The linker
What is a linker?
A linker is a program that links object files generated by a compiler into an executable or shared library. It resolves external symbols and relocations, and generates the final output file.
What are External symbols?
External symbols are variables that get declared in one object file, but they also get used in another different object file. For example, I may declare a global variable 'X' in a header file. If I reference this header file in another file (eg main.rs) and use variable X... that means X has become an external symbol.
What does resolving Exernal symbols mean?
In the context of the linker, resolving External symbols means finding the actual memory location or address of a symbol that is referenced by another module, and updating the reference in the module that uses the symbol to point to the correct memory location.
What is relocation?
Relocation is the act of changing the memory address pointed to by a variable. In the case of a linker, after it has resolved all external symbols, it changes the memory addresses of those external symbols and makes them point to different memory addresses that were specified by the linking script.
So the relocation process adjusts the addresses of symbols in an object file to reflect their final location in memory. In this case, we mean virtual memory addresses... NOT Physical memory addresses.
What is this linking script?
A linker script is a text file that provides additional instructions to the linker about how to link the input files. It can specify the layout of the output file or the order in which the input files should be linked.