Header Files in C

01What are Header Files?

Header files (with .h extension) contain declarations that are shared between multiple source files. They are the foundation of modular C programming.

What Goes in Header Files?

Include in .h

* Function prototypes
* Type definitions (struct, enum, typedef)
* Macro definitions (#define)
* extern variable declarations
* Include guards

Keep in .c

* Function definitions (body)
* Variable definitions
* static functions
* The main() function

02Creating Your Own Header Files

Let's create a simple math library with a header file:

mymath.h- The header file

mymath.h

1#ifndef MYMATH_H    // Include guard - prevents double inclusion
2#define MYMATH_H
3
4// Function prototypes (declarations)
5int add(int a, int b);
6int subtract(int a, int b);
7int multiply(int a, int b);
8double divide(int a, int b);
9
10// Macro definitions
11#define PI 3.14159265359
12#define SQUARE(x) ((x) * (x))
13
14// Type definitions
15typedef struct {
16    double x;
17    double y;
18} Point;
19
20#endif  // MYMATH_H

mymath.c- The source file with implementations

mymath.c

1#include "mymath.h"   // Include our header
2
3// Function definitions (implementations)
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int multiply(int a, int b) {
13    return a * b;
14}
15
16double divide(int a, int b) {
17    if (b == 0) {
18        return 0;  // Handle division by zero
19    }
20    return (double)a / b;
21}

main.c- Using the library

main.c

1#include <stdio.h>
2#include "mymath.h"   // Include our custom header
3
4int main() {
5    printf("5 + 3 = %d\n", add(5, 3));
6    printf("5 - 3 = %d\n", subtract(5, 3));
7    printf("5 * 3 = %d\n", multiply(5, 3));
8    printf("5 / 3 = %.2f\n", divide(5, 3));
9    
10    printf("PI = %.5f\n", PI);
11    printf("SQUARE(5) = %d\n", SQUARE(5));
12    
13    Point p = {3.0, 4.0};
14    printf("Point: (%.1f, %.1f)\n", p.x, p.y);
15    
16    return 0;
17}

Expected Output:

5 + 3 = 8
5 - 3 = 2
5 * 3 = 15
5 / 3 = 1.67
PI = 3.14159
SQUARE(5) = 25
Point: (3.0, 4.0)

03Include Guards

Include guards prevent a header file from being included multiple times, which would cause redefinition errors.

Traditional #ifndef Guard

main.c

1#ifndef MYHEADER_H
2#define MYHEADER_H
3
4// Your declarations here
5
6#endif

Modern #pragma once

main.c

1#pragma once
2
3// Your declarations here
4
5// Simpler but not 100% portable

Why include guards matter:

Without guards, if a.h includes b.h, and main.c includes both, you get duplicate definitions = compiler errors!

04System vs User Headers

#include <header.h>

Searches system directories first (like /usr/include)

main.c

1// System/standard library headers
2#include <stdio.h>
3#include <stdlib.h>
4#include <string.h>
5#include <math.h>

#include "header.h"

Searches current directory first, then system directories

main.c

1// Your own headers
2#include "mymath.h"
3#include "utils.h"
4#include "../lib/helper.h"

05Best Practices

1. One header per module

Each .c file should have a corresponding .h file with its public interface.

2. Include what you use

Each file should include all headers it needs directly, not rely on transitive includes.

3. Minimize includes in headers

Use forward declarations when possible to reduce compilation time.

4. Self-contained headers

A header should compile on its own - include everything it needs.

5. Use descriptive guard names

Convention: PROJECT_FOLDER_FILENAME_H (e.g., MYAPP_UTILS_MATH_H)

06Summary

Header files enable modular programming:

* Put declarations in .h, definitions in .c
* Always use include guards (#ifndef or #pragma once)
* Use angle brackets for system headers, quotes for your own
* Keep headers minimal and self-contained

01What are Header Files?

Header files (with .h extension) contain declarations that are shared between multiple source files. They are the foundation of modular C programming.

What Goes in Header Files?

Include in .h

* Function prototypes
* Type definitions (struct, enum, typedef)
* Macro definitions (#define)
* extern variable declarations
* Include guards

Keep in .c

* Function definitions (body)
* Variable definitions
* static functions
* The main() function

02Creating Your Own Header Files

Let's create a simple math library with a header file:

mymath.h- The header file

mymath.h

1#ifndef MYMATH_H    // Include guard - prevents double inclusion
2#define MYMATH_H
3
4// Function prototypes (declarations)
5int add(int a, int b);
6int subtract(int a, int b);
7int multiply(int a, int b);
8double divide(int a, int b);
9
10// Macro definitions
11#define PI 3.14159265359
12#define SQUARE(x) ((x) * (x))
13
14// Type definitions
15typedef struct {
16    double x;
17    double y;
18} Point;
19
20#endif  // MYMATH_H

mymath.c- The source file with implementations

mymath.c

1#include "mymath.h"   // Include our header
2
3// Function definitions (implementations)
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int multiply(int a, int b) {
13    return a * b;
14}
15
16double divide(int a, int b) {
17    if (b == 0) {
18        return 0;  // Handle division by zero
19    }
20    return (double)a / b;
21}

main.c- Using the library

main.c

1#include <stdio.h>
2#include "mymath.h"   // Include our custom header
3
4int main() {
5    printf("5 + 3 = %d\n", add(5, 3));
6    printf("5 - 3 = %d\n", subtract(5, 3));
7    printf("5 * 3 = %d\n", multiply(5, 3));
8    printf("5 / 3 = %.2f\n", divide(5, 3));
9    
10    printf("PI = %.5f\n", PI);
11    printf("SQUARE(5) = %d\n", SQUARE(5));
12    
13    Point p = {3.0, 4.0};
14    printf("Point: (%.1f, %.1f)\n", p.x, p.y);
15    
16    return 0;
17}

Expected Output:

5 + 3 = 8
5 - 3 = 2
5 * 3 = 15
5 / 3 = 1.67
PI = 3.14159
SQUARE(5) = 25
Point: (3.0, 4.0)

03Include Guards

Include guards prevent a header file from being included multiple times, which would cause redefinition errors.

Traditional #ifndef Guard

main.c

1#ifndef MYHEADER_H
2#define MYHEADER_H
3
4// Your declarations here
5
6#endif

Modern #pragma once

main.c

1#pragma once
2
3// Your declarations here
4
5// Simpler but not 100% portable

Why include guards matter:

Without guards, if a.h includes b.h, and main.c includes both, you get duplicate definitions = compiler errors!

04System vs User Headers

#include <header.h>

Searches system directories first (like /usr/include)

main.c

1// System/standard library headers
2#include <stdio.h>
3#include <stdlib.h>
4#include <string.h>
5#include <math.h>

#include "header.h"

Searches current directory first, then system directories

main.c

1// Your own headers
2#include "mymath.h"
3#include "utils.h"
4#include "../lib/helper.h"

05Best Practices

1. One header per module

Each .c file should have a corresponding .h file with its public interface.

2. Include what you use

Each file should include all headers it needs directly, not rely on transitive includes.

3. Minimize includes in headers

Use forward declarations when possible to reduce compilation time.

4. Self-contained headers

A header should compile on its own - include everything it needs.

5. Use descriptive guard names

Convention: PROJECT_FOLDER_FILENAME_H (e.g., MYAPP_UTILS_MATH_H)

06Summary

Header files enable modular programming:

* Put declarations in .h, definitions in .c
* Always use include guards (#ifndef or #pragma once)
* Use angle brackets for system headers, quotes for your own
* Keep headers minimal and self-contained

What You Will Learn

01What are Header Files?

What Goes in Header Files?

02Creating Your Own Header Files

Expected Output:

03Include Guards

Traditional #ifndef Guard

Modern #pragma once

04System vs User Headers

#include <header.h>

#include "header.h"

05Best Practices

1. One header per module

2. Include what you use

3. Minimize includes in headers

4. Self-contained headers

5. Use descriptive guard names

06Summary

Test Your Knowledge

Related Tutorials

C Preprocessor Directives

Multi-File Programs in C

Debugging with LLDB

Have Feedback?

What You Will Learn

01What are Header Files?

What Goes in Header Files?

02Creating Your Own Header Files

Expected Output:

03Include Guards

Traditional #ifndef Guard

Modern #pragma once

04System vs User Headers

#include <header.h>

#include "header.h"

05Best Practices

1. One header per module

2. Include what you use

3. Minimize includes in headers

4. Self-contained headers

5. Use descriptive guard names

06Summary

Test Your Knowledge

Related Tutorials

C Preprocessor Directives

Multi-File Programs in C

Debugging with LLDB

Have Feedback?