Protothreads provides linear code execution for event-driven systems implemented in C. Protothreads can be used with or without an RTOS.
Protothreads are a extremely lightweight, stackless type of 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 complex state machines or full multi-threading. Protothreads provides conditional blocking inside C functions.
The advantage of protothreads over a purely event-driven approach is that protothreads provides a sequential code structure that allows for blocking functions. In purely event-driven systems, blocking must be implemented by manually breaking the function into two pieces - one for the piece of code before the blocking call and one for the code after the blocking call. This makes it hard to use control structures such as if() conditionals and while() loops.
The advantage of protothreads over ordinary threads is that a protothread do not require a separate stack. In memory constrained systems, the overhead of allocating multiple stacks can consume large amounts of the available memory. In contrast, each protothread only requires between two and twelve bytes of state, depending on the architecture.
Examples applications:
The protothreads API consists of four basic operations: initialization: PT_INIT(), execution: PT_BEGIN(), conditional blocking: PT_WAIT_UNTIL() and exit: PT_END(). On top of these, two convenience functions are built: reversed condition blocking: PT_WAIT_WHILE() and protothread blocking: PT_WAIT_THREAD().
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 block that which functions the never blocks.
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.
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. 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.
Files | |
| file | pt.h |
| Protothreads implementation. | |
Modules | |
| Local continuations | |
| Local continuations form the basis for implementing protothreads. | |
Data Structures | |
| struct | pt |
Initialization | |
| #define | PT_INIT(pt) |
| Initialize a protothread. | |
Declaration and definition | |
| #define | PT_THREAD(name_args) |
| Declaration of a protothread. | |
| #define | PT_BEGIN(pt) |
| Declare the start of a protothread inside the C function implementing the protothread. | |
| #define | PT_END(pt) |
| Declare the end of a protothread. | |
Blocked wait | |
| #define | PT_WAIT_UNTIL(pt, condition) |
| Block and wait until condition is true. | |
| #define | PT_WAIT_WHILE(pt, cond) |
| Block and wait while condition is true. | |
Hierarchical protothreads | |
| #define | PT_WAIT_THREAD(pt, thread) |
| Block and wait until a child protothread completes. | |
| #define | PT_SPAWN(pt, child, thread) |
| Spawn a child protothread and wait until it exits. | |
Exiting and restarting | |
| #define | PT_RESTART(pt) |
| Restart the protothread. | |
| #define | PT_EXIT(pt) |
| Exit the protothread. | |
Calling a protothread | |
| #define | PT_SCHEDULE(f) |
| Schedule a protothread. | |
Yielding from a protothread | |
| #define | PT_YIELD(pt) |
| Yield from the current protothread. | |
| #define | PT_YIELD_UNTIL(pt, cond) |
| Yield from the protothread until a condition occurs. | |
Defines | |
| #define | PT_WAITING 0 |
| #define | PT_EXITED 1 |
| #define | PT_ENDED 2 |
| #define | PT_YIELDED 3 |
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Declare the start of a protothread inside the C function implementing the protothread. This macro is used to declare the starting point of a protothread. It should be placed at the start of the function in which the protothread runs. All C statements above the PT_BEGIN() invokation will be executed each time the protothread is scheduled.
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Declare the end of a protothread. This macro is used for declaring that a protothread ends. It must always be used together with a matching PT_BEGIN() macro.
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Exit the protothread. This macro causes the protothread to exit. If the protothread was spawned by another protothread, the parent protothread will become unblocked and can continue to run.
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Initialize a protothread. Initializes a protothread. Initialization must be done prior to starting to execute the protothread.
Definition at line 80 of file pt.h. Referenced by httpd_appcall(). |
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Restart the protothread. This macro will block and cause the running protothread to restart its execution at the place of the PT_BEGIN() call.
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Schedule a protothread. This function shedules a protothread. The return value of the function is non-zero if the protothread is running or zero if the protothread has exited.
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Spawn a child protothread and wait until it exits. This macro spawns a child protothread and waits until it exits. The macro can only be used within a protothread.
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Declaration of a protothread. This macro is used to declare a protothread. All protothreads must be declared with this macro.
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Block and wait until a child protothread completes. This macro schedules a child protothread. The current protothread will block until the child protothread completes.
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Block and wait until condition is true. This macro blocks the protothread until the specified condition is true.
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Block and wait while condition is true. This function blocks and waits while condition is true. See PT_WAIT_UNTIL().
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Yield from the current protothread. This function will yield the protothread, thereby allowing other processing to take place in the system.
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Yield from the protothread until a condition occurs.
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1.4.6