2.10 Storage Classes

Storage classes apply only to data objects and function parameters. However, storage class keywords in C are also used to affect the visibility of functions. Every data object and parameter used in a program has exactly one storage class, either assigned explicitly or by default. There are four storage classes:

• auto

• register

• static

• extern

An object's storage class determines its availability to the linker and its storage duration. An object with external or internal linkage, or with the storage-class specifier static , has static storage duration, which means that storage for the object is reserved and initialized to 0 only once, before main begins execution. An object with no linkage and without the storage-class specifier static has automatic storage duration; for such an object, storage is automatically allocated on entry to the block in which it is declared, and automatically deallocated on exiting from the block. An automatic object is not initialized.

When applied to functions, the storage-class specifier extern makes the function visible from other compilation units, and the storage-class specifier static makes the function visible only to other functions in the same compilation unit. For example:

static int tree(void);

The following sections describe these storage classes.

2.10.1 The auto Class

The auto class specifies that storage for an object is created upon entry to the block defining the object, and destroyed when the block is exited. This class can be declared only at the beginning of a block, such as at the beginning of a function's body. For example:

auto int a;        /*  Illegal -- auto must be within a block  */

main ()
{
    auto int b;               /*  Valid auto declaration   */
    for (b = 0; b < 10; b++)
      {
        auto int a = b + a;   /*   Valid inner block declaration    */
      }
}

When you use an initializer with an auto object (see Section 4.2), the object is initialized each time it is created. Storage is reserved for the object whether the block containing the object is entered through normal processing of the block or through a jump statement into the block. However, if the block is entered through a jump statement, initialization of the object is not guaranteed.

The auto class is the default for objects with block scope. Objects with the auto class are not available to the linker.

Note

Entering an enclosed block suspends, but does not end, execution of the enclosing block. Calling a function from within a block suspends, but does not end, execution of the block containing the call. Automatic objects with reserved storage maintain their storage in these cases.

2.10.2 The register Class

The register class identifies the assigned object as frequently used, suggesting to the compiler that the object should be assigned a register to minimize access time. register is never the default class; it must be explicitly specified.

The register class has the same storage duration as the auto class; that is, storage is created for a register object upon entry to the block defining the object, and destroyed when the block is exited.

The register class is the only storage class that can be explicitly specified for function parameters.

The DEC C compiler uses sophisticated register allocation techniques that make the use of the register keyword unnecessary.

2.10.3 The static Class

The static class specifies that space for the identifier is maintained for the duration of the program. Static objects are not available to the linker. Therefore, another compilation unit can contain an identical declaration that refers to a different object.

A static object can be declared anywhere a declaration may appear in the program; it does not have to be at the beginning of a block, as with the auto class. If a data object is declared outside a function, it has static duration by default-it is initialized only once at the beginning of the program.

Expressions used to initialize static objects must be constant expressions. If the object with static storage duration is not explicitly initialized, every arithmetic member of that object is initialized to 0, and every pointer member is initialized as a null pointer constant. See Section 4.2 for more information on initializing objects of various data types.

2.10.4 The extern Class

The extern class is the default class for objects with file scope. Objects outside of any function (an external definition) receive the extern class storage unless explicitly assigned the static keyword in the declaration. The extern class specifies the same storage duration as static objects, but the object or function name is not hidden from the linker. Using the extern keyword in a declaration results in external linkage in most cases (see Section 2.8), and results in static duration of the object.