Page 170 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes We discover the value pointed to by a pointer using the “contents-of” operator, *. Placed in front
of a pointer, the * operator accesses the value pointed to by that pointer. In other words, if ip is
a pointer, then the expression *ip gives us whatever it is that's in the variable or location pointed
to by ip.

Example: We could write something like
printf("%d\n", *ip);
which would print 5, since ip points to i, and i is (at the moment) 5.

(You may wonder how the asterisk * can be the pointer contents-of operator when it is also the
multiplication operator. There is no ambiguity here: it is the multiplication operator when it
sits between two variables, and it is the contents-of operator when it sits in front of a single
variable. The situation is analogous to the minus sign: between two variables or expressions it's
the subtraction operator, but in front of a single operator or expression it's the negation operator.
Technical terms you may hear for these distinct roles are unary and binary: a binary operator
applies to two operands, usually on either side of it, while a unary operator applies to a single
operand.)
The contents-of operator * does not merely fetch values through pointers; it can also set values
through pointers. We can write something like
*ip = 7;
which means ``set whatever ip points to 7.'' Again, the * tells us to go to the location pointed to
by ip, but this time, the location isn't the one to fetch from--we're on the left-hand sign of an
assignment operator, so *ip tells us the location to store to.

If we called printf("%d\n", *ip) again, it would now print 7.
At this point, you may be wondering why we're going through this rigmarole--if we wanted to
set i to 7, why didn't we do it directly? We'll begin to explore that next, but first let's notice the
difference between changing a pointer (that is, changing what variable it points to) and changing
the value at the location it points to. When we wrote *ip = 7, we changed the value pointed to by
ip, but if we declare another variable j:
int j = 3;
and write

ip = &j;
we've changed ip itself
We have to be careful when we say that a pointer assignment changes ''what the pointer points
to.'' Our earlier assignment
*ip = 7;
changed the value pointed to by ip, but this more recent assignment
ip = &j;

has changed what variable ip points to. It's true that ''what ip points to'' has changed, but this
time, it has changed for a different reason. Neither i (which is still 7) nor j (which is still 3) has
changed. (What has changed is ip's value.) If we again call
printf("%d\n", *ip);
this time it will print 3.

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