what is linking and loading

Linking and loading are two essential processes that occur during the execution of computer programs. Understanding these processes is crucial for programmers, as it affects the memory usage, performance, and overall functioning of their applications. In this article, we will explore in detail what is linking and loading, and how they work in practice.

What is Linking?

Linking is the process of combining one or more object files generated by a compiler into a single executable file. Object files contain machine code and data that the program uses. The linker's primary task is to resolve any cross-references between the object files and create a single, coherent program image that the operating system can load into memory. There are three types of linking: static, dynamic, and hybrid.

Static Linking

Static linking involves merging all the necessary object files into a single, standalone executable file. The executable file contains all the required libraries, functions, and data needed to run the program. Because everything the program needs is contained in a single file, it is easy to distribute and run on other systems without the need for additional dependencies. However, this approach has some disadvantages, for instance, the program can take up more storage space, and updates require recompiling the whole program.

Dynamic Linking

Dynamic linking, on the other hand, only merges the code needed at run-time, leaving the rest of the program's code to be resolved during loading. Dynamic libraries are loaded and linked at run-time by the operating system's dynamic linker. This method allows programs to share libraries, reducing memory usage, updating libraries without recompiling the program, and reducing launch time. However, programs that rely on dynamic linking may fail if the required libraries are missing or not compatible, resulting in crashes or undefined behavior.

Hybrid Linking

Hybrid linking, also called mixed linking, is a combination of static and dynamic linking. It involves statically linking libraries necessary for the program to function and dynamically linking optional libraries that the program may leverage. This approach results in a smaller executable file and the benefits of dynamic linking.

What is Loading?

Loading is the process of bringing the compiled and linked code to run in memory at runtime. When the operating system starts a program, it loads the program code and data from the executable file to its virtual memory space. The virtual memory allows the operating system to separate the memory space between different programs to avoid conflicts and improve security. The loading process involves several steps, including:

Address Binding

Address binding is the process of assigning physical memory addresses to the program's virtual memory space. The virtual memory is broken into smaller units, called pages, that can be resident in memory or paged out to the hard drive when they are not in use. The binding process can be done either at load-time or run-time.

Relocation

Relocation is the process of adjusting the program's memory addresses when the program is loaded. The relocation process is necessary when a program is loaded at a different address than where it was linked. For instance, if the program is linked to start at memory address 0x1000, and the operating system loads it at address 0x2000, the relocation process adjusts all the addresses in the program to match the new location.

Symbol Resolution

Symbol resolution is the process of matching symbolic references, such as function calls or variable references, with their corresponding memory addresses. Symbol resolution can be done statically or dynamically. Static symbol resolution is done at link time, and dynamic symbol resolution is done during run-time.

Conclusion

In summary, linking and loading are two vital processes that enable computer programs to run correctly. Linking combines object files into a single executable file, while loading brings the compiled and linked code into memory at run-time. Understanding the differences between static, dynamic, and hybrid linking and the nuances of loading is essential for building efficient and robust computer programs.