c - Condition variables and mutex_unlock -

code:

void *inc_func(void *arg) {     pthread_mutex_lock(&mutex);     pthread_cond_signal(&count_threshold_cv);     sleep(1);     pthread_mutex_unlock(&mutex); } void *watch(void *arg) {     pthread_mutex_lock(&mutex);     pthread_cond_wait(&count_threshold_cv,&mutex);     sleep(1);     pthread_mutex_unlock(&mutex); } int main() {     pthread_t id[2];     pthread_create(&id[0],null,watch,null);     pthread_create(&id[1],null,inc_func,null);     int i;     for(i=0;i<2;i++)         pthread_join(id[i],null); } 

now have 1 mutex_unlock function executed in each thread. , 1 locked mutex. why doesn't lead undefined behaviour. since both threads try unlock same mutex leads 1 trying unlock unlocked mutex.

edit: pthread_cond_wait releases mutex second thread use. consider second thread executes pthread_cond_signal leads first thread reacquire mutex. have 2 threads having same mutex lock,because both didn't execute mutex_unlock because of 'sleep' function. understanding wrong?

• pthread_mutex_lock() blocks if mutex locked locked. returns if mutex got unlocked.

• pthread_cond_wait() unlocks mutex when starting wait , locks before returning. returning might delayed if mutex in question still locked. returning delay until mutex got unlocked.

putting above , applying code show 1 sees each thread function nicely starts locking followed unlocking (and on), fine.

referring example code: pthread_cond_wait() returns when inc_func() had called pthread_mutex_unlock().

---

to handle scenario described example code need consider 2 special cases

1. the case of signal coming first and
2. the case of called "spurious wake-ups", pthread_cond_wait() returning without having been signalled.

to handle both such case each condition should have watch variable.

pthread_mutex_t mutex = ... pthread_cond_t count_threshold_cv = ...  int signalled = 0;  void *inc_func(void *arg)    {   pthread_mutex_lock(&mutex);    pthread_cond_signal(&count_threshold_cv);   signalled = 1;    pthread_mutex_unlock(&mutex);   }  void *watch(void *arg) {   pthread_mutex_lock(&mutex);    while (0 == signalled)   {     pthread_cond_wait(&count_threshold_cv,&mutex);   }    pthread_mutex_unlock(&mutex); }  int main(void) {   pthread_t id[2];   pthread_create(&id[0],null,watch,null);   pthread_create(&id[1],null,inc_func,null);   int i;   for(i=0;i<2;i++)     pthread_join(id[i],null); }