Runtime

Runtime, or run time, in computing denotes the period during which a program is executing, from launch to termination. It also refers to the execution environment that supports a program’s operation. This usage is distinct from compile time, which is when source code is translated into executable form.

A program’s runtime consists of the runtime environment and, for many languages, a runtime system. The runtime environment provides access to libraries, services, and resources the program needs at execution. The runtime system manages core facilities such as memory allocation, task scheduling, input/output, and error handling.

Performance and behavior at runtime depend on the implementation. Runtime metrics commonly include elapsed wall-clock time and CPU time. Features such as dynamic memory management (including garbage collection), just-in-time compilation, dynamic linking, and sandboxing can affect startup time, throughput, and latency.

Examples of runtime infrastructures include the Java Virtual Machine and the .NET Common Language Runtime, which provide managed environments. Other languages run on interpreters or virtual machines, such as Python, Ruby, or JavaScript, or on server-side runtimes like Node.js. Some languages also support ahead-of-time compilation alongside a runtime.

In language design, the term runtime can refer to the actual environment provided by a platform, or to a specific subsystem that implements language features at execution. Runtimes enable features such as reflection, dynamic loading of modules, and cross-language interoperability, while also defining security and resource boundaries.