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Concurrent Systems/notes/4 - Semaphores.md

@@ -130,3 +130,27 @@ Thanks to the semaphores, we are sure that while loops will not go on forever! T
 EXERCISE - will do later
 
 #### The Readers/Writers problem
+- Several processes want to access a file
+- Readers may simultaneously access the file
+- At most one writer at a time
+- Reads and writes are mutually exclusive
+
+>[!note] Remark
+>this generalizes the MUTEX problem (MUTEX = RW with only writers)
+
+The read/write operations on the file will all have the following shape:
+```
+conc_read() :=
+	begin_read()
+	read()
+	end_read()
+
+conc_write() :=
+	begin_write()
+	write()
+	end_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»)