Pointers

Key Principles of Pointers in C

Example:

int a = 10;
char c = 'a';
char *cp = &c;

int *ip;
char **cpp;

# creating pointers
ip = &a;
cpp = &cp;

void *p;
p = &a;
p = &c;

*ip = 100; # dereferencing

Every pointer has an associated type

ip is a pointer to an int and cpp is pointer to an object that itself is a pointer to a char.

Every pointer has a value

The pointer holds the value of an address of some object. If pointer equals NULL or 0, then the pointer is not pointing to anything. Each pointer is created using the & operator, which can be used on any lvalue (anything to the left of the = sign). Pointers can also be dereferenced using the * operator.

Array and Pointer and Casting

Array referencing (e.g., a[3]) has the exact same effect as pointer dereferencing (e.g., *(a+3)). Both array referencing and pointer arithmetic require scaling the offsets by the object size.

(int *) p+7 offsets the address by 28 bytes (because int has 4 bytes), but (int *) (p+7) will offset the address by 7 bytes. Note that (char *) p+1 is just offsetting the address by 1 byte. In general if p is a pointer, then p+i (no casting) means the resulting address is $p+Li$ where $L$ is the size of the data type associated with the pointer.

Function Pointers

Example:

int foo(int x, int *p);

Assign a function pointer

(int) (*fp)(int, int *);
fp = foo;

Invoke the function using the pointer

int x = 1;
int result = fp(3, &x);

Note that int *fp(int, int *) would be read as (int *) fp(int, int *);, which means that theres a function fp that takes an int and int * as its arguments and returns an int *. (int) (*fp)(int, int *) means that fp is a pointer to a function that takes an int and int * as its arguments and returns an int.