Lisp compiler in Lisp, testing it inside an existing Lisp Interpreter. Once they had improved the compiler to the point where it could compile its own source code, it was self-hosting. This technique is usually only practicable when an interpreter already exists for the very same language that is to be compiled; though possible, it is extremely uncommon to humanly compile a compiler with itself. The concept borrows directly from and is an example of the broader notion of running a program on itself as input, used also in various proofs in theoretical computer science, such as the proof that the halting problem is undecidable. Examples Ken Thompson started development on Unix in 1968 by writing and compiling programs on the GE-635 and carrying them over to the PDP-7 for testing. After the initial Unix kernel, a command interpreter, an editor, an assembler, and a few utilities were completed, the Unix operating system was self-hosting - programs could be written and tested on the PDP-7 itself. Douglas McIlroy wrote TMG (a compiler-compiler) in TMG on a piece of paper and "decided to give his piece of paper to his piece of paper," doing the computation himself, thus compiling a TMG compiler into assembly, which he typed up and assembled on Ken Thompson's PDP-7. Development of the GNU system relies largely on GCC (the GNU C Compiler) and GNU Emacs (a popular editor), making possible the self contained, maintained and sustained development of free software for the GNU Project. Many programming languages have self-hosted implementations: compilers that are both in and for the same language. In some of these cases, the initial implementation was developed using bootstrapping, i.e. using another high-level language, assembler, or even machine language. List of languages having self-hosting compilers The following programming languages have self-hosting compilers: See also Bootstrapping (compilers) Compiler-compiler Cross-compiler Dogfooding Futamura projection Self-interpreter Self-reference Indirect self-modification References Computer programming Self-hosting software