Classes
class	modm::pt::Protothread
	A very lightweight, stackless thread More...

class	modm::pt::Semaphore
	Counting semaphore More...

#define	PT_BEGIN()

#define	PT_END()

#define	PT_YIELD()

#define	PT_WAIT_WHILE(...)

#define	PT_WAIT_UNTIL(...)

#define	PT_WAIT_THREAD(...)

#define	PT_SPAWN(...)

#define	PT_CALL(...)

#define	PT_RESTART()

#define	PT_EXIT()

Detailed Description

lbuild module: modm:processing:protothread

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.

Since they implement some kind of cooperative multi-threading, Protothreads are non-preemptable. Therefore, a context switch can only take place on blocking operations, which means you don't need complex synchronization.

Protothreads are also stackless, so local variables are not preserved across context switches, and must instead become member variables of the modm::Protothread subclass

A protothread runs within a single function (modm::Protothread::run()) and cannot span over other functions. A protothread may call normal 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 protothread concept was developed by Adam Dunkels and Oliver Schmidt: http://dunkels.com/adam/pt

Originally ported to C++ for use by Hamilton Jet (www.hamiltonjet.co.nz) by Ben Hoyt, but stripped down for public release.

Example

#include <modm/processing/protothread.hpp>
 
using Led = GpioB0;
 
class BlinkingLight : public modm::pt::Protothread
{
public:
    bool
    run()
    {
        PT_BEGIN();
 
        // set everything up
        Led::setOutput();
        Led::set();
 
        while (true)
        {
            timeout.restart(100ms);
            Led::set();
            PT_WAIT_UNTIL(timeout.isExpired());
 
            timeout.restart(200ms);
            Led::reset();
            PT_WAIT_UNTIL(timeout.isExpired());
        }
 
        PT_END();
    }
 
private:
    modm::ShortTimeout timeout;
};
// ...
 
BlinkingLight light;
 
while (true) {
    light.run();
}

Using Fibers

Protothreads can be implemented using stackful fibers by setting the use_fiber option, which replaces the preprocessor macros and C++ implementations of this and the modm:processing:resumable module with a fiber version.

Specifically, the PT_* and RF_* macros are now forwarding their arguments unmodified and instead relying on modm::this_fiber::yield() for context switching:

#define PT_YIELD() modm::this_fiber::yield()
#define PT_WAIT_WHILE(cond) while(cond) { modm::this_fiber::yield(); }
#define PT_CALL(func) func

The modm::pt::Protothread class is implemented using modm::Fiber<> with the default stack size MODM_PROTOTHREAD_STACK_SIZE. It automatically runs the two virtual methods bool run() and bool update() if they are defined in the protothread class.

There should be no modification of the existing code necessary with the exception that you must replace the main loop calling all protothreads with the fiber scheduler:

int main()
{
    /*
    while(true)
    {
        protothread1.update();
        protothread2.update();
    }
    */
    modm::fiber::Scheduler::run();
    return 0;
}

Restrictions

If the default stack size is too low, you can set MODM_PROTOTHREAD_STACK_SIZE to a higher value, however, this will apply to all protothreads, consuming a lot more memory. Instead, we recommend refactoring the protothread into a fiber function.

All functions and macros work as expected, except for the Protothread::stop() function, which not implementable using fibers. It is left unimplemented, so that you may define it with the behavior most suitable to your use case.

See the modm:processing:resumable module for additional restrictions when calling resumable functions from a protothread.

Module Options

modm:processing:protothread:use_fiber: Implement via Fibers

Generated with: no in [yes, no]

Macro Definition Documentation

#define PT_BEGIN

Declare start of protothread

Warning: Use at start of the run() implementation!

#define PT_CALL

Calls a given resumable function and returns whether it completed successfully or not.

#define PT_END

Stop protothread and end it

Warning: Use at end of the run() implementation!

#define PT_EXIT

Stop and exit from protothread.

#define PT_RESTART

Reset protothread to start from the beginning

In the next executing cycle the protothread will restart its execution at its PT_BEGIN.

#define PT_SPAWN

Restart and spawn given child protothread and wait until it completes.

#define PT_WAIT_THREAD

Cause protothread to wait until given child protothread completes.

#define PT_WAIT_UNTIL

Cause protothread to wait until given condition is true.

#define PT_WAIT_WHILE

Cause protothread to wait while given condition is true.

#define PT_YIELD

Yield protothread till next call to its run().