Pointers

What are Pointers

Essentially, a pointer is an address to memory on your machine.

The space taken up by a pointer is determined by your machine’s word size.

Think of your memory as an array:

Lets say we have

int b = 6;
int *c = &b

This means b is a reference to the 6 in memory, as shown below. *c denotes a pointer to the address of b (that’s what the & operator denotes.) As a result, b is pointing to that 6 in the memory, and *c is the address of that 6 in memory.

-	-	6	-

Segmentation Faults

These occur when the code is unable to access a memory location specified in the program runtime. This can occur because the memory address doesn’t exists or if the program doesn’t have permission to read that memory address.

Casting Pointers

Casting the type of a pointer changes the type but not the value

This means when we cast the type of a pointer, we change the sizeof of it, because it is interpreted as whatever the new type is.

Pointers to Functions

(Function Pointers)

Function types are just another derived type in C, so we can have pointers to them

For a function object f, ADDR(f) is simply the memory address to where its code starts.

Note that in C you cannot use sizeof on a function

To set a pointer, no need to use the unary & for the address, you can simply set fp = fun and it would set fp to the pointer.

When calling a function pointer, you just call it as usual, no need to de-reference.

Example

Say we declare:

// example.h
 
int fun(int x, int *p); // a function that takes in an int and int pointer and returns an int
int (*fp) (int, int*); // a pointer to a function that takes in an int and an int pointer and returns an int
int y = 1;

TYPE(fun): int & int ptr -> int TYPE(fp): (int & int ptr -> int)ptr