Header and source file organization - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of program with separated header and source files

Given the following files, what is the output when the program is compiled and run?

C

// file: math_ops.h
#ifndef MATH_OPS_H
#define MATH_OPS_H

int add(int a, int b);
int multiply(int a, int b);

#endif

// file: math_ops.c
#include "math_ops.h"

int add(int a, int b) {
    return a + b;
}

int multiply(int a, int b) {
    return a * b;
}

// file: main.c
#include <stdio.h>
#include "math_ops.h"

int main() {
    int x = 3, y = 4;
    printf("Sum: %d\n", add(x, y));
    printf("Product: %d\n", multiply(x, y));
    return 0;
}

A Compilation error due to missing function definitions

B Sum: 12\nProduct: 7

C Sum: 34\nProduct: 12

D Sum: 7\nProduct: 12

💡 Hint

Remember that the header file declares functions and the source file defines them.

The header file math_ops.h declares the functions add and multiply. The source file math_ops.c defines these functions. The main.c file includes the header and calls these functions. The output is the sum and product of 3 and 4, which are 7 and 12 respectively.

❓ Predict Output

intermediate

Result of including header file multiple times

What will happen if a header file without include guards is included multiple times in a source file?

C

// file: utils.h
int square(int x) { return x * x; }

// file: main.c
#include "utils.h"
#include "utils.h"

int main() {
    int val = 5;
    return square(val);
}

A Compilation error due to multiple definitions of square

B Program runs and returns 25

C Runtime error due to duplicate function

D Warning but program compiles and runs

💡 Hint

Think about what happens when the same function is defined twice in one translation unit.

Including the header file twice without include guards causes the function square to be defined twice in the same source file, leading to a compilation error due to multiple definitions.

🔧 Debug

advanced

Identify the cause of linker error in multi-file project

You have these files. Why does the linker give an 'undefined reference' error for function greet() when compiling main.c?

C

// file: greet.h
#ifndef GREET_H
#define GREET_H

void greet(void);

#endif

// file: main.c
#include "greet.h"

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

A greet() function is declared but not defined in any source file linked

B Missing include guard in greet.h causes multiple definitions

C greet() is defined static in another source file

D main.c is missing #include <stdio.h>

💡 Hint

Check if the function greet() has a definition linked during compilation.

The header greet.h declares greet(), but no source file defines it. The linker cannot find the function implementation, causing an undefined reference error.

📝 Syntax

advanced

Correct syntax for include guards in header files

Which option shows the correct way to write include guards in a header file named config.h?

A #define CONFIG_H\n#ifndef CONFIG_H\n// declarations\n#endif

B #ifndef CONFIG_H\n// declarations\n#define CONFIG_H\n#endif

C #ifndef CONFIG_H\n#define CONFIG_H\n// declarations\n#endif

D #ifdef CONFIG_H\n#define CONFIG_H\n// declarations\n#endif

💡 Hint

Include guards start with #ifndef and then #define the same macro.

Option C correctly uses #ifndef to check if CONFIG_H is not defined, then defines it, and ends with #endif. This prevents multiple inclusion.

🧠 Conceptual

expert

Best practice for function definitions in header files

Which statement about placing function definitions in header files is correct?

A Function definitions must always be in header files to allow inlining by the compiler.

B Function definitions should be in source files; header files only have declarations to avoid multiple definition errors.

C It's best to put all function definitions in header files to simplify compilation.

D Header files should contain only macro definitions and no functions.

💡 Hint

Think about what happens if a function is defined in a header included by multiple source files.

Defining functions in header files included by multiple source files causes multiple definitions at link time. Best practice is to put only declarations in headers and definitions in source files.

Practice

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

easy

A. To execute the program's main logic

B. To declare functions and variables for sharing between source files

C. To contain the actual implementation code of functions

D. To store compiled machine code

Solution

1. Step 1: Understand header file role

   Header files declare functions and variables so multiple source files can use them.

2. Step 2: Differentiate from source files

   Source files contain the actual code, not declarations.

3. Final Answer:

   To declare functions and variables for sharing between source files -> Option B

4. Quick Check:

   Header files declare, source files define [OK]

Hint: Headers declare, sources define code [OK]

Common Mistakes:

• Confusing header files as containing code
• Thinking header files execute code
• Mixing compiled code with header files

2. Which of the following is the correct way to include a header file named math_utils.h in a C source file?

easy

A. #include

B. import math_utils.h

C. #include "math_utils.h"

D. include math_utils.h

Solution

1. Step 1: Identify correct include syntax for user headers

   User-defined headers use double quotes: #include "filename".

2. Step 2: Differentiate from system headers

   System headers use angle brackets, but user headers like math_utils.h use quotes.

3. Final Answer:

   #include "math_utils.h" -> Option C

4. Quick Check:

   User headers use quotes [OK]

Hint: Use quotes for your own headers [OK]

Common Mistakes:

• Using angle brackets for user headers
• Using import instead of #include
• Omitting # in include directive

3. Given these files:

// math_utils.h
int add(int a, int b);

// math_utils.c
#include "math_utils.h"
int add(int a, int b) { return a + b; }

// main.c
#include 
#include "math_utils.h"
int main() {
    printf("%d\n", add(3, 4));
    return 0;
}

What will be the output when running the compiled program?

medium

A. 7

B. 34

C. 0

D. Compilation error

Solution

1. Step 1: Understand function declaration and definition

   math_utils.h declares add(), math_utils.c defines it to return sum of a and b.

2. Step 2: Trace main function call

   main.c calls add(3, 4), which returns 7, then prints it.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Function returns sum, so output is 7 [OK]

Common Mistakes:

• Thinking add concatenates numbers
• Expecting compilation error due to missing code
• Confusing printf format

4. Consider this code snippet:

// utils.h
void print_message();

// utils.c
#include "utils.h"
void print_message() {
    printf("Hello!\n");
}

// main.c
#include "utils.h"
int main() {
    print_message();
    return 0;
}

When compiling, an error says implicit declaration of function 'printf'. What is the fix?

medium

A. Add #include <stdio.h> in utils.c

B. Add #include <stdio.h> in utils.h

C. Add #include <stdio.h> in main.c

D. Remove print_message function

Solution

1. Step 1: Identify cause of error

   printf is declared in stdio.h; utils.c uses printf but lacks this include.

2. Step 2: Fix by including stdio.h in utils.c

   Adding #include <stdio.h> in utils.c provides printf declaration, fixing error.

3. Final Answer:

   Add #include <stdio.h> in utils.c -> Option A

4. Quick Check:

   Include stdio.h where printf is used [OK]

Hint: Include stdio.h in files using printf [OK]

Common Mistakes:

• Including stdio.h only in header file
• Including stdio.h only in main.c
• Ignoring missing printf declaration

5. You want to organize a C project with multiple source files sharing utility functions. Which is the best practice for header and source file organization?

hard

A. Put function definitions in header files and include them everywhere

B. Put function declarations and definitions together in each source file without headers

C. Put all code in a single source file without headers

D. Put all function declarations in one header file and definitions in one source file, then include the header where needed

Solution

1. Step 1: Understand modular organization

   Separating declarations in headers and definitions in source files allows sharing and reuse.

2. Step 2: Avoid putting definitions in headers

   Definitions in headers cause multiple definition errors when included in many files.

3. Final Answer:

   Put all function declarations in one header file and definitions in one source file, then include the header where needed -> Option D

4. Quick Check:

   Headers declare, sources define, include headers [OK]

Hint: Declare in headers, define in sources, include headers [OK]

Common Mistakes:

• Defining functions in headers causing linker errors
• Not using headers causing duplicate declarations
• Putting all code in one file reducing modularity