Page 171 - DCAP507_SYSTEM_SOFTWARE

Unit 10: Programming Languages Concept (I)

We can also assign pointer values to other pointer variables. If we declare a second pointer Notes
variable:
int *ip2;

then we can say
ip2 = ip;
Now ip2 points where ip does;
Now, if we set ip to point back to i again:

ip = &i;
We can now see that the two assignments
ip2 = ip;
and

*ip2 = *ip;
do two very different things. The first would make ip2 again point to where ip points (in other
words, back to i again). The second would store, at the location pointed to by ip2, a copy of the
value pointed to by ip; in other words (if ip and ip2 still point to i and j respectively) it would set
j to i's value, or 7.

It's important to keep very clear in your mind the distinction between a pointer and what it
points to. The two are like apples and oranges (or perhaps oil and water); you can't mix them.
You can't ``set ip to 5'' by writing something like
ip = 5; /* WRONG */
5 is an integer, but ip is a pointer. You probably wanted to ``set the value pointed to by ip to 5,''
which you express by writing
*ip = 5;
Similarly, you can't ``see what ip is'' by writing
printf("%d\n", ip); /* WRONG */

Again, ip is a pointer-to-int, but %d expects an int. To print what ip points to, use
printf("%d\n", *ip);
Finally, a few more notes about pointer declarations. The * in a pointer declaration is related to,
but different from, the contents-of operator *. After we declare a pointer variable
int *ip;
the expression
ip = &i

sets what ip points to (that is, which location it points to), while the expression
*ip = 5
sets the value of the location pointed to by ip. On the other hand, if we declare a pointer variable
and include an initializer:
int *ip3 = &i;

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