Chibi-Scheme is a very small library intended for use as an extension and
scripting language in C programs. In addition to support for lightweight
VM-based threads, each VM itself runs in an isolated heap allowing multiple VMs
to run simultaneously in different OS threads.

The default language is an extended subset of the current draft R7RS Scheme,
with support for all libraries.

The system is designed in optional layers, beginning with a VM based on a small
set of opcodes, a set of primitives implemented in C, a default language, a
module system implementation, and a set of standard modules. You can choose
whichever layer suits your needs best and customize the rest. Adding your own
primitives or wrappers around existing C libraries is easy with the C FFI.

Chibi is known to build and run on 32 and 64-bit Linux, FreeBSD, DragonFly, OS
X, iOS, Windows (under Cygwin) and Plan9.