• C

# difference in a pointer and a pointer to a pointer

can someone explain to me what the difference is in a pointer and a pointer to a pointer? also include an example please.
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A pointer is a variable which contains the address of some other variable. The other variable is of some "final"/"usable" type. Consider the following:

int i,j,*pi;

i=1;
pi=&i;
j=2;
j=*pi;

Here i is an integer, as is j, and pi is a pointer to an integer. In the first line, i is assigned 1, pi is then assigned the address of i, j is assigned 2, then j is assigned the value of what pi points to (which is i), so j will become 1.

On the other hand the following:

int i,j,*pi,**ppi;

i=1;
pi=&i;
ppi=&pi;
j=2;
j=**ppi;

i is an integer and is assigned 1, pi is a pointer to an integer and is assigned the address of i, ppi is a pointer to a pointer to an integer, which is assigned the address of pi, j is assigned 2. Now j is assigned **ppi. This will evaluate as follows: first ppi is dereferenced. Since ppi contains the address of pi, derefencing it will find pi, which is the address of i. Now the second "*" will cause that to be dereferenced which means that it will use the contents of pi as an address (as it should) and, since it is the address of i, j will be assigned 1.

This does not appear very useful, I realize, in the above example. But there are a number of uses for such constructs in C. For instance, just like you pass the address of a variable into a function, so that the variable can be changed, you need to pass the address of a pointer into a function, if you want the pointer to be changed.
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