Race Condition

A simple example of a race condition is a light switch. In some homes

there are multiple light switches connected to a common ceiling light.

 When these types of circuits are   used, the switch position   becomes irrelevant. If the light is   on, moving either switch from its   current position turns the light off.   Similarly, if the light is off, then   moving either switch from its   current position turns the light on.   With that in mind, imagine what   might happen if two people tried to   turn on the light using two different   switches at exactly the same time.   One instruction might cancel the other or the two actions might trip the   circuit breaker.

#include<pthread.h>

#include<stdio.h>

void *fun1();

void *fun2();

int shared=1; //shared variable

int main()

{

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;

    x=shared;//thread one reads value of shared variable

    x++;  //thread one increments its value

    sleep(1);  //thread one is preempted by thread 2

    shared=x; //thread one updates the value of shared variable

}

void *fun2()

{

    int y;

    y=shared;//thread two reads value of shared variable

    y--;  //thread two increments its value

    sleep(3); //thread two is preempted by thread 1

    shared=y; //thread one updates the value of shared variable

}