Dynamic Link Libraries (DLLs) are files containing application functions or instructions called and used by other applications. It enables multiple applications to simultaneously access the necessary functions embedded on a single DLL.

Software developers use a file extension DLL to create programs written in different programming languages capable of using a library or importing DLLs functions. Such programming languages include:

• Java;
• C;
• Visual Basic;
• and C++.

Linux is a modular and free Unix-like operating system. It uses the Linux kernel. Linux gained much of its fundamental design from the values established in UNIX during the 1970s up to the 1980s.

Linux-based libraries and utilities usually come from the OS and seldom from applications. Linux is utilized as an operating system for a wide range of computer hardware. These include:

• Desktop computers
• Video game systems
• Arcade games
• Supercomputers
• Embedded devices (mobile phones, stage lighting systems and routers)

DLLs are executed in Linux as dynamic libraries. Linux links programs to an external function by statically linking these programs at build time or dynamically linking such applications at runtime with shared libraries.

Linux dynamic linking loader maps are stored into the program implementation memory. This means all necessary shared object libraries are initially mapped by the dynamic linking loader into the program's memory before a program starts in Linux. It also uses system shared objects before it tries to resolve the required external references for the programs.

Linux implements the equivalent dynamic linking loader when mapping a DLL into the program's memory after it has been executed. Its dynamic loader routines call the program controls where the dynamic libraries are loaded. This is also where such routines work with the libraries to initiate loading and linking processes. It also tries to extract the addresses of the entry points needed by the applications.

• dlopen - this library function is responsible for mapping the shared objects file into the memory;
• dlerror - this library function is use to return a pointer or NULL to an ASCII string describing latest errors;
• dlsym - this library function extracts a pointer to the entry point that needs it; and
• dlclose - this library function is responsible for locking the handle and unmapping the shared objects.

It is necessary to identify the filename as well as its extension when loading Windows-based DLLs into Linux. This makes attribute access function properly.

The LoadLibrary routine of the DLL loaders can be utilized in loading a DLL. Simply loading the DLL by making an instance of CDLL can also be done. Indirect function calls are then made to the DLLs by the system.

If you need to write DLLs for Linux applications you can check out these instructions from IBM too.