Splitting code into multiple files - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a multi-file C program

Consider a C program split into two files: main.c and helper.c. The helper.c file defines a function int add(int a, int b) that returns the sum of two integers. The main.c file calls this function and prints the result.

What will be the output when the program is compiled and run?

C

/* helper.c */
int add(int a, int b) {
    return a + b;
}

/* main.c */
#include <stdio.h>

int add(int a, int b); // function prototype

int main() {
    int result = add(3, 4);
    printf("Result: %d\n", result);
    return 0;
}

A Compilation error: undefined reference to 'add'

B Result: 34

C Runtime error: segmentation fault

D Result: 7

💡 Hint

Remember that the function add is defined in another file and declared in main.c.

The function add correctly returns the sum of 3 and 4, which is 7. The program prints this value.

❓ Predict Output

intermediate

Effect of missing header file in multi-file C program

Given two files, main.c and math_ops.c. The math_ops.c file defines a function int multiply(int x, int y). The main.c file calls multiply but does not include a header file declaring it.

What is the most likely outcome when compiling and running this program?

C

/* math_ops.c */
int multiply(int x, int y) {
    return x * y;
}

/* main.c */
#include <stdio.h>

int main() {
    int result = multiply(5, 6);
    printf("Product: %d\n", result);
    return 0;
}

A Product: 30

B Compilation error: implicit declaration of function 'multiply'

C Runtime error: segmentation fault

D Product: 0

💡 Hint

Check if the function multiply is declared before use in main.c.

Without a function prototype or header file, the compiler will warn or error about implicit declaration of multiply.

🔧 Debug

advanced

Debugging linker error in multi-file C program

You have two files: main.c and utils.c. The main.c calls a function void greet() defined in utils.c. You compile with gcc main.c only. What error will you get?

C

/* utils.c */
#include <stdio.h>

void greet() {
    printf("Hello!\n");
}

/* main.c */

void greet();

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

A Compilation error: missing prototype for greet

B Program runs and prints 'Hello!'

C Linker error: undefined reference to 'greet'

D Runtime error: segmentation fault

💡 Hint

Think about what happens if you compile only one source file but call a function defined in another.

Compiling only main.c does not link utils.c, so the linker cannot find greet.

📝 Syntax

advanced

Correct way to declare external function in multi-file C program

Which of the following is the correct way to declare a function int compute(int a, int b) defined in another file so it can be used in main.c?

A int compute(int a, int b);

B extern int compute();

C int compute();

D void compute(int a, int b);

💡 Hint

Function declarations should match the function signature exactly.

Option A declares the function with correct return type and parameter types. Option A lacks parameter types, C is old style without parameters, A has wrong return type.

🚀 Application

expert

Number of object files needed for multi-file C project

You have a C project with 4 source files: main.c, math.c, io.c, and utils.c. You compile each source file separately into object files and then link them into an executable.

How many object files will you have before linking?

A 4

B 1

C 3

D 5

💡 Hint

Each source file produces one object file when compiled separately.

Each of the 4 source files compiles into one object file, so total 4 object files before linking.

Practice

1. What is the main reason to split C code into multiple files?

easy

A. To reduce the number of variables

B. To make the program run faster

C. To organize code and make it reusable

D. To avoid using header files

Solution

1. Step 1: Understand code organization

   Splitting code into multiple files helps keep related code parts separate and easier to manage.

2. Step 2: Recognize code reuse benefit

   It also allows reusing code in different programs without rewriting it.

3. Final Answer:

   To organize code and make it reusable -> Option C

4. Quick Check:

   Splitting code = Organize and reuse [OK]

Hint: Think about code clarity and reuse benefits [OK]

Common Mistakes:

• Assuming splitting files makes code run faster
• Thinking header files are not needed
• Believing it reduces variables automatically

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

easy

A. #include utils.h

B. #include

C. include "utils.h"

D. #include "utils.h"

Solution

1. Step 1: Understand header file inclusion syntax

   To include your own header files, use double quotes around the filename.

2. Step 2: Differentiate system and user headers

   Angle brackets are for system headers, double quotes for user headers like utils.h.

3. Final Answer:

   #include "utils.h" -> Option D

4. Quick Check:

   User header files use "filename" [OK]

Hint: Use quotes for your headers, angle brackets for system [OK]

Common Mistakes:

• Using angle brackets for user headers
• Omitting # in include directive
• Not using quotes or brackets around filename

3. Given two files:

// file1.c
#include 
void greet();
int main() {
    greet();
    return 0;
}

// file2.c
#include 
void greet() {
    printf("Hello from file2!\n");
}

What is the output when compiled and run together?

medium

A. Hello from file2!

B. Compilation error: greet not declared

C. No output

D. Runtime error

Solution

1. Step 1: Check function declaration and definition

   file1.c declares greet() and calls it; file2.c defines greet() printing the message.

2. Step 2: Understand linking multiple files

   Compiling both files together links greet() correctly, so the program prints the message.

3. Final Answer:

   Hello from file2! -> Option A

4. Quick Check:

   Function declared and defined across files prints message [OK]

Hint: Declare before use, define in another file, link both [OK]

Common Mistakes:

• Forgetting to compile both files together
• Missing function declaration in file1.c
• Expecting compilation error due to separate files

4. You have two files:

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

// helper.c
#include 
void print_message() {
    printf("Hi!\n");
}

// helper.h
void print_message();

Compiling with gcc main.c gives an error: undefined reference to 'print_message'. What is the likely cause?

medium

A. Missing semicolon in helper.h

B. helper.c was not compiled or linked properly

C. print_message() is not declared in main.c

D. main.c does not include stdio.h

Solution

1. Step 1: Understand the error message

   "undefined reference" means the linker can't find the function definition.

2. Step 2: Check compilation command

   If helper.c is not compiled or linked, the function print_message() is missing at link time.

3. Final Answer:

   helper.c was not compiled or linked properly -> Option B

4. Quick Check:

   Linker error = missing compiled file [OK]

Hint: Always compile and link all source files together [OK]

Common Mistakes:

• Not compiling helper.c with main.c
• Ignoring linker errors thinking it's a syntax error
• Assuming declaration alone is enough

5. You want to split a program into main.c and math_ops.c. math_ops.c defines int add(int a, int b). How should you organize files and compile to use add in main.c correctly?

hard

A. Declare add in math_ops.h, include it in main.c, compile with gcc main.c math_ops.c

B. Define add in main.c and call it from math_ops.c, compile separately

C. Include math_ops.c directly in main.c with #include, compile main.c only

D. Declare add in main.c only, compile math_ops.c separately

Solution

1. Step 1: Create a header file math_ops.h

   Declare the function prototype int add(int a, int b); in math_ops.h.

2. Step 2: Include math_ops.h in main.c and compile both files

   Include math_ops.h in main.c to use add(), then compile with gcc main.c math_ops.c to link both.

3. Final Answer:

   Declare add in math_ops.h, include it in main.c, compile with gcc main.c math_ops.c -> Option A

4. Quick Check:

   Header declares, source defines, compile both [OK]

Hint: Use header for declarations, compile all sources together [OK]

Common Mistakes:

• Including .c files instead of headers
• Not declaring functions before use
• Compiling only one source file