Run-time System

A run-time system (also called runtime system, runtime environment, or just runtime) implements the basic behavior of a computer language, whether the language is a compiled language, interpreted language, embedded domain-specific language, or is invoked via an API as is pthreads. When an API is provided to interact with a runtime, calls to that API look the same as calls to a software library, however it is the existence of a runtime that identify those calls as commands of an embedded style language, and not calls to a software library. The run-time system implements the basic low-level behavior of the language, and may also implement higher-level behavior and even support type checking, debugging, or code generation and optimization.

For embedded style languages, behavior of the run-time system is invoked through an application programming interface. Some compiled or interpreted languages also provide an interface that allows application code to directly interact with the runtime system. An example is the Thread class in the Java language. Normally core aspects of a language's behavior such as task scheduling and resource management are not accessible in this fashion.

A run-time system may implement behavior of tasks such as drawing text on the screen or making an Internet connection. It also typically acts as an abstraction layer that hides the complexity or variations in the services offered by the operating system. In the limit, the run-time system may provide services such as a P-code machine or virtual machine, that hide even the processor's instruction set. This is the approach followed by many interpreted languages such as AWK, and some languages like Java, which are meant to be compiled into some machine-independent pseudocode (bytecode). This arrangement greatly simplifies the task of language implementation and its adaptation to different machines, and allows sophisticated language features such as reflection. It also allows the same program to be executed on any machine without recompiling, a feature that has become very important since the diffusion of the World Wide Web.

A runtime may be modularized according to the proto-runtime approach.