Linking multiple files in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a program with two linked files

Given two C files linked together, what is the output when the program runs?

C

/* file1.c */
#include <stdio.h>

void greet();

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

/* file2.c */
#include <stdio.h>

void greet() {
    printf("Hello from file2!\n");
}

A Runtime error: segmentation fault

B Compilation error: undefined reference to greet

C Hello from file1!

D Hello from file2!

💡 Hint

Remember that functions declared in one file can be defined in another and linked together.

The function greet() is declared in file1.c and defined in file2.c. When linked, the program calls the correct function and prints the message.

❓ Predict Output

intermediate

Value of a global variable shared across files

Consider two C files sharing a global variable. What is the value printed?

C

/* file1.c */
#include <stdio.h>

extern int counter;

int main() {
    counter = 5;
    print_counter();
    return 0;
}

/* file2.c */
#include <stdio.h>

int counter = 0;

void print_counter() {
    printf("Counter: %d\n", counter);
}

A Counter: 5

B Counter: 0

C Compilation error: multiple definitions of counter

D Runtime error: uninitialized variable

💡 Hint

Global variables declared with extern in one file and defined in another share the same memory.

The variable counter is defined in file2.c and declared as extern in file1.c. Setting it to 5 in main changes the shared variable, so print_counter() prints 5.

🔧 Debug

advanced

Identify the linking error in multiple files

Why does this program fail to link?

C

/* file1.c */
#include <stdio.h>

void print_message();

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

/* file2.c */
#include <stdio.h>

static void print_message() {
    printf("Hello from file2!\n");
}

A Compilation error: static function cannot be declared

B Runtime error: segmentation fault

C Linker error: undefined reference to print_message

D Program runs and prints 'Hello from file2!'

💡 Hint

Think about the meaning of static for functions in C files.

The static keyword limits the function's visibility to its own file. So print_message in file2.c is not visible to file1.c. The linker cannot find the function definition, causing an undefined reference error.

📝 Syntax

advanced

Correct declaration for a function defined in another file

Which option correctly declares a function defined in another file to avoid linking errors?

C

/* file1.c */
#include <stdio.h>

// Function declaration here

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

/* file2.c */
#include <stdio.h>

void display() {
    printf("Display function called\n");
}

A void display();

B extern void display();

C void display(void);

D static void display();

💡 Hint

Consider how to tell the compiler the function exists elsewhere.

Using extern void display(); explicitly declares the function is defined in another file. While void display(); and void display(void); are valid declarations, extern clarifies linkage. static would restrict visibility to the current file, causing linking errors.

🚀 Application

expert

Number of symbols visible after linking multiple files

Given three C files below, how many functions are visible to main() after linking?

C

/* file1.c */
#include <stdio.h>

void funcA();

int main() {
    funcA();
    funcB();
    funcC();
    return 0;
}

/* file2.c */
#include <stdio.h>

void funcA() {
    printf("Function A\n");
}

static void funcB() {
    printf("Function B\n");
}

/* file3.c */
#include <stdio.h>

void funcC() {
    printf("Function C\n");
}

A 2

B 3

C 1

D 0

💡 Hint

Remember that static functions are only visible inside their own file.

funcA and funcC are visible to main() because they are non-static and defined in other files. funcB is static in file2.c, so it is not visible outside that file. Therefore, only 2 functions are visible to main().

Practice

1. What is the main purpose of linking multiple files in a C program?

easy

A. To organize code into separate files and combine them into one program

B. To run multiple programs at the same time

C. To convert C code into machine language

D. To write comments in different files

Solution

1. Step 1: Understand code organization

   Linking multiple files allows splitting code into parts for better organization and reuse.

2. Step 2: Understand linking process

   After compiling separately, linking combines these parts into one executable program.

3. Final Answer:

   To organize code into separate files and combine them into one program -> Option A

4. Quick Check:

   Linking = Combining files [OK]

Hint: Linking combines separate code files into one program [OK]

Common Mistakes:

• Confusing linking with running multiple programs
• Thinking linking converts code to machine language
• Believing linking is for writing comments

2. Which command correctly compiles two C files main.c and utils.c together using GCC?

easy

A. gcc main.c -o program

B. gcc -o program -c main.c utils.c

C. gcc main.c utils.c -o program

D. gcc program -o main.c utils.c

Solution

1. Step 1: Understand GCC command structure

   The correct syntax is: gcc [source files] -o [output file].

2. Step 2: Check options order

   The standard GCC syntax is gcc [source files] -o [output], as in gcc main.c utils.c -o program.

3. Final Answer:

   gcc main.c utils.c -o program -> Option C

4. Quick Check:

   gcc source files -o output [OK]

Hint: Put source files first, then -o output [OK]

Common Mistakes:

• Placing -o before source files
• Mixing order of files and output option
• Using output file name as source file

3. Given two files:

// file1.c
#include 
void greet() { printf("Hello from file1\n"); }

// file2.c
void greet();
int main() { greet(); return 0; }

What is the output after compiling and linking both files?

medium

A. Compilation error: greet not declared

B. Hello from file1

C. Runtime error: undefined reference to greet

D. No output

Solution

1. Step 1: Check function declaration and definition

   file1.c defines greet() printing "Hello from file1"; file2.c declares greet() and calls it in main.

2. Step 2: Linking files

   Compiling and linking both files connects the call to greet() with its definition, so it prints the message.

3. Final Answer:

   Hello from file1 -> Option B

4. Quick Check:

   Function call linked correctly = prints message [OK]

Hint: Declare functions in headers or files to link correctly [OK]

Common Mistakes:

• Forgetting to link both files
• Missing function declaration causing errors
• Expecting runtime errors without linking

4. What is wrong with this setup?

// file1.c
void greet() { printf("Hi\n"); }

// file2.c
int main() { greet(); return 0; }

When compiling separately and linking, it gives an error. Why?

medium

A. Cannot compile multiple files separately

B. Missing #include in file1.c

C. main() should be in file1.c

D. greet() is not declared before use in file2.c

Solution

1. Step 1: Check function declaration in file2.c

   file2.c calls greet() but does not declare it or include a header with its prototype.

2. Step 2: Understand compiler error

   Without declaration, compiler errors occur because it doesn't know greet() signature.

3. Final Answer:

   greet() is not declared before use in file2.c -> Option D

4. Quick Check:

   Declare functions before use [OK]

Hint: Always declare functions before calling them [OK]

Common Mistakes:

• Assuming definition is enough without declaration
• Ignoring missing #include (not cause here)
• Thinking main() must be in same file as greet()

5. You have three files:

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

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

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

Which is the correct way to compile and link these files to produce an executable named calc?

hard

A. gcc main.c -c; gcc math_ops.c -c; gcc main.o math_ops.o -o calc

B. gcc main.c -o calc; gcc math_ops.c -o calc

C. gcc main.c math_ops.h -o calc

D. gcc main.c math_ops.c -c

Solution

1. Step 1: Compile source files separately

   Use gcc -c to compile main.c and math_ops.c into object files main.o and math_ops.o.

2. Step 2: Link object files

   Link main.o and math_ops.o together with gcc to create executable calc.

3. Final Answer:

   gcc main.c -c; gcc math_ops.c -c; gcc main.o math_ops.o -o calc -> Option A

4. Quick Check:

   Compile separately, then link object files [OK]

Hint: Compile with -c, then link object files [OK]

Common Mistakes:

• Compiling header files directly
• Overwriting executable by compiling twice
• Not linking object files after compiling