C Pocket Reference

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 ...