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A runtime system, also called run-time system or just runtime, exhibits the behavior of the constructs of a computer language. Every programming language has some form of a runtime system (as explained further below), whether the language is a compiled language, interpreted language, embedded domain-specific language, or is invoked via an API as is pthreads. In addition to the behavior of the language constructs, a runtime system may also perform support services such as type checking, debugging, or code generation and optimization.
The runtime system is also the gateway by which a running program interacts with the runtime environment, which contains state values that are accessible during program execution, as well as active entities that can be interacted with during program execution. For example, environment variables are features of many operating systems, and are part of the runtime environment; a running program can access them via the runtime system. Likewise, hardware devices such as a DVD drive are active entities that a program can interact with via a runtime system.
As a simple example of a basic runtime, the runtime system of the C language is a particular set of instructions inserted into the executable image by the compiler. Among other things, these instructions manage the processor stack, create space for local variables, and copy function-call parameters onto the top of the stack. There are often no clear criteria for deciding which language behavior is considered inside the runtime system versus which behavior is "compiled". In this case, the reason that C's stack behavior is part of the runtime, as opposed to part of a keyword of the language, is that it is systematic, maintaining the state of the stack throughout a program's execution. The systematic behavior implements the execution model of the language, as opposed to implementing semantics of particular keywords which are directly translated into code that computes results.
Another example, which illuminates the nature of a runtime system, is the case of using an application programming interface (API) to interact with a runtime system. The calls to that API look the same as calls to a software library, however the runtime implements an execution model that is different from that of the language the library is written in terms of. A person reading the code of a library would be able to understand the library's behavior by just knowing the language the library was written in. However, a person reading the code of the API that invokes a runtime would not be able to understand the behavior of the API call just by knowing the language the call was written in. At some point, via some mechanism, the execution model stops being that of the language the call is written in and switches over to being the execution model implemented by the runtime system. For example, the trap instruction is one method of switching execution models. This difference is what distinguishes an API invoked language, such as posix threads, from a software library. Both posix-threads calls and software library calls are invoked via an API, but posix-threads behavior cannot be understood in terms of the language of the call. Rather, posix-threads calls bring into play an outside execution model, which is implemented by the posix-threads runtime (this runtime is often the OS kernel).
Some compiled or interpreted languages 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.
Higher level behaviors implemented by a run-time system may include tasks such as drawing text on the screen or making an Internet connection. It is often the case that operating systems provide these kinds of behaviors as well, and when available, the runtime is implemented as an abstraction layer, that translates the invocation of the runtime into an invocation of the operating system. This hides the complexity or variations in the services offered by different operating systems. This also implies that the OS kernel can be viewed as a runtime system, and that the set of OS calls that invoke OS behaviors may be viewed as an API invoked language.
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. To speed up execution, some run-time systems feature just-in-time compilation to machine code.
At the other extreme, for assembly language, the physical CPU itself can be viewed as an implementation of the runtime system for a programming language. This can be better understood by considering that language features such as macros are implemented by the assembler tool to manipulate the code structure, while the execution-time behavior is directed by the physical CPU and memory systems. This also helps in illustrating the fact that runtime systems for higher-level languages are themselves implemented by using some other languages. Also, this creates a hierarchy of runtime systems, with the CPU itself—or actually its inner workings such as interconnections and microcode—acting as the lowest-level runtime.
A modern aspect of runtime systems is parallel execution behaviors, such as the behaviors exhibited by mutex constructs in pthreads and parallel section constructs in OpenMP. A runtime with such parallel execution behaviors may be modularized according to the proto-runtime approach.
Notable early examples of run-time systems are the interpreters for BASIC and Lisp. The latter also included a garbage collector. Forth is an early example of a language that was designed to be compiled into pseudocode; its run-time system was a virtual machine that interpreted that pseudocode. Another popular, if theoretical, example is Donald Knuth's MIX computer.
In C and later languages that supported dynamic memory allocation, the runtime also included a library that managed the program's memory pool.
In the object-oriented programming languages, the run-time system was often also responsible for dynamic type checking and resolving method references.
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- Andrew W. Appel (May 1989). "A Runtime System" (PDF). Princeton University. Retrieved 2013-12-30.