If a thread blocks on pthread_cond_timedwait, and other
threads block on pthread_cond_wait and the first thread
times out, a pthread_cond_signal will fail to wake any
thread.

Here is a piece of sample code:

/**********************************************************/

#include <stdio.h>
#include <errno.h>
#include <pthread.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

char buf[512];

int timeout = 1;

void *transient_worker (void *arg)
{
   struct timespec tspec;

   pthread_mutex_lock(&mutex);

   while (1) {
      tspec.tv_sec = time(NULL) + timeout;
      tspec.tv_nsec = 0;

      if (pthread_cond_timedwait(&cond, &mutex, &tspec) ==
ETIMEDOUT)
         break;

      printf("transient worker: %s\n", buf);
   }

   pthread_mutex_unlock(&mutex);
}

void *worker (void *arg)
{
   pthread_mutex_lock(&mutex);

   while (1) {
      pthread_cond_wait(&cond, &mutex);

      printf("worker: %s\n", buf);
   }
}

int main(int argc, char *argv[])
{
   pthread_t tid;

   if (argc > 1)
      timeout = atoi(argv[1]);

   pthread_create(&tid, NULL, transient_worker, NULL);
   pthread_create(&tid, NULL, worker, NULL);

   while (1) {

      scanf("%s", buf);

      pthread_cond_signal(&cond);

   }
}

/**********************************************************/

Ignore the synchronization of "buf" - it's not really an
issue for this example.

If you compile this code and run it with no arguments and
then type a string, nothing is printed out because the
transient worker has exited. However, if you run it with an
argument of say, 20, and then type in a string it then works
fine. Then if you wait 20 seconds before typing in anything
else, the transient thread will die and then it will work
correctly with the one remaining thread. If you type
something in before the 20 seconds is up, then the transient
thread becomes the first thread waiting on the condition
variable and when it dies the program fails...