This post will explain how to create a program to simulate a race donition.

What is a race condition?

When multiple processes attempt to change the value of shared data at the same time, there is a race condition. The final value of the shared data is unpredictable because the order in which the processes perform their updates on the data is not controlled.

The Program to Simulate this

The C code below has two threads, t1 and t2. They both attempt to change the value of the shared variable var. t1 is trying to multiply var by 10 and t2 is trying to subtract 10 from var. When running the code multiple times, the final value of the shared variable will change between 10 and -9. This is because the order in which t1 and t2 apply their operations to the value can be different each time the program runs.

#include<pthread.h>
#include<stdio.h>
#include<unistd.h>

//Declare functions
void *function1();
void *function2();

//decalre and initialise shared variable
int var=1;

int main()
{
    //create 2 threads
    pthread_t t1, t2; 
    pthread_create(&t1, NULL, function1, NULL);
    pthread_create(&t2, NULL, function2, NULL);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    //output the value of shared variable after the threads have finished their operations
    printf("Shared Variable value is %d\n", var);
}

void *function1() 
{
    // read the shared value, store it in a local variable
    int x;
    x=var;

    // multiply the local variable by 10
    printf("T1 has read value of shared variable as %d\n", x);
    x=x*10;
    printf("T1 has updated it's copy of the shared variable to be %d\n", x);
    // sleep to allow next thread, t2 to run
    sleep(1); 

    // write the value of the local variable to the shared variable
    var = x;
    printf("Value of shared variable updated by T1 is %d\n", var);
}
void *function2()
{
    // read the shared value, store in in a local variable 
    int y;
    y=var;

    // subtract 10 from local variable
    printf("T2 has read value of shared variable as %d\n", y);
    y=y-10;
    printf("T2 has updated it's copy of the shared variable to be %d\n", y);
    sleep(1);

    // write the value of the local variable to the shared variable
    var = y;
    printf("Value of shared variable updated by T2 is %d\n", var);
}

Here is the output when running the program. Note the changes in the final value of the shared variable due to the race condition: