1	Concepts and Terminology
2	Concepts and Terminology Contents Passive Objects Active Objects Threading Concurrency Concurrent Operations Sequential Operations Guarded Operations Synchronisation Critical Sections Locking Monitors
3	Terminology: Passive Object A passive object never initiates a call It rests passively Responding to calls made on it
4	Terminology: Active Object An active object initiates calls It actively calls functions
5	What is a Thread? A thread is an independent path of execution Each thread has its own stack Threads in a process share the same heap segment Threads in a process share the same data segment
6	Single-Threading One thread, one execution path Thread starts at start of main, ends at end of main
7	Multi-Threading Two or more threads!
8	Concurrency Consider... 2+ threads on the same path at the same time
9	Terminology: Concurrent Operations Operations that are safe when concurrent Accessing immutable state via const methods Atomic queries
10	Non-Atomic What's the Time?
11	Atomic What's the Time?
12	Synchronisation Most operations are not safe when concurrent Unless you design them to be concurrent, assume operations are not concurrent Concurrency is a huge design context, not just an implementation detail
13	Terminology: Critical Section A section of code that is unsafe when concurrent Access to critical section must be synchronised Failure to synchronise causes undefined behaviour Race conditions, deadlocks Hair-pulling, teeth-gnashing
14	Question How are critical sections enforced?
15	Locking Lock access to critical section
16	Software Locks Large variety of software locks semaphore binary, counting, blocked-set, blocked-queue, ... mutex read-write mutex, recursive mutex “lock-free” synchronisation primitives InterlockedIncrement( ), InterlockedDecrement( )
17	Mutex Common synchronisation primitive intra-process, inter-process
18	Question Who is responsible for enforcement?
19	Terminology: Sequential Operation Concurrent callers synchronise operation access External complexity
20	External Locking
21	Terminology: Guarded Operation Operation synchronises access to itself Internal complexity
22	Internal Locking
23	Terminology: Monitors Shared objects whose operations are guarded Simplifies programming from class users POV Don't call other public member functions from within the class if the synchronisation object is not re-entrant