An atomic operation is one that cannot be interrupted. These operations are typically used for acquiring locks.
Consider the following code, intended to set a "lock" in shared memory so that only one thread or process performs a particular action at a time:
if (lock == 0) {
lock++;
/* Perform exclusive action */
lock--;
} else {
abort();
}
If another process or thread writes to the lock variable between the lock comparison (lock == 0) and setting the lock (lock++), then the lock could fail to ensure exclusive action.
There are two ways to accomplish an atomic operation:
- Use a single machine instruction.
- Tell the operating system in advance not to interrupt the operation.
GCC provides intrinsics for some atomic operations.
(Note that in the above example, even the lock++ statement may not be atomic - it will typically compile into the three-operation sequence "load-increment-store", and its possible that two processes or threads could each perform this operation in an interleaved fashion and increment lock by just one instead of two).