Linker

Learn the linker and linking process.

Linker

The linker combines compiled object files (.o) into an executable. Each object file carries a symbol table; this table lists defined and referenced symbols.

The linker's job is to find a definition for every referenced symbol. If the definition is in another file, the linker takes it from there and points the reference to the real address. If no definition is found, an undefined reference error is raised. This is why large projects split into multiple .cpp files; each compiles separately and they link together.

Symbol resolution

If a .cpp file calls compute but does not define it, the object file keeps an undefined reference mark for compute. The linker searches other .o files for the definition of compute.

If found, the linker points the reference to the real address. If not found, it raises undefined reference to compute. This is why a forward declaration (int compute(int);) goes in a header; the definition goes in a .cpp file. Each file does not see the definition while compiling but the linker resolves it.

Exercise Write the compute function

Write int compute(int x) with body return x * 2;. In main write std::cout << compute(5);.

Show example solution

#include <iostream>

int compute(int x) {
    return x * 2;
}

int main() {
    std::cout << compute(5);
    return 0;
}

Test yourself

What does the linker do?
```
g++ main.o utils.o -o program
```
- Compiles source code
- Starts the operating system
- Garbage collects
- Combines object files and resolves symbols
Show answer

Combines object files and resolves symbols — The linker combines .o files and produces an executable.

Complete the forward declaration

int compute(int ____);  // forward declaration

Show answer

int compute(int x);  // forward declaration

Match each concept to its role

undefined reference

Link error
object file (.o)

Symbol table
symbol table

Symbol addresses
linker

Combines objects

Next lesson: Optimization

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