What are conditional wait and signal in multi-threading?
Explanation: When you want to sleep a thread, condition variable can be used. In C under Linux, there is a function pthread_cond_wait() to wait or sleep.
On the other hand, there is a function pthread_cond_signal() to wake up sleeping or waiting thread.
Threads can wait on a condition variable.
Prerequisite : Multithreading
Syntax of pthread_cond_wait() :
int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex);
Parameter :
cond : condition variable mutex : is mutex lock
Return Value :
On success, 0 is returned ; otherwise, an error number shall be returned to indicate the error.
The pthread_cond_wait() release a lock specified by mutex and wait on condition cond variable.
Syntax of pthread_cond_signal() :
int pthread_cond_signal(pthread_cond_t *cond);
Parameter :
cond : condition variable
Return Value :
On success, 0 is returned ; otherwise, an error number shall be returned to indicate the error.
The pthread_cond_signal() wake up threads waiting for the condition variable.
Note : The above two functions works together.
Below is the implementation of condition, wait and signal functions.
// C program to implement cond(), signal() // and wait() functions #include <pthread.h> #include <stdio.h> #include <unistd.h> // Declaration of thread condition variable pthread_cond_t cond1 = PTHREAD_COND_INITIALIZER; // declaring mutex pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; int done = 1;
// Thread function void * foo()
{ // acquire a lock
pthread_mutex_lock(&lock);
if (done == 1) {
// let's wait on condition variable cond1
done = 2;
printf ( "Waiting on condition variable cond1\n" );
pthread_cond_wait(&cond1, &lock);
}
else {
// Let's signal condition variable cond1
printf ( "Signaling condition variable cond1\n" );
pthread_cond_signal(&cond1);
}
// release lock
pthread_mutex_unlock(&lock);
printf ( "Returning thread\n" );
return NULL;
} // Driver code int main()
{ pthread_t tid1, tid2;
// Create thread 1
pthread_create(&tid1, NULL, foo, NULL);
// sleep for 1 sec so that thread 1
// would get a chance to run first
sleep(1);
// Create thread 2
pthread_create(&tid2, NULL, foo, NULL);
// wait for the completion of thread 2
pthread_join(tid2, NULL);
return 0;
} |
Output:
Time Complexity:
The time complexity of this program is O(1), as the program only requires one iteration of each thread.
Space Complexity:
The space complexity of this program is O(1), as the program only requires a few variables to be stored in memory.