Peterson's Algorithm for Mutual Exclusion | Set 1 (Basic C implementation)
Last Updated :
13 Jan, 2025
Problem: Given 2 processes i and j, you need to write a program that can guarantee mutual exclusion between the two without any additional hardware support.
Solution: There can be multiple ways to solve this problem, but most of them require additional hardware support. The simplest and the most popular way to do this is by using Peterson's Algorithm for mutual Exclusion. It was developed by Peterson in 1981 though the initial work in this direction was done by Theodorus Jozef Dekker who came up with Dekker's algorithm in 1960, which was later refined by Peterson and came to be known as Peterson's Algorithm.
Basically, Peterson’s algorithm provides guaranteed mutual exclusion by using only the shared memory. It uses two ideas in the algorithm:
- Willingness to acquire lock.
- Turn to acquire lock.
Prerequisite: Multithreading in C
Explanation: The idea is that first a thread expresses its desire to acquire a lock and sets flag[self] = 1 and then gives the other thread a chance to acquire the lock. If the thread desires to acquire the lock, then, it gets the lock and passes the chance to the 1st thread. If it does not desire to get the lock then the while loop breaks and the 1st thread gets the chance.
The below code implementation uses the POSIX threads (pthread), which is common in C programs and lower-level system programming but requires more manual work and typecasting.
C++
#include <iostream>
#include <pthread.h>
using namespace std;
int flag[2];
int turn;
const int MAX = 1e9;
int ans = 0;
void lock_init()
{
flag[0] = flag[1] = 0;
turn = 0;
}
void lock(int self)
{
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] == 1 && turn == 1 - self);
}
void unlock(int self)
{
flag[self] = 0;
}
void* func(void* s)
{
int i = 0;
int self = (int)s;
cout << "Thread Entered: " << self << endl;
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
return nullptr;
}
int main()
{
pthread_t p1, p2;
lock_init();
pthread_create(&p1, nullptr, func, (void*)0);
pthread_create(&p2, nullptr, func, (void*)1);
pthread_join(p1, nullptr);
pthread_join(p2, nullptr);
cout << "Actual Count: " << ans << " | Expected Count: " << MAX * 2 << endl;
return 0;
// mythread.h (A wrapper header file with assert
// statements)
#ifndef __MYTHREADS_h__
#define __MYTHREADS_h__
#include <pthread.h>
#include <assert.h>
#include <sched.h>
void Pthread_mutex_lock(pthread_mutex_t *m)
{
int rc = pthread_mutex_lock(m);
assert(rc == 0);
}
void Pthread_mutex_unlock(pthread_mutex_t *m)
{
int rc = pthread_mutex_unlock(m);
assert(rc == 0);
}
void Pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine)(void*), void *arg)
{
int rc = pthread_create(thread, attr, start_routine, arg);
assert(rc == 0);
}
void Pthread_join(pthread_t thread, void **value_ptr)
{
int rc = pthread_join(thread, value_ptr);
assert(rc == 0);
}
#endif // __MYTHREADS_h__
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PetersonSpinlockThread {
// Shared variables for mutual exclusion
private static int[] flag = new int[2];
private static int turn;
private static final int MAX = (int) 1e9;
private static int ans = 0;
private static Lock mutex = new ReentrantLock();
// Initialize lock variables
private static void lockInit() {
flag[0] = flag[1] = 0;
turn = 0;
}
// Acquire lock
private static void lock(int self) {
flag[self] = 1;
turn = 1 - self;
// Spin until the other thread releases the lock
while (flag[1 - self] == 1 && turn == 1 - self);
}
// Release lock
private static void unlock(int self) {
flag[self] = 0;
}
// Function representing the critical section
private static void func(int self) {
int i = 0;
System.out.println("Thread Entered: " + self);
lock(self); // Acquire the lock
for (i = 0; i < MAX; i++)
ans++;
unlock(self); // Release the lock
}
// Main method
public static void main(String[] args) throws InterruptedException {
// Create two threads
Thread t1 = new Thread(() -> func(0));
Thread t2 = new Thread(() -> func(1));
lockInit(); // Initialize lock variables
t1.start(); // Start thread 1
t2.start(); // Start thread 2
t1.join(); // Wait for thread 1 to finish
t2.join(); // Wait for thread 2 to finish
// Print the final count
System.out.println("Actual Count: " + ans + " | Expected Count: " + MAX * 2);
}
}
import threading
# Shared variables for mutual exclusion
flag = [0, 0]
turn = 0
MAX = int(1e9)
ans = 0
mutex = threading.Lock()
# Initialize lock variables
def lock_init():
global flag, turn
flag = [0, 0]
turn = 0
# Acquire lock
def lock(self):
global flag, turn
flag[self] = 1
turn = 1 - self
# Spin until the other thread releases the lock
while flag[1 - self] == 1 and turn == 1 - self:
pass
# Release lock
def unlock(self):
global flag
flag[self] = 0
# Function representing the critical section
def func(self):
global ans
i = 0
print(f"Thread Entered: {self}")
with mutex:
lock(self) # Acquire the lock
for i in range(MAX):
ans += 1
unlock(self) # Release the lock
# Main method
def main():
# Create two threads
t1 = threading.Thread(target=lambda: func(0))
t2 = threading.Thread(target=lambda: func(1))
lock_init() # Initialize lock variables
t1.start() # Start thread 1
t2.start() # Start thread 2
t1.join() # Wait for thread 1 to finish
t2.join() # Wait for thread 2 to finish
# Print the final count
print(f"Actual Count: {ans} | Expected Count: {MAX * 2}")
if __name__ == "__main__":
main()
const flag = [0, 0];
let turn = 0;
const MAX = 1e9;
let ans = 0;
function lock_init() {
flag[0] = flag[1] = 0;
turn = 0;
}
function lock(self) {
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] === 1 && turn === 1 - self);
}
function unlock(self) {
flag[self] = 0;
}
async function func(self) {
let i = 0;
console.log("Thread Entered:", self);
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
}
async function main() {
lock_init();
const promise1 = func(0);
const promise2 = func(1);
await Promise.all([promise1, promise2]);
console.log("Actual Count:", ans, "| Expected Count:", MAX * 2);
}
main();
//This code is contribuited by Prachi.
Output:
Thread Entered: 1
Thread Entered: 0
Actual Count: 2000000000 | Expected Count: 2000000000
The produced output is 2*109 where 109 is incremented by both threads.
Read more about Peterson's Algorithm for Mutual Exclusion | Set 2