How Does C Linking Combine Object Files and Libraries into an Executable?

How C Linking Works in Practice

Overview

C linking is the process of combining multiple object files (.o) and libraries (.a) into a single executable file. Unlike compilation, which translates source code into object files, linking resolves unresolved symbols, relocates addresses, and ensures that the final executable is ready for execution.

Address Relocation

One of the crucial functions of linking is address relocation. Each object file contains references to external symbols, such as functions and data declared in other files. During linking, the linker calculates the final address of these symbols and adjusts the object file's code and data sections accordingly.

Relocation Information

Object files provide relocation information to the linker. Each relocation entry includes the address within the object file that needs to be relocated and the target symbol.

Linker's Role

The linker reads relocation entries from all object files and libraries. It resolves unresolved symbols by finding their definitions in other input files or libraries. It then calculates the final addresses of all symbols and updates the relocation entries.

Executable File

Once all symbols have been resolved and addresses have been relocated, the linker combines the individual object files into a single executable file. This executable contains the necessary code and data for the program to run on the target platform.

Example

Consider the following C program:

// main.cpp
#include <iostream>

int main() {
  std::cout <p>When this program is compiled, it produces two object files: main.o and cout.o. The object file main.o contains the code for the main function, while cout.o contains the implementation of the </p>
<p>During linking, the linker reads the relocation entries from both object files. It resolves the reference to std::cout by finding its definition in cout.o. The linker also updates the address references in main.o to point to the correct address of std::cout in the executable file.</p>
<p>The final executable, named a.out, contains the combined code from both object files and is ready to be executed.</p></iostream>

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