Structures are sequences of usually heterogeneous elements grouped so that they can be treated together as a single coherent datatype. Pointers to structures are an important part of building data structures. Whereas structures allow us to group data into convenient bundles, pointers let us link these bundles to one another in memory. By linking structures together, we can organize them in meaningful ways to help solve real problems.

As an example, consider chaining a number of elements together in memory to form a linked list (see Chapter 5). To do this, we might use a structure like ListElmt in the following code. Using a ListElmt structure for each element in the list, to link a sequence of list elements together, we set the next member of each element to point to the element that comes after it. We set the next member of the last element to NULL to mark the end of the list. We set the data member of each element to point to the data the element contains. Once we have a list containing elements linked in this way, we can traverse the list by following one next pointer after another.

typedef struct ListElmt_ {

void               *data;
struct ListElmt_   *next;

} ListElmt;

The ListElmt structure illustrates another important aspect about pointers with structures: structures are not permitted to contain instances of themselves, but they may contain pointers to instances of themselves. This is an important idea in building data structures because many data structures are built from components that are self-referential. In a linked list, for example, each ListElmt structure points to another ListElmt structure. Some data structures are even built from structures containing multiple pointers to structures of the same type. In a binary tree (see Chapter 9), for example, each node has pointers to two other binary tree nodes.

Arrays are sequences of homogeneous elements arranged consecutively in memory. In C, arrays are closely related to pointers. In fact, when an array identifier occurs in an expression, C converts the array transparently into an unmodifiable pointer that points to the array’s first element. Considering this, the two following functions are equivalent.

int f() {

int a[10], *iptr;
iptr = a;
iptr[0] = 5;

return 0;

}

int g() {

int a[10], *iptr;
iptr = a;
*iptr = 5;

return 0;

}

To understand the relationship between arrays and pointers in C, recall that to access the i th element in an array a, we use the expression:

a[i]

The reason that this expression accesses the i th element of a is that C treats a in this expression the same as a pointer that points to the first element of a. The expression as a whole is equivalent to:

*(a + i)

which is evaluated using the rules of pointer arithmetic. Simply stated, when we add an integer i to a pointer, the result is the address, plus i times the number of bytes in the datatype the pointer references; it is not simply the address stored in the pointer plus i bytes. An analogous operation is performed when we subtract an integer from a pointer. This explains why arrays are zero-indexed in C; that is, the first element in an array is at position 0.

For example, if an array or pointer contains the address 0x10000000, at which a sequence of five 4-byte integers is stored, a[3] accesses the integer at address 0x1000000c. This address is obtained by adding (3)(4) = 12₁₀ = c₁₆ to the address 0x10000000 (see Figure 2.3a). On the other hand, for an array or pointer referencing twenty characters (a string), a[3] accesses the character at address 0x10000003. This address is obtained by adding (3)(1) = 3₁₀ = 3₁₆ to the address 0x10000000 (see Figure 2.3b). Of course, an array or pointer referencing one piece of data looks no different from an array or pointer referencing many pieces. Therefore, it is important to keep track of the amount of storage that a pointer or array references and to not access addresses beyond this.

The conversion of a multidimensional array to a pointer is analogous to converting a one-dimensional array. However, we also must remember that in C, multi-dimensional arrays are stored in row-major order. This means that subscripts to the right vary more rapidly than those to the left. To access the element at row i and column j in a two-dimensional array, we use the expression:

a[i][j]

C treats a in this expression as a pointer that points to the element at row 0, column in a. The expression as a whole is equivalent to:

*(*(a + i) + j)

Figure 2.3. Using pointer arithmetic to reference an array of (a) integers and (b) characters