Headers and sources
- C course
- Intermediate
- Lesson 59/116
- 5 min
Learn the difference between .h and .c files.
Headers and sources
C projects split into two file types: headers (.h) and sources (.c). Headers carry function declarations and type definitions; sources contain function definitions. Another .c file includes the header with #include and calls the declared functions.
Example: math_utils.h has int add(int, int); declaration, math_utils.c has int add(int a, int b) { return a + b; } definition. This split cuts compile time in big projects and gives modularity; each module ships with its own header.
Include guard
When a header is included multiple times, double declaration errors arise. To prevent this, use an include guard: #ifndef MATH_UTILS_H, #define MATH_UTILS_H, content, #endif.
On first include MATH_UTILS_H is undefined; #ifndef is true, the macro is defined and content is processed. On second include the macro is defined; #ifndef is false and content is skipped. Modern C also uses #pragma once; shorter but compiler-specific. Include guards are standard and portable.
Practice Write header and main
Simulate in one file: #ifndef MATH_H, #define MATH_H, the declaration int add(int, int);, #endif. Then the definition int add(int a, int b) { return a + b; }. In int main(void) add printf("%d\n", add(3, 4));. Don't forget <stdio.h>.
Starter code
#include <stdio.h>
// Start MATH_H include guard
// Declare add
// Close guard
// Define add
int main(void) {
// Call add(3, 4) and print
return 0;
}Sample solution
#include <stdio.h>
#ifndef MATH_H
#define MATH_H
int add(int, int);
#endif
int add(int a, int b) {
return a + b;
}
int main(void) {
printf("%d\n", add(3, 4));
return 0;
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Where does a function definition live?
- Only in .c Correct answer
- Only in .h
- In both .h and .c
- In the Makefile
Explanation: The definition is in .c; the declaration is in .h.
2 Complete the include guard macro
#ifndef UTILS_H
#____ UTILS_H
int util_add(int, int);
#endif#ifndef UTILS_H
#define UTILS_H
int util_add(int, int);
#endifExplanation: #define UTILS_H defines the macro and prevents re-inclusion.
3 Match file types to their content
| Term | Meaning |
|---|---|
math_utils.h |
Declarations |
math_utils.c |
Definitions |
main.c |
Program entry |
Makefile |
Build recipes |