Pointers to Pointers

Pointer to Pointers

In C, you can also use pointers that point to pointers. The pointers in turn point to data or even to other pointers. To declare pointers to pointers, just add an asterisk * for each level of reference.

Fig: Pointer to Pointer

For example, consider the following code:

int x=10;

int *px, **ppx;

px = &x;

ppx = &px;

Let us assume, the memory locations of these variables are as shown in Fig.above

Now if we write,

printf("\n %d", **ppx);

Then, it would print 10, the value of x.

Drawbacks of Pointers

Although pointers are very useful in C, they are not free from limitations. If used incorrectly, pointers can lead to bugs that are difficult to unearth. For example, if you use a pointer to read a memory location but that pointer is pointing to an incorrect location, then you may end up reading

a wrong value. An erroneous input always leads to an erroneous output. Thus however efficient your program code may be, the output will always be disastrous. Same is the case when writing a value to a particular memory location.

Let us try to find some common errors when using pointers.

int x, *px;

x=10;

*px = 20;

Error: Un-initialized pointer. px is pointing to an unknown memory location. Hence it willoverwrite that location’s contents and store 20 in it.

int x, *px;

x=10;

px = x;

Error: It should be px = &x;

int x=10, y=20, *px, *py;

px = &x, py = &y;

if(px<py)