ProtoThreads For Arduino

Protothreads are extremely lightweight stackless threads designed for severely memory constrained systems, such as small embedded systems or wireless sensor network nodes.

Protothreads provide linear code execution for event-driven systems implemented in C. Protothreads can be used with or without an underlying operating system to provide blocking event-handlers.

Protothreads provide sequential flow of control without complex state machines or full multi-threading.

Main features:

Very small RAM overhead - only two bytes per protothread and no extra stacks
Highly portable - the protothreads library is 100% pure C and no architecture specific assembly code
Can be used with or without an OS
Provides blocking wait without full multi-threading or stack-switching
Freely available under a BSD-like open source license