Q44: Concurrent Threads

Answer the question below based on the following code:

Global Variable:

int count = 0;

Thread1:

for(int i = 1; i < 50000; i++)

count++;

Thread2:

for(int i = 1; i < 20000; i++)

count++;

 

Question #44: Given that the two threads run concurrently, what will be the value of count at the end of execution of the two threads?

Options:

• Value of count will be 70000.
• Value of count can’t be predicted.
• There is a race condition in the above program.
• Both B and C.

Solution: count++ is not an atomic statement and is broken down into many instructions while executing. Following is an illustration of how things can go wrong:

Thread 1	Thread 2		Integer value
			0
read value		←	0
	read value	←	0
increase value			0
	increase value		0
write back		→	1
	write back	→	1

Hence, 4th option is the correct solution.