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Concurrent Systems/notes/4 - Semaphores.md

@@ -4,5 +4,29 @@
 **Semaphore:** is a shared counter S accessed via primitives $up$ and $down$ s.t.:
 - is initialized at s0 >= 0
 - it is alwayz >= 0
-- up atomically increases S
-- down atomically decreases S if it is not 0, otherwise the proce
+- *up* atomically increases S
+- *down* atomically decreases S if it is not 0, otherwise the calling process is blocked and waits.
+
+Main use: **prevent busy waiting**: suspend processes that cannot perform *down*.
+
+**Strong semaphore:** if uses a FIFO policy for blocking/unblocking, otherwise it's **weak**.
+**Binary semaphore:** if it is at most 1 (also *up* are blocking).
+
+A semaphore needs two underlying objects:
+- a counter initialized at s0 that can also become negative
+- a data structure (typically a queue), initially empty, to store suspended processes.
+
+#### Ideal implementation
+```
+S.down() :=
+	S.counter--
+	if S.counter < 0 then
+		enter into S.queue
+		SUSPEND
+	return
+
+S.up() :=
+	S.counter++
+	if S.counter <= 0 then
+		activate a proc from S.queue
+	return