/* Program for process synchronization using locks
Program create two threads: one to increment the value of a shared variable and second to decrement the value of shared variable. Both the threads make use of locks so that only one of the threads is executing in its critical section */
#include<pthread.h>
#include<stdio.h>
#include<unistd.h>
void *fun1();
void *fun2();
int shared=1; //shared variable
pthread_mutex_t l; //mutex lock
int main()
{
pthread_mutex_init(&l, NULL); //initializing mutex locks
pthread_t thread1, thread2;
pthread_create(&thread1, NULL, fun1, NULL);
pthread_create(&thread2, NULL, fun2, NULL);
pthread_join(thread1, NULL);
pthread_join(thread2,NULL);
printf("Final value of shared is %d\n",shared); //prints the last updated value of shared variable
}
void *fun1()
{
int x;
printf("Thread1 trying to acquire lock\n");
pthread_mutex_lock(&l); //thread one acquires the lock. Now thread 2 will not be able to acquire the lock //until it is unlocked by thread 1
printf("Thread1 acquired lock\n");
x=shared;//thread one reads value of shared variable
printf("Thread1 reads the value of shared variable as %d\n",x);
x++; //thread one increments its value
printf("Local updation by Thread1: %d\n",x);
sleep(1); //thread one is preempted by thread 2
shared=x; //thread one updates the value of shared variable
printf("Value of shared variable updated by Thread1 is: %d\n",shared);
pthread_mutex_unlock(&l);
printf("Thread1 released the lock\n");
}
void *fun2()
{
int y;
printf("Thread2 trying to acquire lock\n");
pthread_mutex_lock(&l);
printf("Thread2 acquired lock\n");
y=shared;//thread two reads value of shared variable
printf("Thread2 reads the value as %d\n",y);
y--; //thread two increments its value
printf("Local updation by Thread2: %d\n",y);
sleep(1); //thread two is preempted by thread 1
shared=y; //thread one updates the value of shared variable
printf("Value of shared variable updated by Thread2 is: %d\n",shared);
pthread_mutex_unlock(&l);
printf("Thread2 released the lock\n");
}
The final value of shared variable will be 1. When any one of the threads acquires the lock and is making changes to shared variable the other thread (even if it preempts the running thread) is not able to acquire the lock and thus not able to read the inconsistent value of shared variable. Thus only one of the thread is running in its critical section at any given time */
Sample Output
Viva Questions on Program for Process Synchronization using mutex locks
Q1. What is the initial value of the lock variable?
Q2. Why we use pthread_join() function in the above program?
Q3. Why is the second parameter in pthread_mutex_init() NULL?
Q4. What is the significance of using sleep(1) function in the functions fun1() and fun2()?
Q5. What is the use of pthread_mutex_lock() function?
Q6. What is the use of pthread_mutex_unlock() function?