Header files and include directive - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of program with standard and user header includes

What is the output of this C program?

C

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

int main() {
    printMessage();
    return 0;
}

// Contents of myheader.h
// void printMessage() { printf("Hello from header!\n"); }

A Linker error: undefined reference to printMessage

B Compilation error: function printMessage not declared

C Hello from header!\nHello from header!

D Hello from header!

💡 Hint

Remember that function definitions should be in source files, not headers.

Including a header with only a function definition inside a comment does not provide the function implementation to the linker. The program compiles but fails linking with undefined reference error.

❓ Predict Output

intermediate

Effect of include guards in header files

What will be the output of this program?

C

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

int main() {
    printf("%d\n", VALUE);
    return 0;
}

// guarded.h content:
// #ifndef GUARDED_H
// #define GUARDED_H
// #define VALUE 10
// #endif

A Compilation error: redefinition of VALUE

B 0

C Runtime error

D 10

💡 Hint

Include guards prevent multiple definitions.

The include guard ensures the header content is included only once, so VALUE is defined once as 10.

🔧 Debug

advanced

Identify the error with include directive

Why does this code fail to compile?

C

#include <stdio.h>
#include <myheader.h>

int main() {
    printHello();
    return 0;
}

// myheader.h content:
// void printHello() { printf("Hello!\n"); }

A Because function printHello is not declared before use

B Because angle brackets <> are used for user headers instead of quotes ""

C Because function printHello is defined in header, causing multiple definition error

D Because header file content is commented out

💡 Hint

Angle brackets are for system headers, quotes for user headers.

Using angle brackets tells the compiler to look in system directories, so it cannot find user header myheader.h and compilation fails.

❓ Predict Output

advanced

Output when including header with macro and function

What is the output of this program?

C

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

int main() {
    printf("%d\n", SQUARE(5));
    printSquare(5);
    return 0;
}

// macrofunc.h content:
// #define SQUARE(x) ((x)*(x))
// void printSquare(int x) { printf("%d\n", x*x); }

A 25\n25

B Compilation error: multiple definitions of printSquare

C Runtime error

D 25\n10

💡 Hint

Macros are replaced by the preprocessor, functions need to be linked properly.

The macro SQUARE(5) expands to (5*5) = 25. The function printSquare prints 25. Both print 25.

🧠 Conceptual

expert

Why use include guards or #pragma once?

What is the main reason to use include guards or #pragma once in header files?

A To allow headers to be included only in main.c file

B To speed up compilation by skipping system headers

C To prevent multiple inclusion of the same header causing redefinition errors

D To automatically link libraries during compilation

💡 Hint

Think about what happens if a header is included multiple times.

Include guards or #pragma once prevent the compiler from processing the same header multiple times, avoiding errors from repeated definitions.