# Protothread

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A **protothread** is a low-overhead mechanism for [concurrent programming](/source/Concurrent_programming).

Protothreads function as [stackless](/source/Call_stack), lightweight [threads](/source/Thread_(computer_science)), or [coroutines](/source/Coroutines), providing a blocking context cheaply using minimal memory per protothread (on the order of single bytes).

Protothreads are used to accomplish a [non-preempted](/source/Preemption_(computing)) form of [concurrency](/source/Concurrency_(computer_science)) known as [cooperative multitasking](/source/Cooperative_multitasking) and, therefore, do not incur [context switch](/source/Context_switch) when yielding to another thread. Within a protothread, yielding is accomplished by utilizing [Duff's device](/source/Duff's_device) within a thread's function and an external variable used in within the [switch statement](/source/Switch_statement). This allows jumping (resuming) from a yield upon another function call. In order to [block](/source/Blocking_(computing)) threads, these yields may be guarded by a [conditional](/source/Conditional_(computer_programming)) so that successive calls to the same function will yield unless the guard conditional is true.

A feature of protothreads relative to other implementations of [coroutines](/source/Coroutines), or proper threads, is that they are stackless. This has advantages and disadvantages. A disadvantage is that local variables within the protothread cannot be trusted to have retained their values across a yield to another context. They must retain their state through the use of static or external, often [global](/source/Global_variable), variables.[1] An advantage is that they are very lightweight and therefore useful on severely memory constrained systems like small microcontrollers where other solutions are impractical or less desirable.

[Tom Duff](/source/Tom_Duff), of [Duff's device](/source/Duff's_device) fame, had this to say about the shortcomings of the method: "a similar trick for interrupt-driven state machines that is too horrible to go into. [...] I never thought it was an adequate general-purpose coroutine implementation because it’s not easy to have multiple simultaneous activations of a coroutine and it’s not possible using this method to have coroutines give up control anywhere but in their top-level routine. A simple assembly-language stack-switching library lets you do both of those."[2]

The protothread concept was developed by [Adam Dunkels](/source/Adam_Dunkels) and Oliver Schmidt,[3] based on prior work by [Simon Tatham](/source/Simon_Tatham)[4] and [Tom Duff](/source/Tom_Duff).[2]

## See also

- [Coroutine](/source/Coroutine)

- [Microthread](/source/Microthread)

- [Virtual thread](/source/Virtual_thread)

## References

1. **[^](#cite_ref-1)** A. Dunkels, O. Schmidt, T. Voigt, and M. Ali, [*Protothreads: Simplifying Event-Driven Programming of Memory-Constrained Embedded Systems*](http://doi.acm.org/10.1145/1182807.1182811), Proc. ACM SenSys, Boulder, CO, USA, Nov 2006. ([PDF](http://dunkels.com/adam/dunkels06protothreads.pdf), [Presentation slides](http://dunkels.com/adam/dunkels06protothreads.ppt))

1. ^ [***a***](#cite_ref-tatham_duff_2-0) [***b***](#cite_ref-tatham_duff_2-1) ["Brainwagon » Coroutines in C"](https://brainwagon.org/2005/03/05/coroutines-in-c/#comment-1878).

1. **[^](#cite_ref-3)** Adam Dunkels. ["Protothreads - Lightweight, Stackless Threads in C"](http://dunkels.com/adam/pt/). *Dunkels.com*. Retrieved April 21, 2017.

1. **[^](#cite_ref-4)** ["Coroutines in C"](https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html).

## External links

- [Protothread library in C](http://dunkels.com/adam/pt/) used by [Contiki](/source/Contiki)

- [Using Protothreads for Sensor Node Programming](http://dunkels.com/adam/dunkels05using.pdf) A PDF with some in-depth discussion about the use of Protothreads

- [Protothread library](https://code.google.com/p/protothread/) requiring [GCC](/source/GNU_Compiler_Collection), includes a [thread scheduler](/source/Scheduling_(computing)) modeled on the [UNIX kernel](/source/UNIX_kernel) (a simplified form of [POSIX](/source/POSIX_Threads) [condition variables](/source/Monitor_(synchronization)#Condition_variables)). This source base also includes a version that supports multiple CPU cores (processors).

- eigenclass.org performed a comparison of protothreads against POSIX threads, and found a 400x improvement in speed [\[1\]](https://web.archive.org/web/20100725045509/http://eigenclass.org/hiki/threadring-with-protothreads) Retrieved from Archive.org Oct 2014

- [C++ Protothread](https://web.archive.org/web/20140604192611/http://www.manishshakya.com.np/index.php/page/code)

- [Symbian C++ Protothread Implementation](https://web.archive.org/web/20160419190507/http://www.mind-flip.com/theBlog/2007/12/12/protothreads-removing-the-state-from-state-machine/)

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Adapted from the Wikipedia article [Protothread](https://en.wikipedia.org/wiki/Protothread) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Protothread?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.