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

.obsidian/workspace.json

@@ -36,7 +36,7 @@
                 "file": "Concurrent Systems/slides/class 4.pdf",
                 "page": 8,
                 "left": -23,
-                "top": 421,
+                "top": 187,
                 "zoom": 0.652019002375297
               },
               "icon": "lucide-file-text",

Concurrent Systems/notes/4 - Semaphores.md

@@ -154,3 +154,29 @@ 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»)
+
+```
+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