Mastering Algorithms with C

Storage Allocation

When we declare a pointer in C, a certain amount of space is allocated for it, 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 This situation occurs because once f returns, a is no longer valid on the program stack (see Chapter 3).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; ...