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.

#include "pt.h"
struct pt pt;
struct timer timer;
PT_THREAD(example(struct pt *pt))
  while(1) {
    if(initiate_io()) {
         io_completed() ||
Example protothreads code.

While protothreads originally were created for memory-constrained embedded systems, it has found many uses as a general purpose library too. Examples include multimedia streaming server software, grid computing research software, and MPEG decoding software for Internet TVs.


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

Example applications:

  • Memory constrained systems
  • Event-driven protocol stacks
  • Small embedded systems
  • Sensor network nodes
  • Portable C applications

For example usages, see the Examples page.

The protothreads library is released under an open source BSD-style license that freely allows for both non-commercial and commercial usage. The only requirement is that credit is given. Download the full source code here.

Protothreads were invented by Adam Dunkels with support from Oliver Schmidt <>.