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Program Linking

Linking is the process of merging multiple target modules (compiled source code) into a complete executable file. It not only involves merging target modules, but also involves resolving address references and symbol resolution between modules.

Static linking is to combine the target modules and required library functions into a complete executable file after the program is compiled and generated before the program is executed.

  • Process: After each target module is compiled, its address is relative, usually calculated relative to the module's starting address of 0. Through the linking process, the starting addresses of all target modules will be adjusted to actual memory addresses.
  • Symbol parsing: Static links not only need to modify the starting address in each target module, but also parse external call symbols (such as function calls, external variables, etc.) and convert them into actual memory addresses.
  • Advantages: The executable files generated by static linking are usually relatively independent and contain all required library codes, so they do not depend on external shared libraries or dynamic libraries during execution.

Load-time dynamic linking refers to linking on demand when the operating system loads the program into memory while the program is running.

  • Process: The user's target modules are loaded into the memory one by one through the loader. Each time the target module is loaded, if the module depends on other modules or libraries, the operating system will automatically load these dependencies and modify the address references in the target module.
  • Advantages: Dynamic links allow the program to load and connect target modules as needed, reducing the initial loading time of the program and allowing module updates and sharing. For example, after updating a certain module, you only need to update the corresponding target module without recompiling the entire program.

Runtime dynamic linking means that when a module is needed during program execution, the operating system will dynamically load it into the memory and link it.

  • Process: This linking method is usually handled by the operating system or execution time environment when the program is executed. When the program encounters a target module that has not yet been loaded, the operating system loads it into memory and establishes a link. This method can significantly improve the startup speed of the program and save main memory space.
  • Advantages: The most significant advantage of runtime dynamic linking is the memory space saved, since modules are only loaded when needed. In addition, runtime dynamic linking can effectively share common libraries, reduce repeated loading, and improve memory usage.

Summarize

  • Static linking: Completed at compile time, producing an independent executable file without relying on external modules. The disadvantage is that the executable file is larger and the module update is more difficult.
  • Dynamic linking at load time: The program dynamically resolves dependencies between modules when loading, allowing modules to be shared and updated, which is better than static linking, but may increase loading time.
  • Runtime dynamic link: Load modules on demand while the program is running, saving memory space and speeding up program startup. However, it depends on the support of the operating system and may cause delays during execution.