more C pointer confusion

zk/Char_data_type_in_C.md

@@ -80,3 +80,17 @@ The more formal way of doing this would be:
 ```c
 char word[6] = {'H', 'e', 'l', 'l', 'o', '\0'}
 ```
+
+However, in practice, if you just want quick access to a string, where you don't
+need to modify the individual characters later, you can just use a direct
+pointer definition:
+
+```c
+char *name = "Thomas";
+```
+
+And then to use it later just use `name` (no `*`). In this scenario, the string
+is a string literal and is stored as read-only. It can be redefined (the pointer
+can point to something else) but it's not mutable itself.
+
+More info in [Pointers in C](./Pointers_in_C.md).

zk/Pointers_in_C.md

@@ -37,3 +37,54 @@ becomes equal to `27`.
 
 Pointers are necessary because C uses a [call by value](./C_is_call_by_value.md)
 system for function arguments.
+
+## To use a pointer you don't have to declare the value first (confusing)
+
+It is totally legitimate to declare a pointer in one go, especially with
+strings:
+
+```c
+char *name = "Thomas";
+```
+
+Then to reference the value (_not_ the address, the actual value) you just use
+`name`.
+
+This is confusing but I just have to accept for now.
+
+Note that if you do this, it creates the string in read-only memory. You can't
+modify the individual characters of `"Thomas"`. To do this you would need to
+define it as:
+
+```c
+char name[] = "Thomas";
+name[0] = "t";
+```
+
+Here is a real example of me doing this:
+
+```c
+void mqtt_publish(esp_mqtt_client_handle_t client, char *topic,
+                  const char *payload)
+{
+  if (MQTT_CONNECTED) {
+    esp_mqtt_client_publish(client, topic, payload, strlen(payload), 0, 0);
+  }
+}
+```
+
+And then when calling the function:
+
+```c
+mqtt_publish(mqtt_client, "test_topic", "Test message");
+```
+
+Or, using a variable:
+
+```c
+static char *topic = "test_topic";
+mqtt_publish(mqtt_client, topic, "Test message");
+```
+
+> Note in the above the `*` is part of the type definition, _not_ the variable.
+> Even more confusing!

zk/mosquitto_mqtt_client.md

@@ -5,6 +5,20 @@ tags:
   - mqtt
 ---
 
+## Install
+
+```sh
+sudo apt install mosquitto
+```
+
+This just installs the broker and you can start publishing straight away but if
+you want to view the incoming requests to the given topic you need to install
+the client package:
+
+```sh
+sudo apt install mosquitto-clients
+```
+
 ## Config
 
 At `/etc/mosquitto/mosquitto.conf`.