Protothread

Article summary

A protothread is a low-overhead mechanism for concurrent programming. Protothreads function as stackless, lightweight threads, or 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 form of concurrency known as cooperative multitasking and, therefore, do not incur context switch when yielding to another thread. Within a protothread, yielding is accomplished by utilizing Duff's device within a thread's function and an external variable used in within the switch statement. This allows jumping (resuming) from a yield upon another function call. In order to block threads, these yields may be guarded by a conditional 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, 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, variables.…

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