WIn C programming, linking is a critical part of turning your code into a working program. After you compile your code, linking combines all the parts into one executable file. This step connects different object files and libraries, allowing your program to run correctly.

In this guide, you’ll learn what linking is, how it works, and the differences between static and dynamic linking. We’ll also cover real-world code examples, a small project, and interview questions asked by top tech companies.

What is Linking in C?

After you compile your .c file using a compiler like GCC, it produces an object file (e.g., main.o). However, this file isn’t enough to run the program. You need to link this object file with others (like library files) to create an executable (e.g., main.exe or a.out).

The process of combining these files is known as linking.

Two Types of Linking in C

1. Static Linking

In static linking, all necessary code from libraries is added to the final executable during compilation.

Pros:

• No need for external libraries at runtime.
• Faster execution (libraries are already included).

Cons:

• Larger executable file size.
• Updating library code means recompiling the entire program.

Example:

gcc main.c libmath.a -o output

Here, libmath.a is a static library.

2. Dynamic Linking

In dynamic linking, the executable contains references to external libraries (e.g., .so files on Linux or .dll files on Windows). These are loaded when the program runs.

Pros:

• Smaller executable.
• Easy to update shared libraries without recompiling.

Cons:

• The required library must be present on the system.
• Slightly slower due to runtime loading.

Example:

gcc main.c -lm -o output

This links the standard math library dynamically.

Internal and External Linking

Internal Linking

Symbols (like functions or variables) marked as static are only visible within the same source file.

static int count = 0; // This has internal linkage

External Linking

Symbols not marked as static are accessible across different files.

int count = 0; // External by default

Linking Errors and How to Fix Them

1. Undefined Reference

textCopy codeundefined reference to `myFunction'

Fix: Make sure you’re linking all necessary object files or libraries:

gcc main.c myFunction.c -o output

2. Duplicate Symbol

textCopy codemultiple definition of `x'

Fix: Declare variables in only one file, and use extern in others.

Mini Project: Modular C Calculator

Goal: Build a basic calculator that separates logic into multiple C files.

Structure:

• main.c – handles user input
• calc.c – contains functions like add, subtract, etc.
• calc.h – header file with function declarations

Files:

calc.h

#ifndef CALC_H
#define CALC_H

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

#endif

calc.c

#include "calc.h"

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

int subtract(int a, int b) {
    return a - b;
}

main.c

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

int main() {
    int x = 10, y = 5;
    printf("Add: %d\n", add(x, y));
    printf("Subtract: %d\n", subtract(x, y));
    return 0;
}

Compile and Link:

bashCopy codegcc main.c calc.c -o calculator

Interview Questions & Answers

Google

Q: What is the purpose of linking in C?
A: Linking combines object files and libraries into a final executable. It resolves references between code units.

TCS

Q: What is the difference between static and dynamic linking?
A: Static linking copies library code into the executable. Dynamic linking loads libraries at runtime, saving space.

Infosys

Q: What happens if a symbol is not found during linking?
A: You get an “undefined reference” error. This usually means a required object file or library wasn’t linked properly.

Zoho

Q: How can you link multiple C files together?
A: Compile and link all .c files in one command:

gcc file1.c file2.c -o program

Amazon

Q: How do static and dynamic libraries affect performance?
A: Static libraries may run faster but increase file size. Dynamic libraries save space but load at runtime, which can slow startup.

Conclusion

Linking is a key stage in building a C program. Whether you’re working with a single file or a large modular project, understanding static and dynamic linking is essential. Proper linking not only helps in program execution but also improves code organization and scalability.