What is an atomic operation?

Answer 1

An atomic operation is an operation which is guaranteed to finish in a single unit of time. This is important in synchronization where multiple threads may be trying to access a single variable at the same time.

Theoretically, a situation could occur as follows: Imagine two threads are both running the same code.

int c = 0;

void f() {

c = c + 1;

}

Now assume the following sequence of events takes place:

# Thread0 calls f() # Thread1 calls f() # Thread0 reads c as 0 # Thread1 reads c as 0 # Thread0 increases c to 1 # Thread1 increases c to 1 # Thread0 writes back c as 1 # Thread1 writes back c as 1

We end up with c = 1. This is obviously not what was supposed to happen. Each thread was supposed to execute the code once and increase c by one, ending up with c = 2. If the (c = c + 1) statement had been atomic, each thread would read, increment, and write back c before execution was passed to any other thread.