    printf("Person 2: %s, Age: %d\n", p2.name, p2.age);
27    
28    return 0;
29}

Without typedef

main.c

struct Student s1;
struct Student s2;

With typedef ✓

main.c

Person p1;
Person p2;

05Pointers to Structures & Arrow Operator

When you have a pointer to a structure, you use the -> (arrow) operator to access members instead of the dot.

When to Use . vs ->

. Dot Operator

// Direct variable

Student s;

s.age = 20;

Use with the struct itself

-> Arrow Operator

// Pointer to struct

Student *ptr;

ptr->age = 20;

Use with pointer to struct

What's Happening in Memory

ptr

0x1000

Pointer (stores address)

→

Student struct

name: Alice

age: 20

Actual struct data @0x1000

ptr->age means: Go to address in ptr, then access age member

struct_pointer.c

1#include <stdio.h>
2
3typedef struct {
4    char name[50];
5    int age;
6    float gpa;
7} Student;
8
9int main() {
10    Student alice = {"Alice", 20, 3.85};
11    
12    // Create pointer to struct
13    Student *ptr = &alice;
14    
15    // Access with DOT (using the variable directly)
16    printf("Using dot: %s, %d\n", alice.name, alice.age);
17    
18    // Access with ARROW (using pointer)
19    printf("Using arrow: %s, %d\n", ptr->name, ptr->age);
20    
21    // Modify through pointer
22    ptr->age = 21;
23    ptr->gpa = 3.90;
24    
25    printf("After modify: %s, %d, %.2f\n", 
26           alice.name, alice.age, alice.gpa);
27    
28    return 0;
29}

Output

Using dot: Alice, 20

Using arrow: Alice, 20

After modify: Alice, 21, 3.90

Arrow is Shorthand!

ptr->age is exactly the same as (*ptr).age. The arrow operator just makes it cleaner!

ptr->age = (*ptr).age

06Nested Structures

A structure can contain another structure as a member — this is called nesting.

nested_struct.c

1#include <stdio.h>
2
3// Define a Date structure
4typedef struct {
5    int day;
6    int month;
7    int year;
8} Date;
9
10// Define a Person structure with nested Date
11typedef struct {
12    char name[50];
13    int age;
14    Date birthdate;  // Nested structure!
15} Person;
16
17int main() {
18    // Initialize with nested values
19    Person person1 = {
20        "Alice",
21        25,
22        {15, 6, 1999}  // birthdate: June 15, 1999
23    };
24    
25    // Access nested members with multiple dots
26    printf("Name: %s\n", person1.name);
27    printf("Age: %d\n", person1.age);
28    printf("Birthdate: %d/%d/%d\n", 
29           person1.birthdate.day,
30           person1.birthdate.month,
31           person1.birthdate.year);
32    
33    // Modify nested member
34    person1.birthdate.year = 1998;
35    
36    return 0;
37}

Output:

Name: Alice
Age: 25
Birthdate: 15/6/1999

07Array of Structures

You can create an array where each element is a structure — perfect for storing lists of records.

array_of_struct.c

1#include <stdio.h>
2#include <string.h>
3
4typedef struct {
5    int id;
6    char name[30];
7    float gpa;
8} Student;
9
10int main() {
11    // Array of 3 students
12    Student students[3] = {
13        {101, "Alice", 3.85},
14        {102, "Bob", 3.92},
15        {103, "Carol", 3.78}
16    };
17    
18    // Print all students
19    printf("Student Records:\n");
20    printf("%-5s %-15s %s\n", "ID", "Name", "GPA");
21    printf("----------------------------\n");
22    
23    for (int i = 0; i < 3; i++) {
24        printf("%-5d %-15s %.2f\n", 
25               students[i].id, 
26               students[i].name, 
27               students[i].gpa);
28    }
29    
30    // Find highest GPA
31    float maxGpa = 0;
32    int topStudent = 0;
33    for (int i = 0; i < 3; i++) {
34        if (students[i].gpa > maxGpa) {
35            maxGpa = students[i].gpa;
36            topStudent = i;
37        }
38    }
39    printf("\nTop Student: %s with GPA %.2f\n", 
40           students[topStudent].name, maxGpa);
41    
42    // Memory: 3 * sizeof(Student) bytes
43    printf("\nSize of one Student: %zu bytes\n", sizeof(Student));
44    printf("Size of array: %zu bytes\n", sizeof(students));
45    
46    return 0;
47}

Output:

Student Records:
ID Name GPA
----------------------------
101 Alice 3.85
102 Bob 3.92
103 Carol 3.78

Top Student: Bob with GPA 3.92

Size of one Student: 40 bytes
Size of array: 120 bytes

Array of Structures Memory Layout

id, name, gpa

students[0]

id, name, gpa

students[1]

id, name, gpa

students[2]

Each struct stored contiguously in memory

08Passing Structures to Functions

You can pass structures to functions by value (copy) or by pointer (reference). Each has different use cases.

By Value vs By Pointer

Pass by Value (Copy)

original

Alice, 20

→

copy

Alice, 20

✓ Original stays unchanged

✗ Copies entire struct (slow for large structs)

Use: When you don't need to modify

Pass by Pointer (Reference)

ptr

0x1000

→

@0x1000

Alice, 20

✓ Efficient (only copies address)

✓ Can modify original

Use: For large structs or when modifying

struct_functions.c

1#include <stdio.h>
2
3typedef struct {
4    char name[50];
5    int age;
6} Person;
7
8// Pass by VALUE - receives a COPY
9void birthday_value(Person p) {
10    p.age++;  // Only modifies the copy!
11    printf("Inside function: %s is %d\n", p.name, p.age);
12}
13
14// Pass by POINTER - receives the ADDRESS
15void birthday_pointer(Person *p) {
16    p->age++;  // Modifies the ORIGINAL!
17    printf("Inside function: %s is %d\n", p->name, p->age);
18}
19
20int main() {
21    Person alice = {"Alice", 25};
22    
23    printf("Before: %s is %d\n", alice.name, alice.age);
24    
25    birthday_value(alice);  // Pass by value
26    printf("After value: %s is %d\n", alice.name, alice.age);  // Still 25!
27    
28    birthday_pointer(&alice);  // Pass by pointer
29    printf("After pointer: %s is %d\n", alice.name, alice.age);  // Now 26!
30    
31    return 0;
32}

Output

Before: Alice is 25

Inside function: Alice is 26

After value: Alice is 25 ← Unchanged!

Inside function: Alice is 26

After pointer: Alice is 26 ← Modified!

Best Practice

Use pointer when:

• Struct is large
• Need to modify the original

Use const pointer for read-only:

void print(const Person *p)

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Forgetting Semicolon After struct

main.c

struct Point {
    int x;
    int y;
}  // ERROR! Missing semicolon
struct Point {
    int x;
    int y;
};  // Correct!

Assigning Strings with =

main.c

struct Student s;
s.name = "Alice";  // ERROR! Can't assign strings like this
// Correct way:
strcpy(s.name, "Alice");  // Use strcpy()

Comparing Structures with ==

main.c

struct Point p1 = {1, 2};
struct Point p2 = {1, 2};
if (p1 == p2) {  // ERROR! Can't compare structs with ==
}
// Must compare member by member:
if (p1.x == p2.x && p1.y == p2.y) {  // Correct!
}

Watch Out When Copying Code!

Common Mistakes with Structures!

Structures seem simple but Code often makes subtle mistakes that lead to inefficiency or crashes:

1. Passing Large Structs by Value

Copied code often passes entire structs by value, copying all data — slow for large structures:

main.c

void process(struct BigData data) {  // Copies entire struct!
    // Inefficient for large structs
}
// Better: Pass by pointer
void process(struct BigData *data) {
    data->field = value;  // Fast, modifies original
}

2. Shallow Copy with Pointer Members

Copied code might copy a struct containing pointers — both structs then point to the same data:

main.c

struct Person p1 = {"Alice", 25};
struct Person p2 = p1;  // If name is char*, both point to same string!
// Changing p2.name affects p1 too!

3. Forgetting Struct Padding/Alignment

Copied code might assume sizeof(struct) equals sum of member sizes. Wrong! Compilers add padding for alignment. A struct with charand int may be 8 bytes, not 5.

Best Practices: Pass large structs by pointer, use const for read-only access, and always usesizeof() for struct sizes.

12Frequently Asked Questions

Q:When should I use a struct vs an array?

A: Use an array when you have multiple items of the same type (list of scores, collection of numbers). Use a struct when you need to group different types that belong together (a person with name, age, and salary). Arrays are for "many of the same," structs are for "one thing with multiple properties."

Q:What's the difference between . and -> operators?

A: Use . (dot) when you have the actual struct: person.name. Use-> (arrow) when you have a pointer to a struct:personPtr->name. Arrow is shorthand for(*personPtr).name. Simple rule: variable uses dot, pointer uses arrow.

Q:Why use typedef with structs?

A: typedef creates a shorter name so you don't have to write struct every time. Without: struct Person p; With:Person p; (after typedef struct {...} Person;). It's purely convenience — the struct works the same either way.

Q:Can I have a struct inside a struct?

A: Yes! This is called nesting. Example: A Date struct inside a Person struct for their birthday. Access with chained dots: person.birthday.year. You can nest as deep as needed, but keep it reasonable for readability.

Q:How do I copy a struct to another?

A: Simple assignment works:struct Person p2 = p1; copies all members.But beware: if the struct contains pointers, both copies point to the same data (shallow copy). For deep copies with pointer members, you must manually allocate and copy the pointed-to data.

Q:What is struct padding and why should I care?

A: CPUs access memory most efficiently at aligned addresses. The compiler adds padding bytes between struct members to ensure proper alignment. A struct with char then int may have 3 padding bytes after the char. This matters when: (1) writing binary files, (2) sending data over networks, (3) optimizing memory usage. Use sizeof() to check actual size.

Q:Should I pass structs by value or by pointer?

A: By pointer is usually better: faster (no copying), allows modification of the original. Use constto prevent modification: void print(const struct Person *p). Pass by value only for small structs (<16 bytes) that you want to remain unchanged, or when you specifically need a copy.

12Summary

✓What You Learned

✓Structures: Group different types together
✓Dot operator: Access members directly
✓Arrow operator: Access via pointer
✓typedef: Shorter type names
✓Nesting: Structs inside structs
✓Functions: Pass by value or pointer

Quick Reference

Define:

struct Name { type member; };

Create:

struct Name var = {...};

Access (direct):

var.member

Access (pointer):

ptr->member

Structure Usage Flow

struct {...}

Define

→

var = {...}

Create

→

var.member

Access

→

func(var)

Use

01What is a Structure?

A structure (struct) is a user-defined data type that allows you to group different data types together under one name. Think of it as creating your own custom data type.

WHY Do We Need Structures?

The Problem: Without Structs

// Managing a student needs 4 separate variables!

char name1[50] = "Alice";

int id1 = 101;

float gpa1 = 3.85;

int age1 = 20;

// What about 100 students? 400 variables!

✓The Solution: With Structs

// Everything in one neat package!

struct Student alice = {

"Alice", 101, 3.85, 20

};

// 100 students? Just an array!

Think of It Like a Blueprint

struct Student

• name (text)

• id (number)

• gpa (decimal)

Blueprint (Template)

No memory yet!

→

student1

Alice

101

3.85

Actual Variable

student2

Bob

102

3.92

Actual Variable

🆚 Arrays vs Structures

Array

• All elements same type
• Access by index [0], [1], [2]...
• Example: 5 integers

All same type (int)

Structure

• Different types together
• Access by name .age, .name
• Example: Person data

Alice

3.85

Mixed types (string, int, float)

Real-World Examples

‍Student

name, ID, GPA

Book

title, author, price

Point

x, y coordinates

D Date

day, month, year

02Defining a Structure

structure_basic.c

1#include <stdio.h>
2
3// Define a structure (blueprint)
4struct Student {
5    int id;              // Member 1: integer
6    char name[50];       // Member 2: string (char array)
7    float gpa;           // Member 3: float
8    int age;             // Member 4: integer
9};  // Don't forget the semicolon!
10
11int main() {
12    // Create a structure variable
13    struct Student student1;
14    
15    // Assign values to members using dot (.) operator
16    student1.id = 101;
17    student1.gpa = 3.85;
18    student1.age = 20;
19    // For strings, use strcpy
20    // strcpy(student1.name, "Alice");
21    
22    // Or initialize at declaration
23    struct Student student2 = {102, "Bob", 3.92, 21};
24    
25    // Print values
26    printf("Student ID: %d\n", student2.id);
27    printf("Name: %s\n", student2.name);
28    printf("GPA: %.2f\n", student2.gpa);
29    printf("Age: %d\n", student2.age);
30    
31    return 0;
32}

Output:

Student ID: 102
Name: Bob
GPA: 3.92
Age: 21

How This Program Works

struct Student {...} — Defines a blueprint (template) for Student data. No memory allocated yet.

struct Student student1; — Creates an actual variable using the blueprint. Memory is now allocated.

student1.id = 101 — The . operator accesses member "id" and assigns value 101 to it.

{102, "Bob", 3.92, 21} — Initialize all members at once in order they were declared.

student2.name — Access the name member. Since it's a char array, use %s to print.

Key Points

• struct keyword defines the structure
• Members are listed inside curly braces {}
• Semicolon ; required after closing brace
• Use . (dot) operator to access members

03Structure Memory Layout

Structure members are stored in contiguous memory in the order they are declared. However, the compiler may add padding bytes for alignment.

Memory Layout: struct Point { int x; int y; }

x (int)

4 bytes

y (int)

4 bytes

Total: 8 bytes (contiguous memory)

Memory Padding & Alignment

The compiler adds padding bytes to align data on memory boundaries for faster access:

struct Example { char a; int b; char c; }

pad

b (int)

4 bytes

pad

Total: 12 bytes (not 6!) due to padding

struct_padding.c

1#include <stdio.h>
2
3struct WithPadding {
4    char a;     // 1 byte + 3 padding
5    int b;      // 4 bytes
6    char c;     // 1 byte + 3 padding
7};  // Total: 12 bytes
8
9struct Optimized {
10    int b;      // 4 bytes
11    char a;     // 1 byte
12    char c;     // 1 byte + 2 padding
13};  // Total: 8 bytes (better!)
14
15int main() {
16    printf("WithPadding size: %zu bytes\n", sizeof(struct WithPadding));   // 12
17    printf("Optimized size: %zu bytes\n", sizeof(struct Optimized));       // 8
18    
19    return 0;
20}

Optimization Tip

Order structure members from largest to smallest to minimize padding and save memory.

04typedef with Structures

typedef creates an alias (shorter name) for a type, so you don't have to write struct every time.

typedef_struct.c

1#include <stdio.h>
2
3// Without typedef - must use "struct Student" everywhere
4struct Student {
5    int id;
6    char name[50];
7};
8
9// With typedef - can use just "Person"
10typedef struct {
11    int id;
12    char name[50];
13    int age;
14} Person;  // "Person" is now the type name
15
16int main() {
17    // Without typedef
18    struct Student s1 = {101, "Alice"};
19    
20    // With typedef (cleaner!)
21    Person p1 = {201, "Bob", 25};
22    Person p2 = {202, "Carol", 30};
23    
24    printf("Student: %s\n", s1.name);
25    printf("Person 1: %s, Age: %d\n", p1.name, p1.age);
26    printf("Person 2: %s, Age: %d\n", p2.name, p2.age);
27    
28    return 0;
29}

Without typedef

main.c

struct Student s1;
struct Student s2;

With typedef ✓

main.c

Person p1;
Person p2;

05Pointers to Structures & Arrow Operator

When you have a pointer to a structure, you use the -> (arrow) operator to access members instead of the dot.

When to Use . vs ->

. Dot Operator

// Direct variable

Student s;

s.age = 20;

Use with the struct itself

-> Arrow Operator

// Pointer to struct

Student *ptr;

ptr->age = 20;

Use with pointer to struct

What's Happening in Memory

ptr

0x1000

Pointer (stores address)

→

Student struct

name: Alice

age: 20

Actual struct data @0x1000

ptr->age means: Go to address in ptr, then access age member

struct_pointer.c

1#include <stdio.h>
2
3typedef struct {
4    char name[50];
5    int age;
6    float gpa;
7} Student;
8
9int main() {
10    Student alice = {"Alice", 20, 3.85};
11    
12    // Create pointer to struct
13    Student *ptr = &alice;
14    
15    // Access with DOT (using the variable directly)
16    printf("Using dot: %s, %d\n", alice.name, alice.age);
17    
18    // Access with ARROW (using pointer)
19    printf("Using arrow: %s, %d\n", ptr->name, ptr->age);
20    
21    // Modify through pointer
22    ptr->age = 21;
23    ptr->gpa = 3.90;
24    
25    printf("After modify: %s, %d, %.2f\n", 
26           alice.name, alice.age, alice.gpa);
27    
28    return 0;
29}

Output

Using dot: Alice, 20

Using arrow: Alice, 20

After modify: Alice, 21, 3.90

Arrow is Shorthand!

ptr->age is exactly the same as (*ptr).age. The arrow operator just makes it cleaner!

ptr->age = (*ptr).age

06Nested Structures

A structure can contain another structure as a member — this is called nesting.

nested_struct.c

1#include <stdio.h>
2
3// Define a Date structure
4typedef struct {
5    int day;
6    int month;
7    int year;
8} Date;
9
10// Define a Person structure with nested Date
11typedef struct {
12    char name[50];
13    int age;
14    Date birthdate;  // Nested structure!
15} Person;
16
17int main() {
18    // Initialize with nested values
19    Person person1 = {
20        "Alice",
21        25,
22        {15, 6, 1999}  // birthdate: June 15, 1999
23    };
24    
25    // Access nested members with multiple dots
26    printf("Name: %s\n", person1.name);
27    printf("Age: %d\n", person1.age);
28    printf("Birthdate: %d/%d/%d\n", 
29           person1.birthdate.day,
30           person1.birthdate.month,
31           person1.birthdate.year);
32    
33    // Modify nested member
34    person1.birthdate.year = 1998;
35    
36    return 0;
37}

Output:

Name: Alice
Age: 25
Birthdate: 15/6/1999

07Array of Structures

You can create an array where each element is a structure — perfect for storing lists of records.

array_of_struct.c

1#include <stdio.h>
2#include <string.h>
3
4typedef struct {
5    int id;
6    char name[30];
7    float gpa;
8} Student;
9
10int main() {
11    // Array of 3 students
12    Student students[3] = {
13        {101, "Alice", 3.85},
14        {102, "Bob", 3.92},
15        {103, "Carol", 3.78}
16    };
17    
18    // Print all students
19    printf("Student Records:\n");
20    printf("%-5s %-15s %s\n", "ID", "Name", "GPA");
21    printf("----------------------------\n");
22    
23    for (int i = 0; i < 3; i++) {
24        printf("%-5d %-15s %.2f\n", 
25               students[i].id, 
26               students[i].name, 
27               students[i].gpa);
28    }
29    
30    // Find highest GPA
31    float maxGpa = 0;
32    int topStudent = 0;
33    for (int i = 0; i < 3; i++) {
34        if (students[i].gpa > maxGpa) {
35            maxGpa = students[i].gpa;
36            topStudent = i;
37        }
38    }
39    printf("\nTop Student: %s with GPA %.2f\n", 
40           students[topStudent].name, maxGpa);
41    
42    // Memory: 3 * sizeof(Student) bytes
43    printf("\nSize of one Student: %zu bytes\n", sizeof(Student));
44    printf("Size of array: %zu bytes\n", sizeof(students));
45    
46    return 0;
47}

Output:

Student Records:
ID Name GPA
----------------------------
101 Alice 3.85
102 Bob 3.92
103 Carol 3.78

Top Student: Bob with GPA 3.92

Size of one Student: 40 bytes
Size of array: 120 bytes

Array of Structures Memory Layout

id, name, gpa

students[0]

id, name, gpa

students[1]

id, name, gpa

students[2]

Each struct stored contiguously in memory

08Passing Structures to Functions

You can pass structures to functions by value (copy) or by pointer (reference). Each has different use cases.

By Value vs By Pointer

Pass by Value (Copy)

original

Alice, 20

→

copy

Alice, 20

✓ Original stays unchanged

✗ Copies entire struct (slow for large structs)

Use: When you don't need to modify

Pass by Pointer (Reference)

ptr

0x1000

→

@0x1000

Alice, 20

✓ Efficient (only copies address)

✓ Can modify original

Use: For large structs or when modifying

struct_functions.c

1#include <stdio.h>
2
3typedef struct {
4    char name[50];
5    int age;
6} Person;
7
8// Pass by VALUE - receives a COPY
9void birthday_value(Person p) {
10    p.age++;  // Only modifies the copy!
11    printf("Inside function: %s is %d\n", p.name, p.age);
12}
13
14// Pass by POINTER - receives the ADDRESS
15void birthday_pointer(Person *p) {
16    p->age++;  // Modifies the ORIGINAL!
17    printf("Inside function: %s is %d\n", p->name, p->age);
18}
19
20int main() {
21    Person alice = {"Alice", 25};
22    
23    printf("Before: %s is %d\n", alice.name, alice.age);
24    
25    birthday_value(alice);  // Pass by value
26    printf("After value: %s is %d\n", alice.name, alice.age);  // Still 25!
27    
28    birthday_pointer(&alice);  // Pass by pointer
29    printf("After pointer: %s is %d\n", alice.name, alice.age);  // Now 26!
30    
31    return 0;
32}

Output

Before: Alice is 25

Inside function: Alice is 26

After value: Alice is 25 ← Unchanged!

Inside function: Alice is 26

After pointer: Alice is 26 ← Modified!

Best Practice

Use pointer when:

• Struct is large
• Need to modify the original

Use const pointer for read-only:

void print(const Person *p)

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Forgetting Semicolon After struct

main.c

struct Point {
    int x;
    int y;
}  // ERROR! Missing semicolon
struct Point {
    int x;
    int y;
};  // Correct!

Assigning Strings with =

main.c

struct Student s;
s.name = "Alice";  // ERROR! Can't assign strings like this
// Correct way:
strcpy(s.name, "Alice");  // Use strcpy()

Comparing Structures with ==

main.c

struct Point p1 = {1, 2};
struct Point p2 = {1, 2};
if (p1 == p2) {  // ERROR! Can't compare structs with ==
}
// Must compare member by member:
if (p1.x == p2.x && p1.y == p2.y) {  // Correct!
}

Watch Out When Copying Code!

Common Mistakes with Structures!

Structures seem simple but Code often makes subtle mistakes that lead to inefficiency or crashes:

1. Passing Large Structs by Value

Copied code often passes entire structs by value, copying all data — slow for large structures:

main.c

void process(struct BigData data) {  // Copies entire struct!
    // Inefficient for large structs
}
// Better: Pass by pointer
void process(struct BigData *data) {
    data->field = value;  // Fast, modifies original
}

2. Shallow Copy with Pointer Members

Copied code might copy a struct containing pointers — both structs then point to the same data:

main.c

struct Person p1 = {"Alice", 25};
struct Person p2 = p1;  // If name is char*, both point to same string!
// Changing p2.name affects p1 too!

3. Forgetting Struct Padding/Alignment

Copied code might assume sizeof(struct) equals sum of member sizes. Wrong! Compilers add padding for alignment. A struct with charand int may be 8 bytes, not 5.

Best Practices: Pass large structs by pointer, use const for read-only access, and always usesizeof() for struct sizes.

12Frequently Asked Questions

Q:When should I use a struct vs an array?

Q:What's the difference between . and -> operators?

Q:Why use typedef with structs?

Q:Can I have a struct inside a struct?

Q:How do I copy a struct to another?

Q:What is struct padding and why should I care?

Q:Should I pass structs by value or by pointer?

12Summary

✓What You Learned

✓Structures: Group different types together
✓Dot operator: Access members directly
✓Arrow operator: Access via pointer
✓typedef: Shorter type names
✓Nesting: Structs inside structs
✓Functions: Pass by value or pointer

Quick Reference

Define:

struct Name { type member; };

Create:

struct Name var = {...};

Access (direct):

var.member

Access (pointer):

ptr->member

Structure Usage Flow

struct {...}

Define

→

var = {...}

Create

→

var.member

Access

→

func(var)

Use

What You Will Learn

01What is a Structure?

WHY Do We Need Structures?

The Problem: Without Structs

✓The Solution: With Structs

Think of It Like a Blueprint

🆚 Arrays vs Structures

Array

Structure

Real-World Examples

02Defining a Structure

Output:

How This Program Works

Key Points

03Structure Memory Layout

Memory Layout: struct Point { int x; int y; }

Memory Padding & Alignment

struct Example { char a; int b; char c; }

Optimization Tip

04typedef with Structures

Without typedef

With typedef ✓

05Pointers to Structures & Arrow Operator

When to Use . vs ->

. Dot Operator

-> Arrow Operator

What's Happening in Memory

Arrow is Shorthand!

06Nested Structures

Output:

07Array of Structures

Output:

Array of Structures Memory Layout

08Passing Structures to Functions

By Value vs By Pointer

Pass by Value (Copy)

Pass by Pointer (Reference)

Best Practice

!Code Pitfalls: Common Mistakes & What to Watch For

Forgetting Semicolon After struct

Assigning Strings with =

Comparing Structures with ==

Watch Out When Copying Code!

Common Mistakes with Structures!

1. Passing Large Structs by Value

2. Shallow Copy with Pointer Members

3. Forgetting Struct Padding/Alignment

12Frequently Asked Questions

Q:When should I use a struct vs an array?

Q:What's the difference between . and -> operators?

Q:Why use typedef with structs?

Q:Can I have a struct inside a struct?

Q:How do I copy a struct to another?

Q:What is struct padding and why should I care?

Q:Should I pass structs by value or by pointer?

12Summary

✓What You Learned

Quick Reference

Structure Usage Flow

Test Your Knowledge

Related Tutorials

Unions in C

Structures and Dynamic Memory

Bit Fields in Structures

Have Feedback?

What You Will Learn

01What is a Structure?

WHY Do We Need Structures?

The Problem: Without Structs

✓The Solution: With Structs

Think of It Like a Blueprint

🆚 Arrays vs Structures

Array

Structure

Real-World Examples

02Defining a Structure

Output:

How This Program Works

Key Points

03Structure Memory Layout