Embedded scripting is the practice of building a small interpreter into a larger application so that the application can be controlled and extended in a high-level language. Instead of shipping a standalone scripting tool, the host program, typically written in C or C++, links the interpreter as a library and exposes its own functions to scripts. Users and developers then customize behavior by writing script code that the host runs, with no need to touch or recompile the underlying program.
The Lua Reference Manual describes the mechanism precisely in its section “The Application Program Interface,” which defines the C API a host uses to embed Lua. The host creates a Lua state, pushes and pops values across a shared stack to move data between C and Lua, and registers its own C functions so that scripts can call into the application. This thin, stack-based boundary is what lets a compiled program and an interpreted script cooperate while keeping each side simple.
Tcl was designed around the same idea from the very beginning. In his history of the language, John Ousterhout calls the embeddability of Tcl “one of the most unique aspects of Tcl,” explaining that he set out to build an embeddable command language that any interactive tool could link and reuse rather than reinventing a command syntax for each program. The host registers application-specific commands, and Tcl provides the surrounding language for free.
The benefits are practical. An embedded interpreter gives an application a configuration language, a plug-in system, and a user-automation interface all at once, while keeping the performance-critical core in a fast compiled language. Because the script layer is sandboxed inside the host’s own API, the application controls exactly what scripts are allowed to touch.
This pattern is everywhere in modern software. Game engines embed Lua so designers can script gameplay and players can write mods, servers such as Redis and nginx embed Lua to let operators add custom logic, and countless tools embed Tcl or a JavaScript engine for the same reasons. Embedded scripting is the technical foundation that makes a program extensible, and it is the close cousin of the glue-language idea, since the embedded interpreter is precisely the glue that binds a flexible script layer to a rigid compiled core.