From 96dbd779e786f3801becb565ef50d91c45cbe296 Mon Sep 17 00:00:00 2001
From: darkicewolf50 <brock.tomlinson@ucalgary.ca>
Date: Thu, 13 Mar 2025 17:59:05 -0600
Subject: [PATCH] started ch16.3

---
 ().md                       |  0
 .obsidian/workspace.json    | 40 ++++++++++----------
 Shared State Concurrency.md | 73 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 92 insertions(+), 21 deletions(-)
 create mode 100644 ().md

diff --git a/().md b/().md
new file mode 100644
index 0000000..e69de29
diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index f2bc017..54a0ee1 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -24,20 +24,6 @@
           {
             "id": "53b36d00b704136e",
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-                "file": "Smart Pointers.md",
-                "mode": "source",
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-              },
-              "icon": "lucide-file",
-              "title": "Smart Pointers"
-            }
-          },
-          {
-            "id": "feca3744741d15fc",
-            "type": "leaf",
             "state": {
               "type": "markdown",
               "state": {
@@ -49,6 +35,20 @@
               "title": "Concurrency"
             }
           },
+          {
+            "id": "3d0ca0b1691c4c2f",
+            "type": "leaf",
+            "state": {
+              "type": "markdown",
+              "state": {
+                "file": "Shared State Concurrency.md",
+                "mode": "source",
+                "source": false
+              },
+              "icon": "lucide-file",
+              "title": "Shared State Concurrency"
+            }
+          },
           {
             "id": "b104e4647c0ac328",
             "type": "leaf",
@@ -245,10 +245,14 @@
       "command-palette:Open command palette": false
     }
   },
-  "active": "feca3744741d15fc",
+  "active": "3d0ca0b1691c4c2f",
   "lastOpenFiles": [
-    "Smart Pointers.md",
     "Concurrency.md",
+    "Shared State Concurrency.md",
+    "Smart Pointers.md",
+    "().md",
+    "Simultaneous Code Running.md",
+    "Passing Data Between Threads.md",
     "Leaky Reference Cycles.md",
     "Test Controls.md",
     "Ref Cell Mutability.md",
@@ -270,10 +274,6 @@
     "Lifetimes.md",
     "2025-02-04.md",
     "data_types.md",
-    "Collection of Common Data Structs.md",
-    "Constants.md",
-    "Crates.md",
-    "Data Types.md",
     "does_not_compile.svg",
     "Untitled.canvas",
     "Good and Bad Code/Commenting Pratices",
diff --git a/Shared State Concurrency.md b/Shared State Concurrency.md
index 7b43f58..606f164 100644
--- a/Shared State Concurrency.md	
+++ b/Shared State Concurrency.md	
@@ -1 +1,72 @@
-# Shared-State Concurrency
\ No newline at end of file
+# Shared-State Concurrency
+Message passing is not the only way of handling concurrency.
+
+Another way would be multiple threads to access the same shared data.
+
+Consider this part of the slogan form the Go language documentation:
+
+"Do not communicate by sharing memory."
+
+What would communicating by sharing memory look like?
+
+Why would message-passing enthusiasts caution not to use memory sharing.
+
+Channels in any programming language are similar to single ownership, because once you transfer a value down a channel, you should no longer use that value.
+
+Shared memory concurrency is like multiple ownership: multiple threads can access the same memory location at the same time.
+
+As we saw before [Ch15](./Smart%20Pointers.md), where smart pointers made multiple ownership possible.
+
+This can add complexity because these different owners need managing.
+
+Rust's type system and ownership rules majority assist in getting this management correct.
+
+Lets look at one example mutexes, one of the more common concurrency primitives for shared memory.
+
+## Using Mutexes to Allow Access to Data from One Thread at a Time
+*Mutex* is an abbreviation for *mutual exclusion*, as in, a mutex allows only one thread to access some data at any given time.
+
+To access the data in a mutex, a thread must first signal that it wants access by asking to acquire the mutex's *lock*.
+
+The lock is a data structure that is part of the mutex that keeps track of who is currently gas exclusive access to the data.
+
+The mutex can be described as *guarding* the data it holds via the locking system.
+
+Mutexes have an associated reputation for being difficult to use because to must remember two rules:
+- You must attempt to acquire the lock before using the data
+- When you are done with the data that the mutex guards, you must unlock the data so other threads can acquire the lock
+
+A real-world metaphor for a mutex would be like imagining a panel discussion at a conference with only one microphone.
+
+Before a panelist can speak, they have to ask or signal that they want to use the microphone.
+
+When they get the microphone, they can talk for as long as they want to and then hand the microphone to the next panelist who requests to speak.
+
+If a panelist forgets to hand off the microphone off when they they are finished with it, no one else is able to speak.
+
+If management of the shared microphone goes wrong, the panel won't work as planned.
+
+Management of mutexes can be incredibly tricky to get right.
+
+This is why so many people are enthusiastic about channels.
+
+However due to Rust's type system and ownership rules, you can't get locking and unlocking wrong.
+
+### The API of `Mutex<T>`
+Here is an example of how to use a mutex.
+
+We will start by using a mutex in a single threaded context.
+```rust
+use std::sync::Mutex;
+
+fn main() {
+    let m = Mutex::new(5);
+
+    {
+        let mut num = m.lock().unwrap();
+        *num = 6;
+    }
+
+    println!("m = {m:?}");
+}
+```