DLLs

Dynamic Link Library, or DLL, is a module that contains code and data that can be used by multiple programs concurrently. In Windows, DLLs enable code reuse and modularization by allowing common functionality—such as file I/O, graphics, or utility routines—to reside in a single file that several applications can load as needed. A DLL can also contain resources such as icons or strings.

How it works: Executables that use a DLL declare imports. At load time, the operating system resolves these imports by mapping the DLL into the process address space and resolving symbol addresses via the DLL's export table. A DLL exports functions and data; other code obtains pointers to these using the import table and, optionally, GetProcAddress when loading dynamically.

Usage and benefits: DLLs reduce memory usage and disk space by sharing a single copy of code between processes, simplify updates, and support modular architectures. However, they introduce dependency management challenges, often referred to as DLL Hell, where applications require different versions of shared components.

Dynamic loading and flexibility: Programs can delay loading a DLL until its functionality is needed, using LoadLibrary or equivalent, and can obtain function pointers with GetProcAddress. This enables plugin systems and optional features.

Security and portability: Windows supports features like side-by-side assemblies, ASLR, and code signing to mitigate risks. In cross-platform terms, similar concepts exist as shared libraries (.so on Linux, .dylib on macOS).