C - Linker and Object File Concepts in C

Object File – Definition

An object file is an intermediate file generated by the compiler after successful compilation of a C source file.
It contains machine-level instructions, but it is not executable.

Typical extensions:

• .o (Linux/Unix)

• .obj (Windows)

Contents of an Object File

An object file generally contains:

1. Machine Code

   • Compiled instructions of functions and statements

2. Symbol Table

   • Names of variables and functions

   • Indicates whether symbols are defined or undefined

3. Relocation Information

   • Used by the linker to adjust addresses

4. Data Section

   • Global and static variables

Types of Object Files

1. Relocatable Object File

   • Generated by compiler

   • Needs linking to become executable

2. Executable Object File

   • Final output after linking

   • Ready to run

3. Shared Object File

   • Used for dynamic linking

   • Example: .so, .dll

Linker – Definition

A linker is a system software tool that combines one or more object files and required libraries to produce a final executable file.

Functions of the Linker

1. Symbol Resolution

• Matches function calls with their definitions

• Example: Resolving printf() from standard library

2. Address Binding

• Assigns final memory addresses to variables and functions

3. Combining Object Files

• Merges multiple .o files into one executable

Example:

gcc file1.o file2.o

4. Library Linking

• Links user-defined and standard libraries

• Handles missing external references

Types of Linking

1. Static Linking

• Library code is copied into executable

• Executable size is larger

• No external dependency at runtime

Example:

gcc -static prog.c

2. Dynamic Linking

• Libraries are linked at runtime

• Smaller executable size

• Shared libraries are reused

Example:

gcc prog.c

Linking Errors

Errors detected during linking are called linker errors.

Examples:

• Undefined reference to a function

• Multiple definition of a variable

Example error:

undefined reference to `add`

Difference Between Compiler and Linker

Program Flow (Simplified)

Source File (.c)
       ↓
Compiler
       ↓
Object File (.o)
       ↓
Linker
       ↓
Executable File