In the execution of the program, major role is played by two utility programs known as Linker and Loader.
1. Linker: A linker is special program that combines the object files, generated by compiler/assembler and other pieces of code to originate an executable file has .exe extension. In the object file, linker searches and append all libraries needed for execution of file. It regulates the memory space that will hold the code from each module. It also merges two or more separate object programs and establishes link among them.
Generally, linkers are of two types :
1. Linkage Editor 2. Dynamic Linker
The linker performs several tasks, including:
- Symbol resolution: The linker resolves symbols in the program that are defined in one module and referenced in another.
- Code optimization: The linker optimizes the code generated by the compiler to reduce code size and improve program performance.
- Memory management: The linker assigns memory addresses to the code and data sections of the program and resolves any conflicts that arise.
- Library management: The linker can link external libraries into the executable file to provide additional functionality.
2. Loader: It is special program that takes input of executable files from linker, loads it to main memory, and prepares this code for execution by computer. Loader allocates memory space to program. Even it settles down symbolic reference between objects. It is in charge of loading programs and libraries in operating system. The embedded computer systems don’t have loaders. In them, code is executed through ROM. There are following various loading schemes:
1. Absolute Loaders 2. Relocating Loaders 3. Direct Linking Loaders 4. Bootstrap Loaders
The loader performs several tasks, including:
- Loading: The loader loads the executable file into memory and allocates memory for the program.
- Relocation: The loader adjusts the program’s memory addresses to reflect its location in memory.
- Symbol resolution: The loader resolves any unresolved external symbols that are required by the program.
- Dynamic linking: The loader can dynamically link libraries into the program at runtime to provide additional functionality.
Differences between Linker and Loader are as follows:
| LINKER | LOADER |
|---|---|
| The main function of Linker is to generate executable files. | Whereas main objective of Loader is to load executable files to main memory. |
| The linker takes input of object code generated by compiler/assembler. | And the loader takes input of executable files generated by linker. |
| Linking can be defined as process of combining various pieces of codes and source code to obtain executable code. | Loading can be defined as process of loading executable codes to main memory for further execution. |
| Linkers are of 2 types: Linkage Editor and Dynamic Linker. | Loaders are of 4 types: Absolute, Relocating, Direct Linking, Bootstrap. |
| Another use of linker is to combine all object modules. | It helps in allocating the address to executable codes/files. |
| Linker is also responsible for arranging objects in program’s address space. | Loader is also responsible for adjusting references which are used within the program. |
Conclusion :
Linker and Loader are both important components in the software development process. The Linker is used during the compilation process to link object files into a single executable file, while the Loader is used at runtime to load the executable file into memory and prepare it for execution. Understanding the differences between the two is important for efficient and effective software development.