Atomic Operation

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:

1. Use a single machine instruction.
2. Tell the operating system in advance not to interrupt the operation.

GCC provides intrinsics for some atomic operations.