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is it possible to use only two semaphores to synchronise three or more threads?


is it possible to use only two semaphores to synchronise three or more threads?

By : Sonjoy Kumar Paul
Date : October 25 2020, 07:29 PM
hop of those help? A semaphore is used to protect a shared resource. You need as many semaphores as shared resources, this is not linked to how many threads access these resources.
code :


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Can't synchronise threads in Java (using semaphores)

Can't synchronise threads in Java (using semaphores)


By : Arun Kumar
Date : March 29 2020, 07:55 AM
With these it helps Based on your code and output, it is possible that Thread 3 released semaphore 3 so Thread 2 that was waiting for it - acquired it and printed the message before Thread 3 completed.
I noticed that you didn't synchronized release method.
code :
synchronized(this) {
    available.get(id).acquire();                                            
    available.get(id+1).acquire();                                         
}

synchronized(this) {
   available.get(id).release();                                            
   available.get(id+1).release(); 
}
Synchronise ArrayList over two threads

Synchronise ArrayList over two threads


By : Murugan p
Date : March 29 2020, 07:55 AM
I wish this help you This is something called a producer-consumer task. You can do it with arraylist, but it's honestly just not the right way to approach this problem.
Luckily, Java provides us with some collections, the BlockingQueue collections, which are designed specifically for this reason;
code :
//the collection with the stuff in it
static BlockingQueue<Object> items = new BlockingQueue<Object>();
//(there are a few different types of blocking queues, check javadocs.
//you would want Linked or Array blocking queue

//what happens on the reader thread
public void producer()
{
    //read the data into the collection
    for (all the data in the file)
    {
       //add the next item
       items.put(/* next item from file or w/e */);

       //stop if necessary
       if (atEndOfFile) stillReadingData = false;

       //etc
    }
}
//what happens on the other threads
public void consumer()
{


   //keep this thread alive so long as there is data to process
   //or so long as there might be more data to process
   while (stillReadingData || !items.isEmpty())
   {
       //get the next item from the list
       //while the list is empty, we basically sleep for "timeout" timeunits,
       //then the while-loop would repeat, and so on
       Object o = items.poll(long timeout, int units);
       if (o != null) //process it
   }
}
About threads and condition variable and synchronise

About threads and condition variable and synchronise


By : Γιωργος Παλαιολογοπο
Date : March 29 2020, 07:55 AM
it fixes the issue Here you go. Here I am using 3 mutexs and 3 condition variables. With the below examples you can schedule or control any number of threads in C. First look at the first thread below. Here it locked mutex lock1 (so that other thread could not access the codes) starts executing (codes not added just comments) and finally after completing its task waiting on cond1, likewise second thread locked mutex lock2, starts executing its business logic and finally waits on condition cond2 and 3rd thread locked mutex lock3, starts executing its business logic and finally waits on condition cond3. I am not adding any business logic here because this is just an example. In the commented section you can add your business logic which will execute in parallel mode. Suppose thread3 depends on final output of thread1 which is going to be inserted in a table and thread3 will read that information before creating it final result and thread2 depends on final outcome of thread3 to generate its final outcome. Hence thread1 after inserting the data into table, signals thread3 through condition variable to go ahead with its final process. That means thread1 controls thread3. As thread2 depends on final outcome from thread3, hence thread3 controls the execution of Thread2. Here we can allow thread1 to execute independently as its operation does not depends on any other thread, but for example of thread control we are controlling all the threads here and hence thread1 is being controlled from thread2.
To start the controlling process, we are releasing thread1 first. In the main thread (i.e. main function, every program has one main thread, in C/C++ this main thread is created automatically by operating system once the control pass to the main method/function by kernel) we are calling pthread_cond_signal(&cond1); Once this function called from main thread, thread1 which was waiting on cond1 will be released and it will start executing further. Once it finishes with its final task, it will call pthread_cond_signal(&cond3); now thread which was waiting on condition cond3 i.e. thread3 will be released and it will start to execute it’s final stage and will call pthread_cond_signal(&cond2); and it will release the thread which is waiting on condition cond2 i.e. in this case thread2. This is the way we can schedule and control execution of thread in multi-threaded environment.
code :
#include<pthread.h>

pthread_cond_t cond1 = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond2 = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond3 = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t lock2 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t lock3 = PTHREAD_MUTEX_INITIALIZER;

int TRUE = 1;

void * threadMethod1(void *arg)
{
  printf("In thread1\n");
  do{
    pthread_mutex_lock(&lock1);
    //Add your business logic(parallel execution codes)  here
    pthread_cond_wait(&cond1, &lock1);
    printf("I am thread1  generating the final report and inserting into a table \n");
    pthread_cond_signal(&cond3);/* Now allow 3rd thread to process */
    pthread_mutex_unlock(&lock1);
  }while(TRUE);
  pthread_exit(NULL);
}

void * threadMethod2(void *arg)
{
  printf("In thread2\n");
  do
  {
    pthread_mutex_lock(&lock2);
    //Add your business logic(parallel execution codes)  here
    pthread_cond_wait(&cond2, &lock2);
    printf("I am thread2  generating the final report and inserting into a table \n");
    pthread_cond_signal(&cond1);
    pthread_mutex_unlock(&lock2);
  }while(TRUE);
  pthread_exit(NULL);
}

void * threadMethod3(void *arg)
{
  printf("In thread3\n");
  do
  {
    pthread_mutex_lock(&lock3);
    //Add your business logic(parallel execution codes)  here
    pthread_cond_wait(&cond3, &lock3);
    printf("I am thread3  generating the final report and inserting into a table \n");
    pthread_cond_signal(&cond2);
    pthread_mutex_unlock(&lock3);
  }while(TRUE);
  pthread_exit(NULL);
}

int main(void)
{
  pthread_t tid1, tid2, tid3;
  int i = 0;

  printf("Before creating the threads\n");
  if( pthread_create(&tid1, NULL, threadMethod1, NULL) != 0 )
        printf("Failed to create thread1\n");
  if( pthread_create(&tid2, NULL, threadMethod2, NULL) != 0 )
        printf("Failed to create thread2\n");
  if( pthread_create(&tid3, NULL, threadMethod3, NULL) != 0 )
        printf("Failed to create thread3\n");
  pthread_cond_signal(&cond1);/* Now allow first thread to process first */


  sleep(1);
  TRUE = 0;/* Stop all the thread */
  sleep(3);

 /* this is how we join thread before exit from a system */
  /*  
  pthread_join(tid1,NULL);
  pthread_join(tid2,NULL);
  pthread_join(tid3,NULL);*/

 exit(0);
}
Synchronise two threads passing events between each other

Synchronise two threads passing events between each other


By : Ольга Павлюк
Date : March 29 2020, 07:55 AM
With these it helps The problem is the question you are asking really doesn't make sense. Multiple threads are designed to run at the same time and you're trying to play a game of pass the buck from one thread to another to get sequential serialised behavoir. Its like taking a really complicated tool and ask how it solves what is normally a really easy question.
However, multithreading is a really important topic to learn so I'll try to answer what you need to the best of my ability.
How can i synchronise two threads on Linux C?

How can i synchronise two threads on Linux C?


By : red ranger
Date : March 29 2020, 07:55 AM
may help you . aside from the obvious "why use two threads if one is always waiting anyway?" what you need to do is use some kind of "state" variable with the mutex:
code :
// start here
#define STATE_INITIAL 0
// go here when step 1 finishes
#define STATE_STEP1 1
// go here when step 2 finishes
#define STATE_STEP2 2
pthread_mutex_t my_mutex;
int cur_state;

void wait_my_turn(int desired_state) {
    pthread_mutex_lock(&my_mutex);
    if (cur_state == desired_state) return;
    pthread_mutex_unload(&my_mutex);
}

void finish_turn() {
    ++cur_state;
    if (cur_state == 3) cur_state = 1;
    pthread_mutex_unlock(&my_mutex);
}

// in main, initialize mutex, lock it, and set cur_state to STATE_INITIAL
// until you are ready for threads to start.  Then you need to set it
// to STATE_STEP1 to allow that thread to begin.
// in your threads, begin with wait_my_turn(STATE_STEPn)
// and call finish_turn() when done
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