vault backup: 2025-03-12 11:24:27

Concurrent Systems/notes/4 - Semaphores.md

@@ -153,4 +153,30 @@ conc_write() :=
 
 ##### Weak priority to Readers
 - If a reader arrives during a read, it can surpass possible writers already suspended (risk of starvation for the writes)
-- When a writer terminates, it activates the first suspended process, irrispectively of whether it is a reader or a writer (so, the priority to readers is said «weak»)
+- When a writer terminates, it activates the first suspended process, irrispectively of whether it is a reader or a writer (so, the priority to readers is said «weak»)
+
+```
+GLOB_MUTEX and R_MUTEX semaphores init. at 1
+R a shared register init. at 0
+
+begin_read() :=
+	R_MUTEX.down()
+	R++ # currently active readers
+	if R = 1 then GLOB_MUTEX.down()
+	R_MUTEX.up()
+	return
+
+end_read() :=
+	R_MUTEX.down()
+	R--
+	if R = 0 then GLOB_MUTEX.up()
+	R_MUTEX.up()
+	return
+
+begin_write() :=
+	GLOB_MUTEX.down()
+	return
+
+end_write() :=
+	GLOB_MUTEX.up()
+	return