source: tests/concurrent/park/force_preempt.cfa@ cca034e

#include <thread.hfa>
#include <fstream.hfa>

volatile int global = 0;

thread Poller {};
void main(Poller & this) {
        while(true) {
                waitfor( ^?{} : this ) {
                        break;
                } or else {
                        global = (global + 1) % 10;
                        yield();
                }
        }
}

thread Waiter {};

volatile int count = 0;

void main(Waiter & this) {
        // Get a unique id
        int id = __atomic_fetch_add(&count, 1, __ATOMIC_SEQ_CST);
        int id_hash = id | (id << 8) | (id << 16) | (id << 24);
        int mask = 0xCAFEBABA;

        for(int i = 0; i < 5; i++) {
                assert(mask == 0xCAFEBABA);

                // Unpark this thread, don't force a yield
                unpark( this );
                assert(mask == 0xCAFEBABA);

                // Hash the mask to make sure no one else messes with them
                mask ^= id_hash;
                assert(mask == (id_hash ^ 0xCAFEBABA));

                // Force a preemption before the call to park
                int prev = global;
                while(prev == global) {}

                // Park this thread,
                assert(mask == (id_hash ^ 0xCAFEBABA));
                park();
                assert(mask == (id_hash ^ 0xCAFEBABA));

                // Reset the hash and recheck it
                mask ^= id_hash;
                assert(mask == 0xCAFEBABA);
        }
}

int main() {
        Poller p;
        {
                Waiter waiters[5];
        }
        printf( "done\n" );                             // non-empty .expect file
}