Race condition

Definition

A race condition is a situation where multiple process access and manipulate the same data concurrently, and the outcome of the execution depends on the particular order (interleaving) of such accesses.

Example

Suppose that initially y = 12, and a program with two threads that do the following is run.

Thread A	Thread B
x = y + 1	y = y * 2

• If thread A reads y before thread B updates y, the result is x = 13.
• Otherwise, the result is x = 25.

Suppose that initially x = 0, and a program with two threads that do the following is run.

Thread A	Thread B
x = 1	y = 2
x = y + 1	y = y * 2*

• If thread A runs to completion and then thread B starts and runs to completion, the result is x = 1.
• If thread B runs to completion and then thread A starts and runs to completion, the result is x = 5.
• If thread B executes y = 2 and interleaves - thread A starts and runs to completion - and thread B executes y = y * 2, the result is x = 3.
• If thread A executes x = 1 and interleaves - thread B starts and runs to completion - and thread A executes x = y + 1, the result is x = 5.