eolas/zk/Pointers_in_C.md

---
tags:
  - C
---

A pointer is a reference to the address of a variable in memory.

```c
int x = 27;
int *ptr = &x;

printf("%i\n", x);
// 27


printf("%p\n", *ptr);
// 0x7ffeb44f7eac
```

The `&` and `*` is frankly confusing.

In the previous example, `int *ptr = &x`, `ptr` is a variable that holds the
memory address of `x`. `*` signals that it is a pointer variable, `&` is what
does the retrieval.

In the following:

```c
int x = 27;
int *ptr = &x;
int value = *ptr;
```

We again set `ptr` to the memory address of `x`, but we use `*` on the last line
to **de-reference** the pointer and get the original value back. Thus `value`
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!