This is a header-only stackless coroutine implementation in standard C99 and C++11. coroutine.h contains a C adaptation of the C++ stackless coroutine from Boost.Asio by Christopher M. Kohlhoff. This is itself a variant of Simon Tatham's Coroutines in C, which was inspired by Duff's device. The API is designed to be a more powerfull version of Protothreads with a more natural syntax.

The implementation (ab)uses the switch statement. It is therefore not possible to yield from a coroutine from within a nested switch statement.

Since the implementation is stackless, variables local to the coroutine are not stored between invocations. In C++, this drawback can be partially mitigated by implementing the coroutine as a function object (see coroutine.hpp) and making all local variables (private) data members.

• co_reenter(ctx): Defines the body of a stackless coroutine. When the body is executed at runtime, control jumps to the ___location immediately following the last co_yield or co_fork statement.

  Note that a function MUST NOT contain multiple co_reenter expressions with the same context.

• co_yield <expression>: Stores the context of the current stackless coroutine, evaluates the expression following the co_yield keyword, if any, and exits the scope of the #co_reenter statement. co_yield break terminates the coroutine. co_yield continue is equivalent to co_yield.

  A co_yield expression is valid only within a co_reenter statement. Since co_reenter is implemented using a switch statement, co_yield CANNOT be used from within a nested switch statement.

• co_fork <expression>: "Forks" a coroutine and executes the expression following the co_fork keyword as a child. This expression will typically create a copy of the coroutine context. After the expression completes, the coroutine continues and as a parent. If the coroutine is reentered with (the copy of) the context created by co_fork, the coroutine continues as a child until the next co_yield statement.

  See coroutine.h for an example of co_fork.

#include "coroutine.h"

void my_coroutine(co_ctx_t* ctx) {
        ...
        // statements executed on every invocation of my_coroutine()
        ...
        co_reenter (ctx) {
                assert(!co_is_ready(ctx));
                ...
                // statements executed on the first invocation of
                // my_coroutine()
                ...
                // Store the context and exit the scope of the co_reenter
                // statement.
                co_yield;
                ...
                // statements executed on the second invocation of
                // my_coroutine()
                ...
                co_yield;
                ...
                // statements executed on the third invocation of
                // my_coroutine()
                ...
        }
        ...
        // statements executed on every invocation of my_coroutine() (unless the
        // function returns early)
        ...
}

#include "coroutine.hpp"

class MyCoroutine : public Coroutine {
 public:
  void operator()() {
    ...
    // statements executed on every invocation
    ...
    co_reenter (this) {
      ...
      // statements executed on the first invocation
      ...
      // Store the context and exit the scope of the co_reenter statement.
      co_yield;
      ...
      // statements executed on the second invocation
      ...
      co_yield;
      ...
      // statements executed on the third invocation
      ...
    }
    ...
    // statements executed on every invocation (unless this function returns
    // early)
    ...
  }
};