What are the main linking and loading mechanisms?

The main linking and loading mechanisms are static linking, dynamic linking, load-time dynamic linking, and run-time dynamic linking.

Static linking is the process where the linker combines all the modules of a program, including the main program and any libraries it uses, into a single executable file. This process happens at compile-time, meaning that all the necessary code is included in the executable file before it is run. This results in a larger executable file, but it ensures that all the necessary code is present and that the program can run independently of any external libraries.

Dynamic linking, on the other hand, does not include the library code in the executable file. Instead, references to the library functions are included in the executable file, and the actual linking to the library code is done either at load-time or at run-time. This results in a smaller executable file, but it means that the necessary libraries must be present on the system where the program is run.

Load-time dynamic linking is a type of dynamic linking where the linking to the library code is done when the program is loaded into memory, before it starts running. The operating system's loader is responsible for this linking process. It checks the references to the library functions in the executable file and links them to the actual library code in memory. If the necessary libraries are not present on the system, the loader will report an error and the program will not run.

Run-time dynamic linking is another type of dynamic linking where the linking to the library code is done while the program is running. This is typically used for libraries that are only needed in certain situations, so they are not loaded into memory until they are actually needed. This can save memory and improve performance, but it also means that errors may not be detected until the program is running. If the necessary libraries are not present on the system when they are needed, the program will report an error and may stop running.