1.9 Variables

Every variable must be declared before it can be used. The declaration determines the variable's type, its storage class, and possibly its initial value. The type of a variable determines how much space it occupies in storage and how the bit pattern it stores is interpreted. For example:

float dollars = 2.5F;     // a variable of type float

The variable dollars designates a region in memory with a size of 4 bytes. The contents of these four bytes are interpreted as a floating-point number, and initialized with the value 2.5.

1.9.1 Storage Classes

The storage class of a variable determines its scope, its storage duration, and its linkage. The scope can be either block or file (see Section 1.2.4, earlier in this book). Variables also have one of two storage durations:

Static storage duration

The variable is generated and initialized once, before the program begins. It exists continuously throughout the execution of the program.

Automatic storage duration

The variable is generated anew each time the program flow enters the block in which it is defined. When the block is terminated, the memory occupied by the variable is freed.

The storage class of a variable is determined by the position of its declaration in the source file and by the storage class specifier, if any. A declaration may contain no more than one storage class specifier. Table 1-18 lists the valid storage class specifiers.

Table 1-18. The storage class specifiers
Specifier	Meaning
auto	Variables declared with the storage class specifier auto have automatic storage duration. The specifier auto is applicable only to variables that are declared within a function. Because the automatic storage class is the default for such variables, the specifier auto is rarely used.
register	The storage class specifier register instructs the compiler to store the variable in a CPU register if possible. As a result, the address operator (&) cannot be used with a register variable. In all other respects, however, register variables are treated the same as auto variables.
static	Variables declared as static always have static storage duration. The storage class specifier static is used to declare static variables with a limited scope.
extern	The specifier extern is used to declare variables with static storage duration that can be used throughout the program.

Table 1-19 illustrates the possible storage classes and their effect on the scope and the storage duration of variables.

Table 1-19. Storage class, scope, and storage duration of variables
Position of the declaration	Storage class specifier	Scope	Storage duration
Outside all functions	none, extern, static	File	Static
Within a function	none, auto, register	Block	Automatic
Within a function	extern, static	Block	Static

1.9.2 Initialization

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression. Some examples are:

int index = 0, max = 99, *intptr = NULL;
static char message[20] = "Example!";

Variables are not initialized in declarations that do not cause an object to be created, such as function prototypes and declarations that refer to external variable definitions.

Every initialization is subject to the following rules:

1. A variable declaration with an initializer is always a definition. This means that storage is allocated for the variable.

2. A variable with static storage duration can only be initialized with a value that can be calculated at the time of compiling. Hence the initial value must be a constant expression.

3. For declarations without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined!

The type conversion rules for simple assignments are also applied on initialization.