How does semaphore work?

Question

Can the semaphore be lower than 0? I mean, say I have a semaphore with N=3 and I call \"down\" 4 times, then N will remain 0 but one process will be blocked?

And same th

Answer 1

Using java.util.concurrent.Semaphore with methods acquire() and release(), I think permits will always be >=0. Let's say you want to synchronize threads so that only 1 thread can be inside for loop. If sem is type of Semaphore that has initial value 1, this will not work for more than 2 threads.

while(true){            

    sem.wait(); // wait is acquire

    for(int i=0; i<=5; i++){

        try {
            Thread.sleep(250);
        }catch (InterruptedException e) {}

        System.out.println("Thread "+ threadname+ " " + i);

             }
    sem.signal(); // signal is release }

However, you can implement the Semaphore class from java and make your own class that allows this.

package yourpackage;

import java.util.concurrent.Semaphore;

public class SemaphoreLayer {
public Semaphore s=null;
public String name;
private int val;

public SemaphoreLayer(int i){
    s=new Semaphore(i); val=i;
}

public void wait(){
 try {
     val--;
     s.acquire();

  } catch (InterruptedException e) {
    System.out.println("Error signal semaphorelayer");
}}

public void signal(){
    if(val<0){val++;}{
        s.release();
        val++;
    }
}

}

Now val can be negative. However, I am not sure that this is completely safe, because if we have signal from one thread and wait from the other and they try val++ and val-- this can be bad. (chances for this are very small but stil they exist, so if you are coding and you have to be 100% no error, I don't recommend using this code ) In conclusion this is why it is better to use concept of monitors in java and key word synchronized.