Dynamic loading

Dynamic loading was a common technique for IBM's operating systems for System/360 such as OS/360, particularly for I/O subroutines, and for COBOL and PL/I runtime libraries, and continues to be used in IBM's operating systems for z/Architecture, such as z/OS. As far as the application programmer is concerned, the loading is largely transparent, since it is mostly handled by the operating system (or its I/O subsystem). The main advantages are: IBM's strategic transaction processing system, CICS (1970s onwards) uses dynamic loading extensively both for its kernel and for normal application program loading. Corrections to application programs could be made offline and new copies of changed programs loaded dynamically without needing to restart CICS (which can, and frequently does, run 24/7). Shared libraries were added to Unix in the 1980s, but initially without the ability to let a program load additional libraries after startup. Dynamic loading is most frequently used in implementing software plugins. For example, the Apache Web Server's *.dso 'dynamic shared object' plugin files are libraries which are loaded at runtime with dynamic loading. Dynamic loading is also used in implementing computer programs where multiple different libraries may supply the requisite functionality and where the user has the option to select which library or libraries to provide. Not all systems support dynamic loading. UNIX-like operating systems such as macOS, Linux, and Solaris provide dynamic loading with the C programming language 'dl' library. The Windows operating system provides dynamic loading through the Windows API. Loading the library is accomplished with LoadLibrary or LoadLibraryEx on Windows and with dlopen on UNIX-like operating systems. Examples follow: As a UNIX library: As a macOS Framework: