Contiki is an open source, highly portable, networked, multi-tasking
operating system for memory-constrained systems.

Contiki runs on a variety of tiny systems ranging from embedded 8-bit
microcontrollers to old homecomputers such the Commodore 64. Code
footprint is on the order of kilobytes and memory usage can be
configured to be as low as tens of bytes.

Contiki provides a simple event-driven kernel with per-process
optional preemptive multi-threading, interprocess communication using
message passing through events, a dynamic process structure with
support for loading and unloading programs, native TCP/IP support
using the uIP TCP/IP stack, and a GUI subsystem with either direct
graphic support for locally connected terminals or networked virtual
display with VNC or over Telnet.

The Contiki source code is split into the following directories:

  apps/ - Applications
  conf/ - Example configuration files
  ctk/  - CTK, the Contiki GUI toolkit
  doc/  - Files for building documentation from the sources
  ek/   - Event kernel, multitasking, protothreads
  lib/  - Libraries 
  uip/  - The uIP TCP/IP stack

To build a Contiki system, you also need a Contiki port. This contains
all the architecture specific files needed to build an actual
system. For most ports, the build procedure is simple: place the
port's directory in the same directory as the contiki/ directory and
run "make" (or "gmake" under FreeBSD) in the port's directory.