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# Peterson’s Algorithm for Mutual Exclusion | Set 1 (Basic C implementation)

• Difficulty Level : Medium
• Last Updated : 03 Mar, 2022

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:

1. Willingness to acquire lock.
2. Turn to acquire lock.

#### 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.

Implementation in C language

## C

 `// Filename: peterson_spinlock.c``// Use below command to compile:``// gcc -pthread peterson_spinlock.c -o peterson_spinlock` `#include ``#include ``#include"mythreads.h"` `int` `flag;``int` `turn;``const` `int` `MAX = 1e9;``int` `ans = 0;` `void` `lock_init()``{``    ``// Initialize lock by resetting the desire of``    ``// both the threads to acquire the locks.``    ``// And, giving turn to one of them.``    ``flag = flag = 0;``    ``turn = 0;``}` `// Executed before entering critical section``void` `lock(``int` `self)``{``    ``// Set flag[self] = 1 saying you want to acquire lock``    ``flag[self] = 1;` `    ``// But, first give the other thread the chance to``    ``// acquire lock``    ``turn = 1-self;` `    ``// Wait until the other thread looses the desire``    ``// to acquire lock or it is your turn to get the lock.``    ``while` `(flag[1-self]==1 && turn==1-self) ;``}` `// Executed after leaving critical section``void` `unlock(``int` `self)``{``    ``// You do not desire to acquire lock in future.``    ``// This will allow the other thread to acquire``    ``// the lock.``    ``flag[self] = 0;``}` `// A Sample function run by two threads created``// in main()``void``* func(``void` `*s)``{``    ``int` `i = 0;``    ``int` `self = (``int` `*)s;``    ``printf``(``"Thread Entered: %d\n"``, self);` `    ``lock(self);` `    ``// Critical section (Only one thread``    ``// can enter here at a time)``    ``for` `(i=0; i

## C

 `// mythread.h (A wrapper header file with assert``// statements)``#ifndef __MYTHREADS_h__``#define __MYTHREADS_h__` `#include ``#include ``#include ` `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__`

Output:

```Thread Entered: 1