Include Guards

Part of the Object Oriented Programming section of Coddy's C journey — lesson 2 of 61.

When a header file gets included multiple times in a project, the compiler sees the same declarations repeatedly, causing errors. This commonly happens when multiple files include the same header, or when headers include each other. Include guards solve this problem.

An include guard uses three preprocessor directives to ensure a header's contents are processed only once:

// math_utils.h
#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);

#endif

Here's how it works: #ifndef MATH_UTILS_H checks if the symbol MATH_UTILS_H has not been defined yet. If it hasn't, the preprocessor continues and immediately defines it with #define MATH_UTILS_H. Everything between this point and #endif is included in the compilation.

The second time this header is included, MATH_UTILS_H is already defined, so the entire block is skipped. The guard name is typically the filename in uppercase with underscores replacing dots and special characters.

Every header file you create should have include guards. This is a standard practice that prevents compilation errors and makes your code safe to include from anywhere in your project.

Challenge

Easy

Let's protect a header file from double inclusion by adding include guards. You'll build on the greeting utility from the previous lesson, but this time ensure the header is safe to include multiple times.

You'll create two files:

greet.h: Your header file containing the function declaration for greet. This time, wrap the entire contents with include guards using the symbol GREET_H.
main.c: Your main file that includes the header, implements the greet function, and calls it from main().

The greet function should accept a const char* parameter for the name and print:

Hello, Alice!

Your header file should follow this structure:

#ifndef GREET_H
#define GREET_H

// function declaration here

#endif

You will receive one input: a name (string).

Print the greeting message exactly as shown in the format above, replacing Alice with the provided name.

Cheat sheet

Include guards prevent header files from being processed multiple times during compilation, which would cause errors.

An include guard uses three preprocessor directives:

#ifndef HEADER_NAME_H
#define HEADER_NAME_H

// header contents here

#endif

#ifndef checks if a symbol has not been defined. If undefined, #define defines it and the contents are processed. On subsequent inclusions, the symbol is already defined, so the entire block is skipped.

The guard name is typically the filename in uppercase with underscores replacing dots and special characters.

Every header file should have include guards as standard practice.

Try it yourself

#include <stdio.h>
#include "greet.h"

// TODO: Implement the greet function that prints "Hello, <name>!"

int main() {
    char name[100];
    scanf("%s", name);
    
    // TODO: Call the greet function with the name
    
    return 0;
}