5.1. The Basics
A limitation you may have noticed is that functions can only
affect your program via their return value, so what do you do when you
want a function to alter more than one variable? You use pointers. A
pointer is a special kind of variable. Pointers are designed for
storing memory address i.e. the address of another variable.
Declaring a pointer is the same as declaring a normal variable except
you stick an asterisk '*' in front of the variables identifier. There
are two new operators you will need to know to work with pointers.
The "address of" operator '&' and the
"dereferencing" operator '*'. Both are prefix unary
operators. When you place an ampersand in front of a variable you
will get it's address, this can be store in a pointer. When you
place an asterisk in front of a pointer you will get the value at the
memory address pointed to. As usual, we'll look at a quick code
example to show how simple this is.
Example 5-1. pointers_are_simple.c
#include <stdio.h>
int
main()
{
int my_variable = 6, other_variable = 10;
int *my_pointer;
printf("the address of my_variable is : %p\n", &my_variable);
printf("the address of other_variable is : %p\n", &other_variable);
my_pointer = &my_variable;
printf("\nafter \"my_pointer = &my_variable\":\n");
printf("\tthe value of my_pointer is %p\n", my_pointer);
printf("\tthe value at that address is %d\n", *my_pointer);
my_pointer = &other_variable;
printf("\nafter \"my_pointer = &other_variable\":\n");
printf("\tthe value of my_pointer is %p\n", my_pointer);
printf("\tthe value at that address is %d\n", *my_pointer);
return 0;
}
The output shows you the address of the two variables, the addresses
your system assigns to the variables will be different to mine. In
printf() you'll notice we used
%p
to display the addresses. This is the conversion specifier for all
pointers. Anyway, here's the output I got:
the address of my_variable is : 0xbffffa18
the address of other_variable is : 0xbffffa14
after "my_pointer = &my_variable":
the value of my_pointer is 0xbffffa18
the value at that address is 6
after "my_pointer = &other_variable":
the value of my_pointer is 0xbffffa14
the value at that address is 10
There. That's not too complicated. Once you are comfortable with
pointers you're well on your way to mastering C.
