QUESTION 27 · Storage Classes
Ordinary auto and register local objects have automatic storage duration.
QUESTION 28 · Storage Classes
Storage-class specifiers help describe how declarations relate to storage duration and linkage. Their scope depends on where the declaration appears.
A block-local auto object has automatic storage duration and no linkage. It is normally created for an execution of its block. Without a valid initialisation, an ordinary automatic scalar does not have a useful guaranteed starting value.
A block-local register object is also automatic and has no linkage. It suggests frequent access, but the compiler does not have to place it in a hardware register. In C11, its address cannot be taken with &.
A local static object has block scope, no linkage and a lifetime covering program execution. It retains its value across calls. A file-scope static object instead has file scope and internal linkage. Arithmetic objects with static storage duration are initialised to zero if no explicit initialiser is given.
A common extern declaration refers to an object defined elsewhere. That object can be shared across source files through compatible external-linkage declarations. A declaration without an initialiser does not ordinarily create a separate new object.
Use an automatic temporary for a calculation needed only during one call. Use a local static counter when lasting state is intentional. Use an extern declaration when referring to a shared definition. Do not choose a class solely by a simplified memory-location table: C does not require automatic objects to occupy a particular physical stack or register. State scope, duration and linkage separately.
| Declaration | Scope | Duration | Linkage / main rule |
|---|---|---|---|
| Block auto | Block | Automatic | None; initialise before reading |
| Block register | Block | Automatic | None; address cannot be taken in C11 |
| Block static | Block | Static | None; retains value |
| File static | File | Static | Internal |
| Typical extern declaration | At declaration scope | Static for referred external object | Refers to compatible shared definition |
Related questions in the storage classes category from the supplied papers. The marks below are the printed allocations, including any combined subquestion.
If a variable has auto storage class by default, why was the auto keyword introduced in C? Explain.
Paper P01 · page 1List and explain storage classes in C.
Paper P04 · page 1