The C Programming Language - Pointers

To prevent a pointer from pointing to an unknown, undefined memory address, it
should be assigned an address immediately, or if the address is not yet known, the
value 0 or NULL. When a pointer is assigned the value 0 or NULL, it is called a null
pointer. This prevents a valid memory address from being determined by chance when
the pointer is used, thus causing errors that are difficult to find.
int *ptr1 = NULL;
int *ptr2 = 0;
// NULL pointer
// NULL pointer
To keep track when dealing with pointers, you should always use a meaningful name
for a pointer.
int *ptr_var = &var;
As in this example, _var has been appended to the name of the pointer to make it clear
that this pointer points to the address of var. You can also assign an address to a
pointer after it has been declared.
int *ptr_var; // undefined pointer
…
ptr_var = &var; // defined pointer - ptr_var points to var
1.3.1 Direct Address Assignment
A pointer can also be assigned a direct address in memory during initialization. The
following example shows a direct address assignment:
/* example 129 – pointers */
#include
int main()
{
int *ptr = (int *)0x0061ff14; // typecast to an int* - ptr points to 0x0061ff14
printf("ptr points to: %p\n", ptr);
}
return 0;
ptr points to: 0061ff14
To assign a desired address to a pointer, a number (the desired address) must first be
typed into a pointer, in this case an int pointer. This tells the compiler that it is a pointer
to the constant address 0x0061ff14.
The C Programming Language by Heimo Gaicher – Chapter Pointers 12