What is a Linker?

A linker is a tool that takes one or more object files generated by a compiler and combines them into a single executable file, library, or another type of binary. Object files contain machine code and data generated from source code, but they are not complete programs. The linker's job is to piece these fragments together, resolving references and addresses to create a runnable program.

Why Are Linkers Used?

Linkers are used to handle several critical tasks in the software development process:

1. Combining Object Files: Modern programs are often split into multiple source files for better organization and maintainability. Each source file is compiled into an object file, and the linker combines these object files into a single executable.
2. Symbol Resolution: Functions and variables defined in one source file might be used in another. The linker resolves these references, ensuring that the final executable correctly links each function and variable to its definition.
3. Address Binding: During the linking process, the linker assigns final memory addresses to various parts of the program. This includes code, static data, and dynamically allocated memory.
4. Library Management: Linkers handle the inclusion of libraries, which are collections of pre-compiled code that programs can use. This includes standard libraries provided by the language and third-party libraries.
5. Relocation: If the object files or libraries were compiled to use different memory addresses than what is available at runtime, the linker adjusts the addresses so that they correctly point to the right locations in memory.
6. Optimization: Some linkers perform optimizations, such as removing unused code (dead code elimination) or combining duplicate sections (merging).

Output File Formats

Linkers produce various types of output files depending on the target operating system and architecture. Here are some of the most common formats:

1 ELF (Executable and Linkable Format):

• Usage: Widely used in Unix-like operating systems, including Linux and BSD.
• Features: Supports dynamic linking, contains headers, program headers, and section headers.
• Tools: GNU ld, gold, ld.lld.

2 PE (Portable Executable):

• Usage: Standard format for Windows executables, including .exe and .dll files.
• Features: Derived from COFF (Common Object File Format), supports dynamic linking and runtime linking.
• Tools: Microsoft link.exe, GNU ld with MinGW.

3 Mach-O (Mach Object):

• Usage: Used in macOS and iOS.
• Features: Supports static and dynamic linking, contains headers, load commands, and sections.
• Tools: Apple ld, ld64, ld.lld.

4 COFF (Common Object File Format):

• Usage: Primarily in older Unix systems and as a basis for the PE format.
• Features: Simple structure, used in embedded systems.
• Tools: Older versions of GNU ld, Microsoft link.exe.

5 A.OUT:

• Usage: One of the earliest Unix executable formats.
• Features: Simple structure, largely replaced by more advanced formats.
• Tools: Older versions of GNU ld.