C

C Programming

Master the fundamentals of C programming with comprehensive tutorials, examples, and hands-on exercises

Start Learning Try Online Compiler
C Programming Tutorial
Getting Started
Fundamentals
Control Flow
Functions
Arrays & Strings
Pointers
Advanced Topics

Header Files in C

Header files contain function declarations, macro definitions, and type definitions that can be shared across multiple source files. They promote code reusability and organization.

Creating Custom Header Files

math_utils.h 
// math_utils.h
#ifndef MATH_UTILS_H
#define MATH_UTILS_H

// Function declarations
int add(int a, int b);
int subtract(int a, int b);
int multiply(int a, int b);
float divide(float a, float b);
int power(int base, int exponent);

// Constants
#define PI 3.14159265359
#define E 2.71828182846

// Macro functions
#define SQUARE(x) ((x) * (x))
#define ABS(x) ((x) < 0 ? -(x) : (x))

#endif // MATH_UTILS_H
math_utils.c (Implementation) 
// math_utils.c
#include "math_utils.h"

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

float divide(float a, float b) {
    if (b != 0) {
        return a / b;
    }
    return 0.0;  // Error case
}

int power(int base, int exponent) {
    int result = 1;
    for (int i = 0; i < exponent; i++) {
        result *= base;
    }
    return result;
}
main.c (Using the Header) 
// main.c
#include <stdio.h>
#include "math_utils.h"

int main() {
    int a = 10, b = 5;
    
    printf("Addition: %d + %d = %d\n", a, b, add(a, b));
    printf("Subtraction: %d - %d = %d\n", a, b, subtract(a, b));
    printf("Multiplication: %d * %d = %d\n", a, b, multiply(a, b));
    printf("Division: %d / %d = %.2f\n", a, b, divide(a, b));
    printf("Power: %d^%d = %d\n", a, 3, power(a, 3));
    
    printf("\nUsing macros:\n");
    printf("PI = %.5f\n", PI);
    printf("Square of %d = %d\n", a, SQUARE(a));
    printf("Absolute value of -15 = %d\n", ABS(-15));
    
    return 0;
}

Header Guards

Preventing Multiple Inclusions 
// student.h
#ifndef STUDENT_H
#define STUDENT_H

#include <stdio.h>

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

// Function prototypes
void printStudent(const Student *s);
void initStudent(Student *s, int id, const char *name, float gpa);
int compareStudents(const Student *s1, const Student *s2);

#endif // STUDENT_H

// Alternative: #pragma once (compiler-specific)
// #pragma once

Standard Library Headers

Common Standard Headers 
#include <stdio.h>    // Standard I/O functions
#include <stdlib.h>   // Memory allocation, conversion
#include <string.h>   // String manipulation functions
#include <math.h>     // Mathematical functions
#include <time.h>     // Date and time functions
#include <ctype.h>    // Character classification
#include <limits.h>   // Implementation limits
#include <float.h>    // Floating-point limits

int main() {
    // stdio.h functions
    printf("Hello from stdio.h\n");
    
    // stdlib.h functions
    int *ptr = malloc(sizeof(int) * 5);
    free(ptr);
    
    // string.h functions
    char str1[20] = "Hello";
    char str2[20] = "World";
    strcat(str1, " ");
    strcat(str1, str2);
    printf("Concatenated: %s\n", str1);
    
    // math.h functions (compile with -lm)
    printf("Square root of 16: %.2f\n", sqrt(16.0));
    printf("Power 2^3: %.0f\n", pow(2.0, 3.0));
    
    // time.h functions
    time_t current_time = time(NULL);
    printf("Current time: %s", ctime(¤t_time));
    
    // limits.h constants
    printf("Maximum int value: %d\n", INT_MAX);
    printf("Minimum int value: %d\n", INT_MIN);
    
    return 0;
}

Compilation with Multiple Files

Compilation Commands 
# Compile all files together
gcc main.c math_utils.c -o program

# Compile separately and link
gcc -c main.c          # Creates main.o
gcc -c math_utils.c    # Creates math_utils.o
gcc main.o math_utils.o -o program

# With math library
gcc main.c math_utils.c -lm -o program
Header File Best Practices:
  • Always use header guards or #pragma once
  • Include only necessary headers
  • Use angle brackets <> for system headers
  • Use quotes "" for user-defined headers
  • Declare functions, don't define them in headers
← Preprocessor Storage Classes →

Frequently Asked Questions

C is a powerful, general-purpose programming language that's been around since the 1970s. It's the foundation for many modern languages and is essential for system programming, embedded systems, and understanding how computers work at a low level. Learning C gives you a solid foundation in programming fundamentals.

No prior programming experience is required! Our C programming tutorial is designed for complete beginners. We start with the basics and gradually progress to more advanced topics. However, having some basic computer literacy and logical thinking skills will be helpful.

You need a C compiler to run C programs. For beginners, we recommend using our online C compiler which requires no installation. For local development, you can use GCC (GNU Compiler Collection) which is available on Windows (via MinGW), macOS (via Xcode), and Linux. IDEs like Code::Blocks, Dev-C++, or Visual Studio Code are also helpful.

The time to learn C depends on your dedication and prior experience. With consistent practice (1-2 hours daily), you can grasp the basics in 2-4 weeks and become proficient in 2-3 months. Mastering advanced concepts like pointers, memory management, and data structures may take 6-12 months of regular practice.

C programming skills open doors to various career paths including system programming, embedded systems development, operating system development, device driver programming, game development, and firmware development. Many companies in automotive, aerospace, telecommunications, and IoT sectors actively seek C programmers.

Absolutely! C remains highly relevant and is consistently ranked among the top programming languages. It's essential for system programming, embedded systems, IoT devices, and performance-critical applications. Many modern languages are built on C, and understanding C helps you become a better programmer overall.