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Header files

  • C course
  • Beginner
  • Lesson 110/116
  • 5 min

Learn to create your own header files.

Header files and the include guard

A header file (.h) carries declarations; .c files contain definitions. To prevent double inclusion an include guard is used.

#ifndef CONFIG_H
#define CONFIG_H

#define MAX_USERS 10

#endif

Here once CONFIG_H is defined, when the file is included a second time its content is skipped. This prevents the same declaration from being compiled twice in multi-file projects. #pragma once serves a similar purpose but is not standard; the safest way is to use an include guard.

Header vs source file split

A .h file contains declarations; a .c file contains definitions. This split shortens compile times in large projects.

/* math_utils.h */
#ifndef MATH_UTILS_H
#define MATH_UTILS_H
int add(int a, int b);
#endif

/* math_utils.c */
#include "math_utils.h"
int add(int a, int b) { return a + b; }

Here the .h file only gives the signature of add; the .c writes the body. This way other files can include math_utils.h and use add; math_utils.c is compiled only once.

Practice Print MAX_USERS with the CONFIG_H guard

Write an include guard block with #ifndef CONFIG_H, #define CONFIG_H, #define MAX_USERS 10, #endif. In main print the value of MAX_USERS with printf.

Starter code

main.c
#include <stdio.h>

// Define MAX_USERS 10 with the CONFIG_H include guard
int main(void) {
    // Print the MAX_USERS value with printf
    return 0;
}

Sample solution

main.c
#ifndef CONFIG_H
#define CONFIG_H
#define MAX_USERS 10
#endif
printf("%d\n", MAX_USERS);

Expected output

10

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does an include guard (#ifndef/#define/#endif) do?

  • Speeds up the program
  • Prevents a header from being included twice Correct answer
  • Hides the code
  • Resets compilation time

Explanation: An include guard prevents a header from being included twice.

2 Match file type to content

Term Meaning
.h file Declarations
.c file Definitions
include guard Prevents double inclusion
#endif Ends the block

3 Fill in the blank: include guard

Question 
____ CONFIG_H
#define CONFIG_H
#define MAX_USERS 10
#endif
Answer 
#ifndef CONFIG_H
#define CONFIG_H
#define MAX_USERS 10
#endif

Explanation: A block starting with #ifndef forms an include guard.

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together