91 lines
1.9 KiB
Markdown
91 lines
1.9 KiB
Markdown
---
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tags:
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- C
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---
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A pointer is a reference to the address of a variable in memory.
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```c
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int x = 27;
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int *ptr = &x;
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printf("%i\n", x);
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// 27
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printf("%p\n", *ptr);
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// 0x7ffeb44f7eac
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```
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The `&` and `*` is frankly confusing.
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In the previous example, `int *ptr = &x`, `ptr` is a variable that holds the
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memory address of `x`. `*` signals that it is a pointer variable, `&` is what
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does the retrieval.
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In the following:
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```c
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int x = 27;
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int *ptr = &x;
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int value = *ptr;
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```
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We again set `ptr` to the memory address of `x`, but we use `*` on the last line
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to **de-reference** the pointer and get the original value back. Thus `value`
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becomes equal to `27`.
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Pointers are necessary because C uses a [call by value](./C_is_call_by_value.md)
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system for function arguments.
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## To use a pointer you don't have to declare the value first (confusing)
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It is totally legitimate to declare a pointer in one go, especially with
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strings:
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```c
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char *name = "Thomas";
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```
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Then to reference the value (_not_ the address, the actual value) you just use
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`name`. The `*` is just part of the type definition in the above - it's just
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signalling the type. If you later want to change the value, you then use `*` as
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an operator:
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```c
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*name = 'Tom';
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```
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Note that if you use a pointer to `char`, it creates the string in read-only
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memory. You can't modify the individual characters of `"Thomas"`. To do this you
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would need to define it as:
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```c
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char name[] = "Thomas";
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name[0] = "t";
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```
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Here is a real example of me doing this:
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```c
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void mqtt_publish(esp_mqtt_client_handle_t client, char *topic,
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const char *payload)
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{
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if (MQTT_CONNECTED) {
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esp_mqtt_client_publish(client, topic, payload, strlen(payload), 0, 0);
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}
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}
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```
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And then when calling the function:
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```c
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mqtt_publish(mqtt_client, "test_topic", "Test message");
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```
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Or, using a variable:
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```c
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static char *topic = "test_topic";
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mqtt_publish(mqtt_client, topic, "Test message");
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```
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