C pthread: How to wake it up after some time? - c

Question

I would like to wake up a pthread from another pthread - but after some time. I know signal or pthread_signal with pthread_cond_wait can be used to wake another thread, but I can't see a way to schedule this. The situation would be something like:
THREAD 1:
========
while(1)
recv(low priority msg);
dump msg to buffer
THREAD 2:
========
while(1)
recv(high priority msg);
..do a little bit of processing with msg ..
dump msg to buffer
wake(THREAD3, 5-seconds-later); <-- **HOW TO DO THIS? **
//let some msgs collect for at least a 5 sec window.
//i.e.,Don't wake thread3 immediately for every msg rcvd.
THREAD 3:
=========
while(1)
do some stuff ..
Process all msgs in buffer
sleep(60 seconds).
Any simple way to schedule a wakeup (short of creating a 4th thread that wakes up every second and decides if there is a scheduled entry for thread-3 to wakeup). I really don't want to wakeup thread-3 frequently if there are only low priority msgs in queue. Also, since the messages come in bursts (say 1000 high priority messages in a single burst), I don't want to wake up thread-3 for every single message. It really slows things down (as there is a bunch of other processing stuff it does every time it wakes up).
I am using an ubuntu pc.

Answer 1

How about the use of the pthread_cond_t object available through the pthread API ?
You could share such an object within your threads and let them act on it appropriately.
The resulting code should look like this :
/*
* I lazily chose to make it global.
* You could dynamically allocate the memory for it
* And share the pointer between your threads in
* A data structure through the argument pointer
*/
pthread_cond_t cond_var;
pthread_mutex_t cond_mutex;
int wake_up = 0;
/* To call before creating your threads: */
int err;
if (0 != (err = pthread_cond_init(&cond_var, NULL))) {
/* An error occurred, handle it nicely */
}
if (0 != (err = pthread_mutex_init(&cond_mutex, NULL))) {
/* Error ! */
}
/*****************************************/
/* Within your threads */
void *thread_one(void *arg)
{
int err = 0;
/* Remember you can embed the cond_var
* and the cond_mutex in
* Whatever you get from arg pointer */
/* Some work */
/* Argh ! I want to wake up thread 3 */
pthread_mutex_lock(&cond_mutex);
wake_up = 1; // Tell thread 3 a wake_up rq has been done
pthread_mutex_unlock(&cond_mutex);
if (0 != (err = pthread_cond_broadcast(&cond_var))) {
/* Oops ... Error :S */
} else {
/* Thread 3 should be alright now ! */
}
/* Some work */
pthread_exit(NULL);
return NULL;
}
void *thread_three(void *arg)
{
int err;
/* Some work */
/* Oh, I need to sleep for a while ...
* I'll wait for thread_one to wake me up. */
pthread_mutex_lock(&cond_mutex);
while (!wake_up) {
err = pthread_cond_wait(&cond_var, &cond_mutex);
pthread_mutex_unlock(&cond_mutex);
if (!err || ETIMEDOUT == err) {
/* Woken up or time out */
} else {
/* Oops : error */
/* We might have to break the loop */
}
/* We lock the mutex again before the test */
pthread_mutex_lock(&cond_mutex);
}
/* Since we have acknowledged the wake_up rq
* We set "wake_up" to 0. */
wake_up = 0;
pthread_mutex_unlock(&cond_mutex);
/* Some work */
pthread_exit(NULL);
return NULL;
}
If you want your thread 3 to exit the blocking call to pthread_cond_wait() after a timeout, consider using pthread_cond_timedwait() instead (read the man carefully, the timeout value you supply is the ABSOLUTE time, not the amount of time you don't want to exceed).
If the timeout expires, pthread_cond_timedwait() will return an ETIMEDOUT error.
EDIT : I skipped error checking in the lock / unlock calls, don't forget to handle this potential issue !
EDIT² : I reviewed the code a little bit

Answer 2

You can have the woken thread do the wait itself. In the waking thread:
pthread_mutex_lock(&lock);
if (!wakeup_scheduled) {
wakeup_scheduled = 1;
wakeup_time = time() + 5;
pthread_cond_signal(&cond);
}
pthread_mutex_unlock(&lock);
In the waiting thread:
pthread_mutex_lock(&lock);
while (!wakeup_scheduled)
pthread_cond_wait(&cond, &lock);
pthread_mutex_unlock(&lock);
sleep_until(wakeup_time);
pthread_mutex_lock(&lock);
wakeup_scheduled = 0;
pthread_mutex_unlock(&lock);

Answer 3

Why not just compare the current time to one save earlier?
time_t last_uncond_wakeup = time(NULL);
time_t last_recv = 0;
while (1)
{
if (recv())
{
// Do things
last_recv = time(NULL);
}
// Possible other things
time_t now = time(NULL);
if ((last_recv != 0 && now - last_recv > 5) ||
(now - last_uncond_wakeup > 60))
{
wake(thread3);
last_uncond_wakeup = now;
last_recv = 0;
}
}