======  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:

<code>
 if (lock == 0) {
   lock++;
   /* Perform exclusive action */
   lock--;
 } else {
   abort();
 }</code>

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 [[Compiler Intrinsics|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).