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Complete Referencegetchar() and putchar() in C Programminggets() and puts() in C ProgrammingC Input Output Programs and Examplesprintf() Function in C Programmingscanf() Function in C ProgrammingArrays of Structures in C – Managing CollectionsEnums in C – Enumerated Data TypesNested Structures in C – Struct Within StructPointers to Structures in C – Arrow OperatorStructures in C – User-Defined Data Typestypedef in C – Creating Type AliasesUnions in C – Shared Memory Data Types
Complete guide to structures in C programming — definition, declaration, initialization, accessing members with dot operator, struct memory layout, size and padding, practical examples with output.
Imagine you're building a student management system. Each student has a name, roll number, age, and GPA. Without structures, you'd need separate arrays for each field — messy and error-prone. Structures let you group related variables of different types into a single unit, making your code cleaner and more logical.
What is a Structure?
A structure is a user-defined data type that combines multiple variables (called members) of different types under one name.
Example
struct Student {
int roll_no;
char name[50];
float gpa;
int age;
};
Think of a structure as a blueprint — like a form template. Each variable of that structure type is like a filled-in form.
Declaring Structure Variables
There are multiple ways to declare structure variables:
Example
// Method 1: Declare separately
struct Student s1, s2;
// Method 2: Declare along with definition
struct Student {
int roll_no;
char name[50];
float gpa;
} s1, s2;
// Method 3: Anonymous struct (can't create more variables later)
struct {
int x;
int y;
} point1, point2;
Initialization and Access
Example
#include <stdio.h>
#include <string.h>
struct Student {
int roll_no;
char name[50];
float gpa;
int age;
};
int main() {
// Method 1: Initialize at declaration
struct Student s1 = {101, "Alice Johnson", 3.85, 20};
// Method 2: Initialize member by member
struct Student s2;
s2.roll_no = 102;
strcpy(s2.name, "Bob Smith");
s2.gpa = 3.65;
s2.age = 21;
// Method 3: Designated initializers (C99)
struct Student s3 = {.name = "Charlie", .roll_no = 103, .gpa = 3.90, .age = 19};
// Access with dot (.) operator
printf("Student 1: %s (Roll: %d, GPA: %.2f)\n", s1.name, s1.roll_no, s1.gpa);
printf("Student 2: %s (Roll: %d, GPA: %.2f)\n", s2.name, s2.roll_no, s2.gpa);
printf("Student 3: %s (Roll: %d, GPA: %.2f)\n", s3.name, s3.roll_no, s3.gpa);
return 0;
}
Output
Student 1: Alice Johnson (Roll: 101, GPA: 3.85) Student 2: Bob Smith (Roll: 102, GPA: 3.65) Student 3: Charlie (Roll: 103, GPA: 3.90)
Memory Layout of a Structure
| roll_no | name[50] | gpa | age |
|---|---|---|---|
| 4 bytes | 50 bytes | 4 bytes | 4 bytes |
| 101 | "Alice\0..." | 3.85 | 20 |
Structure Padding and sizeof
Compilers add padding bytes for alignment:
c example
#include <stdio.h>
struct Padded {
char c; // 1 byte + 3 padding
int i; // 4 bytes
char d; // 1 byte + 3 padding
};
struct Packed {
int i; // 4 bytes
char c; // 1 byte
char d; // 1 byte + 2 padding
};
int main() {
printf("struct Padded: %zu bytes\n", sizeof(struct Padded));
printf("struct Packed: %zu bytes\n", sizeof(struct Packed));
return 0;
}Output
struct Padded: 12 bytes struct Packed: 8 bytes
Lesson: Order members from largest to smallest to minimize padding.
Copying Structures
Unlike arrays, structures can be copied with simple assignment:
c example
#include <stdio.h>
struct Point {
int x, y;
};
int main() {
struct Point p1 = {10, 20};
struct Point p2 = p1; // Copies all members!
p2.x = 99; // Changing p2 does NOT affect p1
printf("p1: (%d, %d)\n", p1.x, p1.y);
printf("p2: (%d, %d)\n", p2.x, p2.y);
return 0;
}Output
p1: (10, 20) p2: (99, 20)
Structures and Functions
c example
#include <stdio.h>
#include <math.h>
struct Point {
double x, y;
};
// Pass by value (receives a copy)
double distance(struct Point a, struct Point b) {
double dx = a.x - b.x;
double dy = a.y - b.y;
return sqrt(dx * dx + dy * dy);
}
// Return a structure
struct Point midpoint(struct Point a, struct Point b) {
struct Point mid;
mid.x = (a.x + b.x) / 2.0;
mid.y = (a.y + b.y) / 2.0;
return mid;
}
int main() {
struct Point p1 = {0.0, 0.0};
struct Point p2 = {6.0, 8.0};
printf("Distance: %.2f\n", distance(p1, p2));
struct Point m = midpoint(p1, p2);
printf("Midpoint: (%.1f, %.1f)\n", m.x, m.y);
return 0;
}Output
Distance: 10.00 Midpoint: (3.0, 4.0)
Structures vs Arrays vs Basic Types
| Feature | Basic Types | Arrays | Structures |
|---|---|---|---|
| Data types | Single type | Same type elements | Mixed types |
| Access | By name | By index | By member name |
| Assignment | Yes | No (must copy manually) | Yes (full copy) |
| Pass to function | By value | By pointer (decays) | By value or pointer |
| Size | Fixed | Fixed | Sum of members + padding |
Practical Example: Library Book System
Example
#include <stdio.h>
#include <string.h>
struct Date {
int day, month, year;
};
struct Book {
char title[100];
char author[50];
float price;
int pages;
struct Date published;
};
void print_book(struct Book b) {
printf("Title: %s\n", b.title);
printf("Author: %s\n", b.author);
printf("Price: $%.2f\n", b.price);
printf("Pages: %d\n", b.pages);
printf("Published: %02d/%02d/%d\n",
b.published.day, b.published.month, b.published.year);
printf("---\n");
}
int main() {
struct Book books[2] = {
{"The C Programming Language", "Kernighan & Ritchie", 45.99, 272, {1, 1, 1978}},
{"C: A Modern Approach", "K.N. King", 89.95, 832, {1, 4, 2008}}
};
for (int i = 0; i < 2; i++) {
print_book(books[i]);
}
return 0;
}
Output
Title: The C Programming Language Author: Kernighan & Ritchie Price: $45.99 Pages: 272 Published: 01/01/1978
Interview Questions
Q1: Can a structure contain a pointer to itself?
Yes! This is called a self-referential structure and is the foundation of linked lists, trees, and graphs. Example: struct Node { int data; struct Node *next; };
Q2: What is the difference between structure declaration and definition?
Declaration tells the compiler the structure exists (forward declaration): struct Student;. Definition provides the full layout with members. You can declare a pointer to an incomplete type but can't access members until it's defined.
Q3: Why is padding added to structures?
CPUs access memory most efficiently at aligned addresses (e.g., 4-byte int at addresses divisible by 4). Padding ensures each member starts at a properly aligned address, avoiding slow unaligned access or hardware exceptions on some architectures.
Q4: Can you compare two structures with == operator?
No. C doesn't support == for structures because padding bytes may contain garbage. You must compare member by member, or use memcmp() (but memcmp can give wrong results due to padding). Write a custom comparison function.
Summary
- Structures group related variables of different types into one unit
- Access members with the dot (
.) operator - Structures can be assigned, passed to functions, and returned from functions
- Compiler adds padding for alignment — order members by size to save space
- Use designated initializers (C99) for readable initialization
- Structures form the basis for OOP-like patterns in C
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Structures in C – User-Defined Data Types.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.
Search Terms
c-programming, c programming, programming, structures, and, unions, structures in c – user-defined data types
Related C Programming Topics
Continue learning this concept
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