Here are some coding examples in Python, Java and C++ on how to use Semaphores and Barriers for concurrency.

• (Counting) Semaphores: allow up to $x$ number of threads to acquire the semaphore before blocking
• Barriers: Will block up to $x - 1$ threads. When $x$ number of threads have hit the barrier, the barrier will open to allow all $x$ threads through

To showcase these in practice, I will use the Leetcode problem of

Essentially, you expose 2 functions

• hydrogen()
• oxygen()

And you only release the $H_2O$ molecule when 2 molecules of hydrogen and oxygen are available.

Python

from threading import Semaphore, Barrier

h = Semaphore(2)
b = Barrier(3)
o = Semaphore(1)

def hydrogen(releaseHydrogen):
    with h:
        b.wait()
        releaseHydrogen()

def oxygen(releaseOxygen)
    with o:
        b.wait()
        releaseOxygen()

Java

import java.util.concurrent.*;

CyclicBarrier b = new CyclicBarrier(3);
Semaphore h = new Semaphore(2);
Semaphore o = new Semaphore(1);

void hydrogen(Runnable releaseHydrogen) throws InterruptedException {
    try {
        h.acquire();
        b.await();
        releaseHydrogen.run();
    } catch(Exception ignore) {}
    finally {
        h.release();
    }
}

void oxygen(Runnable releaseOxygen) throws InterruptedException {
    try {
        o.acquire();
        b.await();
        releaseOxygen.run();
    } catch(Exception ignore) {}
    finally {
        o.release();
    }
}

C++

#include <semaphore.h>
#include <pthread.h>

pthread_barrier_t b;
sem_t h;
sem_t o;

pthread_barrier_init(&b, 0, 3); //Need 3 threads to pass the barrier
sem_init(&h, 0, 2); // Two molecules of hydrogen
sem_init(&o, 0, 1); // One molecule of oxygen

void hydrogen(function<void()> releaseHydrogen) {
    sem_wait(&h); // Two hydrogen threads can arrive at a time
    pthread_barrier_wait(&b); // but wait for one more thread , which must be oxygen
    releaseHydrogen();
    sem_post(&h);
}

void oxygen(function<void()> releaseOxygen) {
    sem_wait(&o);
    pthread_barrier_wait(&b);
    releaseOxygen();
    sem_post(&o);
}