How does linking work in C ?

Linking is the process of combining separate object files generated by a compiler into a single executable program. It resolves references between different object files and libraries, assigns memory addresses to the code and data, and generates a final executable that can be loaded and run by the operating system.

The Linking Process

Linking is typically performed by a linker program. The linker takes as input a set of object files, along with any necessary libraries, and produces a single executable file. The linking process involves the following steps:

1. Symbol Resolution: The linker searches for all undefined symbols (functions, variables, etc.) in the object files and tries to match them with corresponding definitions from other object files or libraries.
2. Address Relocation: Once all symbols have been resolved, the linker assigns memory addresses to the code and data in the executable file. This process involves modifying instructions in the object files to point to the correct memory locations for their referenced symbols.
3. Library Resolution: The linker also handles the inclusion of any libraries that are referenced in the object files. These libraries provide additional functionality, such as standard C libraries or third-party modules, and are typically linked in at this stage.
4. Executable Generation: Finally, the linker combines all the modified object files and libraries into a single executable file. This file can then be loaded into memory and executed by the operating system.

Understanding Address Relocation

Address relocation is a crucial step in linking, as it ensures that the executable code can be loaded and run from any memory address. Here's a detailed explanation of how address relocation works:

• Symbol Table: Each object file contains a symbol table that lists all the symbols defined and referenced within the file.
• Relocation Table: Object files also contain a relocation table that lists the addresses within the object file that need to be modified during linking. These addresses typically correspond to instructions that reference unresolved symbols.
• Relocation Calculation: When the linker encounters a relocation entry, it calculates the correct memory address for the referenced symbol. This involves adding a base address to the address stored in the relocation entry. The base address represents the starting memory address of the executable file.
• Address Modification: Once the correct memory address has been calculated, the linker modifies the instruction at the specified address to point to the new address. This ensures that the instruction will execute correctly when the executable is loaded into memory.

Conclusion

Linking is a critical process that enables the creation of executable programs from separate object files. By resolving symbol references, assigning memory addresses, and handling library inclusion, the linker ensures that the executable code can be loaded and executed efficiently and correctly.

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