Errors happen in a parallel program when two tasks that are executing simultaneously contain operations that access the same memory location. If one of them writes to the location and the other one reads from it, then the result of the read depends on whether it happens before or after the write. If they both write to the location, then the final value in the location depends on which write happens last.
We call these errors data sharing problems. They are also sometimes referred to as conflicts, or data races, because the behavior of the program depends on which operation gets to the data first, or as data dependencies, because the result of executing a data access operation depends on other operations that have previously accessed the same data.
To avoid data sharing problems during parallel execution, you need to add a form of synchronization.
