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.

Practice

1. What is the main purpose of a header file in C programming?

easy

A. To store declarations and share them between source files

B. To execute the main program logic

C. To compile the program faster

D. To store variable values during runtime

Solution

1. Step 1: Understand what header files contain

   Header files usually contain declarations like function prototypes and macros, not executable code.

2. Step 2: Understand their role in sharing code

   They allow multiple source files to share these declarations, so the compiler knows about functions and variables.

3. Final Answer:

   To store declarations and share them between source files -> Option A

4. Quick Check:

   Header files = declarations shared [OK]

Hint: Header files hold declarations, not code [OK]

Common Mistakes:

• Thinking header files contain executable code
• Confusing header files with source files
• Believing header files speed up compilation
• Assuming header files store variable values

2. Which of the following is the correct syntax to include a system header file named stdio.h?

easy

A. #include "stdio.h"

B. #include

C. include

D. #include stdio.h

Solution

1. Step 1: Recall syntax for system headers

   System headers use angle brackets <> to tell the compiler to look in system directories.

2. Step 2: Check each option

   #include uses #include with <stdio.h>, which is correct syntax. #include "stdio.h" uses quotes which are for user headers. Options C and D miss the # or angle brackets.

3. Final Answer:

   #include <stdio.h> -> Option B

4. Quick Check:

   System headers use <> [OK]

Hint: Use < > for system headers, " " for your own [OK]

Common Mistakes:

• Using quotes for system headers
• Omitting the # in #include
• Not using angle brackets or quotes
• Writing include without #

3. What will be the output of this C program?

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

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

Assuming myheader.h contains:

void printMessage() {
    printf("Hello from header!\n");
}

medium

A. No output

B. Compilation error: function not declared

C. Runtime error

D. Hello from header!

Solution

1. Step 1: Understand #include "myheader.h" effect

   The content of myheader.h is inserted, so printMessage() is defined before main.

2. Step 2: Check function call and output

   printMessage() prints "Hello from header!\n", so the program outputs that line.

3. Final Answer:

   Hello from header! -> Option D

4. Quick Check:

   Included function runs and prints message [OK]

Hint: Including header inserts code, so functions are available [OK]

Common Mistakes:

• Assuming function is undeclared without prototype
• Thinking header files can't contain function definitions
• Confusing compile and runtime errors
• Ignoring the newline in output

4. Identify the error in this code snippet:

#include 
#include 

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

Assuming myheader.h is a user-defined header file in the project folder.

medium

A. Function printHello() is not defined

B. Missing semicolon after #include directives

C. Using angle brackets for a user-defined header file

D. Missing main function return type

Solution

1. Step 1: Check #include syntax for user headers

   User headers should be included with quotes "myheader.h" to search local directories first.

2. Step 2: Understand effect of wrong include syntax

   Using angle brackets makes compiler look in system directories, so it may not find myheader.h causing an error.

3. Final Answer:

   Using angle brackets for a user-defined header file -> Option C

4. Quick Check:

   User headers use "" not <> [OK]

Hint: Use "" for your headers, < > for system headers [OK]

Common Mistakes:

• Using < > for user headers
• Thinking #include needs semicolons
• Assuming printHello() is always undefined
• Ignoring main return type

5. You want to create a header file mathutils.h that declares a function int add(int a, int b); and use it in two source files main.c and calc.c. Which is the best practice to avoid multiple definition errors?

hard

A. Put only the function declaration in mathutils.h and define the function in one source file

B. Define the function in both source files and include mathutils.h everywhere

C. Put the function definition in mathutils.h and include it in both source files

D. Include mathutils.h only in main.c and define function in calc.c

Solution

1. Step 1: Understand declarations vs definitions in headers

   Headers should contain declarations (function prototypes), not definitions (function bodies), to avoid multiple definitions.

2. Step 2: Apply to multiple source files

   Define the function in one source file (e.g., calc.c) and declare it in mathutils.h included by both files.

3. Final Answer:

   Put only the function declaration in mathutils.h and define the function in one source file -> Option A

4. Quick Check:

   Declarations in headers, definitions in one source file [OK]

Hint: Declare in header, define once in source file [OK]

Common Mistakes:

• Defining functions in headers causing multiple definitions
• Not including header in all source files needing declaration
• Defining function in multiple source files
• Including header only in one source file