Runtime system

A runtime system, also called run-time system, runtime environment or run-time environment, primarily implements portions of an execution model. This is not to be confused with the runtime lifecycle phase of a program, during which the runtime system is in operation. Most languages have some form of runtime system that provides an environment in which programs run. This environment may address a number of issues including the layout of application memory, how the program accesses variables, mechanisms for passing parameters between procedures, interfacing with the operating system, and otherwise. The compiler makes assumptions depending on the specific runtime system to generate correct code. Typically the runtime system will have some responsibility for setting up and managing the stack and heap, and may include features such as garbage collection, threads or other dynamic features built into the language. A runtime system, also called run-time system, runtime environment or run-time environment, primarily implements portions of an execution model. This is not to be confused with the runtime lifecycle phase of a program, during which the runtime system is in operation. Most languages have some form of runtime system that provides an environment in which programs run. This environment may address a number of issues including the layout of application memory, how the program accesses variables, mechanisms for passing parameters between procedures, interfacing with the operating system, and otherwise. The compiler makes assumptions depending on the specific runtime system to generate correct code. Typically the runtime system will have some responsibility for setting up and managing the stack and heap, and may include features such as garbage collection, threads or other dynamic features built into the language. Every programming language specifies an execution model, and many implement at least part of that model in a runtime system. One possible definition of runtime system behavior is, among others, any behavior not directly attributable to the program itself. This definition includes, as part of the runtime system, things such as putting parameters onto the stack before a function call, the behavior of disk I/O, and parallel execution of related behaviors. By this definition, essentially every language has a runtime system, including compiled languages, interpreted languages, and embedded domain-specific languages. Even API invoked stand alone execution models such as Pthreads have a runtime system that is the implementation of execution model's behavior. Most scholarly papers on runtime systems focus on the implementation details of parallel runtime systems. A notable example of a parallel runtime system is that of Cilk, a popular parallel programming model.In addition, the proto-runtime toolkit was created to simplify the creation of parallel runtime systems. In addition to the execution model behavior, 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 not only state values that are accessible during program execution, but also active entities that can be interacted with during program execution like disk drives and people via keyboards. 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. A unique application of a runtime environment (RTE) is within an operating system (OS) that only allows that RTE to run, meaning from boot until power-down the entire OS is dedicated to only the application(s) running within that RTE. Any other code that tries to run or any failures in the application(s) break the RTE which breaks the OS which stops all processing and requires a re-boot. If the boot is from read-only memory, an extremely secure, simple, single-mission system is created. As a simple example of a basic runtime system, 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 part of the source program. For C, the setup of the stack is part of the runtime system, as opposed to part of the semantics of an individual program, because it maintains a global invariant that holds over all executions. This systematic behavior implements the execution model of the language, as opposed to implementing semantics of the particular program text which is directly translated into code that computes results. One way to observe this separation between the semantics of a particular program and the runtime environment is to compile a program into an object file containing all the functions versus compiling an entire program to an executable binary. The object file will only contain assembly code relevant to those functions, while the executable binary will contain additional code used to implement the runtime environment. The object file, on one hand, may be missing information from the runtime environment that will be resolved by linking. On the other hand, the code in the object file still depends on assumptions in the runtime system; for example, a function may read parameters from a particular register or stack location, depending on the calling convention used by the runtime environment.