eolas/zk/Pointers_in_C.md

tags
C

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

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:

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 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:

char *name = "Thomas";

Then to reference the value (not the address, the actual value) you just use name. The * is just part of the type definition in the above - it's just signalling the type. If you later want to change the value, you then use * as an operator:

*name = 'Tom';

Note that if you use a pointer to char, 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:

char name[] = "Thomas";
name[0] = "t";

Here is a real example of me doing this:

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:

mqtt_publish(mqtt_client, "test_topic", "Test message");

Or, using a variable:

static char *topic = "test_topic";
mqtt_publish(mqtt_client, topic, "Test message");