The OS assures that there is little or no interaction between processes. For two processes to interact, some common OS resource must be explicitly shared. This can be a common file, a specific shared memory object, or a semaphore with a shared state between the processes. Processes are inherently independent, interaction is exceptional.

Multiple threads, on the other hand, are part of a single process; all threads of a process share OS resources. We can make an exception to get some thread-local memory that can be freely written without interference from other threads. Outside thread-local memory, operations that write to memory can set the internal state of the process in a potentially unpredictable order. Explicit locking must be used to avoid problems with these stateful updates. As noted previously, the overall sequence of instruction executions is rarely, strictly speaking, concurrent. The instructions from concurrent threads and processes are...