C Header Files

In this page:

Standard vs. Custom Header Files
Header Guards
Declaring Functions in Headers
Variables and Macros in Headers
Preventing Duplicate Definitions

Standard vs. Custom Header Files

Standard headers, included with angle brackets like #include <stdio.h>, are provided by the compiler's own library and searched for in its system include paths, whereas custom headers, included with double quotes like #include "utils.h", are your own files searched for relative to the current project first.

Example: Standard vs. Custom Header Files

#include <stdio.h>
int main() {
	printf("stdio.h is a standard header");
	return 0;
}

Header Guards

A header guard wraps a header file's contents in #ifndef HEADER_NAME, #define HEADER_NAME, and a closing #endif, ensuring the preprocessor skips the file's contents entirely if it has already been included once in the current translation unit, preventing duplicate-definition compile errors.

Example: Header Guards

#ifndef MYHEADER_H
#define MYHEADER_H
#include <stdio.h>
int main() {
	printf("Guarded against duplicate inclusion");
	return 0;
}
#endif

Declaring Functions in Headers

A header file should contain declarations — function prototypes, type definitions, macro constants — describing what exists, not the actual executable code that implements those functions; the implementations belong in a corresponding .c source file instead.

Example: Declaring Functions in Headers

#include <stdio.h>
int add(int a, int b);
int main() {
	printf("%d", add(2, 3));
	return 0;
}
int add(int a, int b) {
	return a + b;
}

Variables and Macros in Headers

Declaring shared constants and macros in a header lets every source file that includes it use the exact same values consistently, so changing a shared setting in one place (the header) automatically updates every file that depends on it, instead of hunting down scattered duplicate definitions.

Example: Variables and Macros in Headers

#include <stdio.h>
#define MAX_USERS 100
int main() {
	printf("%d", MAX_USERS);
	return 0;
}

Preventing Duplicate Definitions

If a header contains an actual function body rather than just a prototype, and that header gets included into more than one .c file, the linker sees the same function defined twice and reports a duplicate-symbol error — which is exactly the failure header guards and declaration-only headers are designed to avoid.

Example: Preventing Duplicate Definitions

#include <stdio.h>
int square(int n) {
	return n * n;
}
int main() {
	printf("%d", square(4));
	return 0;
}

🔒

Chapter Quiz — Complete all 9 topics to unlock

0/9 topics done

Complete these topics first:

← Previous Lesson C goto Statement Next Lesson → C Multi-file Programming