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just as for other types of variables. Pointers generally occupy one machine word, but their size can vary. Therefore, for portability, we should never assume that a pointer has a specific size. Pointers often vary in size as a result of compiler settings and type specifiers allowed by certain C implementations. It is also important to remember that when we declare a pointer, space is allocated only for the pointer itself; no space is allocated for the data the pointer references. Storage for the data is allocated in one of two ways: by declaring a variable for it or by allocating storage dynamically at runtime (using malloc or realloc, for example).

When we declare a variable, its type tells the compiler how much storage to set aside for it as the program runs. Storage for the variable is allocated automatically, but it may not be persistent throughout the life of the program. This is especially important to remember when dealing with pointers to automatic variables. Automatic variables are those for which storage is allocated and deallocated automatically when entering and leaving a block or function. For example, since iptr is set to the address of the automatic variable a in the following function f, iptr becomes a dangling pointer when f returns. This situation occurs because once f returns, a is no longer valid on the program stack (see Chapter 3).

int f(int **iptr) {

int a = 10;

*iptr = &a;

return 0;

}

In C, when we dynamically allocate storage, we get a pointer to some storage on the heap (see Chapter 3). Since it is then our responsibility to manage this storage ourselves, the storage remains valid until we explicitly deallocate it. For example, the storage allocated by malloc in the following code remains valid until we call free at some later time. Thus, it remains valid even after g returns (see Figure 2.2), unlike the storage allocated automatically for a previously. The parameter iptr is a pointer to the object we wish to modify (another pointer) so that when g returns, iptr contains the address returned by malloc. This idea is explored further in the section on pointers as parameters to functions.

#include

int g(int **iptr) {

if ((*iptr = (int *)malloc(sizeof(int))) == NULL)

return -1;

return 0;

}

Figure 2.2. Pointer operations in returning storage dynamically allocated in a function

Pointers and storage allocation are arguably the areas of C that provide the most fodder for the language's sometimes bad reputation. The misuse of dynamically allocated storage, in particular, is a notorious source of memory leaks. Memory leaks are blocks of storage that are allocated but never freed by a program, even when no longer in use. They are particularly detrimental when found in sections of code that are executed repeatedly. Fortunately, we can greatly reduce memory leaks by employing consistent approaches to how we manage storage.

One example of a consistent approach to storage management is the one used for data structures presented in this book. The philosophy followed in every case is that it is the responsibility of the user to manage the storage associated with the actual data that the data structure organizes; the data structure itself allocates storage only for internal structures used to keep the data organized. Consequently, only pointers are maintained to the data inserted into the data structure, rather than private copies of the data. One important implication of this is that a data structure's implementation does not depend on the type and size of the data it stores. Also, multiple data structures are able to operate on a single copy of data, which can be useful when organizing large amounts of data.

In addition, this book provides operations for initializing and destroying data structures. Initialization may involve many steps, one of which may be the allocation of memory. Destroying a data structure generally involves removing all of its data and freeing the memory allocated in the data structure. Destroying a data structure also usually involves freeing all