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An upwaker is something inbetween semaphores and conditionals.
Similarly to semaphores, it guarantees upwaker_sleep() to wake up, even
if the upwaker isn't currently sleeping (the next call to upwaker_sleep() will
not actually go to sleep).
And similarly to conditionals, it will wake the sleeper up _only_ one time
when upwaker_wake_up() is called several times in a row, where
upwaker_sleep() hasn't got a chance to wake up inbetween those calls.
This implementation only supports one thread calling upwaker_sleep() and
one thread calling upwaker_wake_up(). Using several sleepers and upwakers on
the same upwaker_t variable has not been tested or given any thought about
whether it would work.
#include <pthread.h>
#include "atomic.h"
typedef struct {
DEFINE_ATOMIC(bool, please_wakeup);
pthread_mutex_t mutex;
pthread_cond_t cond;
} upwaker_t;
upwaker_t *create_upwaker(void);
void upwaker_sleep(upwaker_t *upwaker);
void upwaker_wake_up(upwaker_t *upwaker);