Why 2 threads incrementing one variable can result in different outputs

Question

I just started learning concurrent programming, and even though I have a vague idea about this, why can two threads incrementing one variable result in different outputs?

Let's say the variable i is global, and there are two threads trying to do

i = i + 1;

Why can the output be either 1 or 2? And assuming the main thread doesn't quit, because I know that it is possible that sometimes the main thread quits before one of the threads is able to do anything.

Answer 1

Because addition operation is not atomic.

From wikipedia:

In concurrent programming, an operation (or set of operations) is atomic if it appears to the rest of the system to occur instantaneously.

Atomicity is a guarantee of isolation from concurrent processes.

Keeping that in mind, consider the following (simplified) illustration. The operation i = i + 1 is made up of three atomic operations:

• read the current value of i from the memory
• add 1 to the value that was read
• write i back to memory

When two threads try to perform i = i + 1, they may both get the same current value of i, and then set it to i + 1, making it appear that only 1 was incremented. Consider the following 2 possible scenarios:

Scenario 1

int i = 0;
i = i + 1;

Thread 1: Reads i       // Reads 0
Thread 1 : i = i + 1    // Adds 1 to i
Thread 1: Write i = 1   // Makes i == 1
Thread 2: Reads i;      // Reads 1
Thread 2: i = i + 1     // Adds 1 to i, making i == 2
Thread 2: Write i = 2   // Makes i == 2

// Finally, i becomes 2

In the above scenario, everything works as expected. But what happens when Thread 2 reads before Thread 1 writes the value of i?

Scenario 2

int i = 0;
i = i + 1;

Thread 1: Reads i       // Reads 0
Thread 1 : i = i + 1    // Adds 1 to i
Thread 2: Reads i;      // Reads 0 - NOT 1 because it's not written to memory yet
Thread 1: Write i = 1   // Makes i == 1
Thread 2: i = i + 1     // Adds 1 to i, making i == 1 (Remember, thread 2 had read 0)
Thread 2: Write i = 1   // Makes i == 1

// i is finally 1, instead of 2