"Many of us use the switch/case construct to explicitly implement concurrent state machines in our code. The [protothread] macros merely provide a level of abstraction above that so that the code appears more linear and the overall logic more visible." Dan Henry summarizes how protothreads work and how they are used in a discussion about protothreads in the 8052 message board at www.8052.com.
Protothreads are a extremely lightweight, stackless threads that provides a blocking context on top of an event-driven system, without the overhead of per-thread stacks. The purpose of protothreads is to implement sequential flow of control without using complex state machines or full multi-threading. Protothreads provides conditional blocking inside a C function.
In memory constrained systems, such as deeply embedded systems, traditional multi-threading may have a too large memory overhead. In traditional multi-threading, each thread requires its own stack, that typically is over-provisioned. The stacks may use large parts of the available memory.
The main advantage of protothreads over ordinary threads is that protothreads are very lightweight: a protothread does not require its own stack. Rather, all protothreads run on the same stack and context switching is done by stack rewinding. This is advantageous in memory constrained systems, where a stack for a thread might use a large part of the available memory. A protothread only requires only two bytes of memory per protothread. Moreover, protothreads are implemented in pure C and do not require any machine-specific assembler code.
A protothread runs within a single C function and cannot span over other functions. A protothread may call normal C functions, but cannot block inside a called function. Blocking inside nested function calls is instead made by spawning a separate protothread for each potentially blocking function. The advantage of this approach is that blocking is explicit: the programmer knows exactly which functions that may block that which functions that are not able block.
Protothreads are similar to asymmetric co-routines. The main difference is that co-routines uses a separate stack for each co-routine, whereas protothreads are stackless. The most similar mechanism to protothreads are Python generators. These are also stackless constructs, but have a different purpose. Protothreads provides blocking contexts inside a C function, whereas Python generators provide multiple exit points from a generator function.
Because protothreads do not save the stack context across a blocking call, local variables are not preserved when the protothread blocks. This means that local variables should be used with utmost care - if in doubt, do not use local variables inside a protothread!
A protothread is driven by repeated calls to the function in which the protothread is running. Each time the function is called, the protothread will run until it blocks or exits. Thus the scheduling of protothreads is done by the application that uses protothreads.
Protothreads are implemented using local continuations. A local continuation represents the current state of execution at a particular place in the program, but does not provide any call history or local variables. A local continuation can be set in a specific function to capture the state of the function. After a local continuation has been set can be resumed in order to restore the state of the function at the point where the local continuation was set.
Local continuations can be implemented in a variety of ways:
The first way works by saving and restoring the processor state, except for stack pointers, and requires between 16 and 32 bytes of memory per protothread. The exact amount of memory required depends on the architecture.
The standard C implementation requires only two bytes of state per protothread and utilizes the C switch() statement in a non-obvious way that is similar to Duff's device and heavily inspired from Simon Tatham's Coroutines in C. This implementation does, however, impose a slight restriction to the code that uses protothreads in that the code cannot use switch() statements itself.
Certain compilers has C extensions that can be used to implement protothreads. GCC supports label pointers that can be used for this purpose. With this implementation, protothreads require 4 bytes of RAM per protothread.
The protothreads library was written by Adam Dunkels <adam@sics.se> with support from Oliver Schmidt <ol.sc@web.de>.