What is a Linker

 A linker is a tool in the process of software development that combines one or more object files into a single executable program or library. It performs the process of linking, where it resolves references between different parts of a program, such as functions, variables, and libraries.

Key Functions of a Linker:

1. Symbol Resolution:

   • When a program is compiled, each source file is translated into an object file (e.g., .obj or .o files). These object files may contain references to functions or variables that are defined in other object files or libraries.
   • The linker resolves these references by matching symbols (like function names or global variables) in the object files with their actual definitions, ensuring that the program calls the right functions or accesses the right data.

2. Address Binding:

   • The linker assigns memory addresses to the symbols (variables, functions, etc.) in the object files. This process is called address binding. The linker adjusts the machine code in the object files to reflect the correct addresses.
   • It ensures that functions and data are located in memory correctly when the program is run.

3. Combining Object Files:

   • A program typically consists of multiple object files. The linker combines these into a single executable file by appending each object file together and adjusting the addresses where necessary.
   • It handles static libraries (e.g., .lib or .a files) or shared libraries (e.g., .dll or .so files), linking them with the object files to include the required external code.

4. Relocation:

   • The linker modifies references in the code so that they point to the correct memory locations. For example, if a function in one object file calls a function in another object file, the linker updates the call instruction to point to the correct memory address of the target function.
   • Relocation is necessary because object files are generated without knowing where they will be loaded into memory.

5. Handling External Dependencies:

   • The linker can also link with external libraries, such as system libraries or third-party libraries. This ensures that all external symbols are properly resolved and included in the final executable.

Types of Linkers:

1. Static Linker:

   • A static linker takes all the object files and static libraries required for the program and combines them into a single executable file. The linking process happens at compile time.
   • Once linked, the program is independent of the libraries, as all the necessary code is included within the executable. For example, linking a C program with standard libraries to produce a static executable.
   • Example: gcc -o myprogram myprogram.o -lm (where -lm links the math library statically).

2. Dynamic Linker:

   • A dynamic linker works at runtime (rather than compile time) to link the executable with shared libraries (like .dll on Windows or .so on Linux).
   • When a program is executed, the dynamic linker loads the required libraries into memory and resolves function calls to the correct locations.
   • Dynamic linking reduces the size of the executable and allows multiple programs to share the same library code, which can save memory.
   • Example: gcc -o myprogram myprogram.o -lm (where -lm links the math library dynamically).

A linker is a critical tool in the software development process. It takes one or more object files and combines them into a complete, executable program or library. It resolves symbol references, assigns memory addresses, handles external dependencies, and ensures that the final executable functions correctly by linking all necessary parts together.