Editing C syntax (section)

====Storage class specifiers====
Every object has a storage class.{{cn|date=April 2025}} This specifies most basically the storage ''duration,'' which may be static (default for global), automatic (default for local), or dynamic (allocated), together with other features (linkage and register hint).{{cn|date=April 2025}}

{| class="wikitable"
|+ Storage classes
|-
! Specifiers
! Lifetime
! Scope
! Default initializer
|-
| {{code|auto}}
| Block (stack)
| Block
| Uninitialized
|-
| {{code|register}}
| Block (stack or CPU register)
| Block
| Uninitialized
|-
| {{code|static}}
| Program
| Block or compilation unit
| Zero
|-
| {{code|extern}}
| Program
| Global (entire program)
| Zero
|-
| {{code|_Thread_local}}
| Thread
| 
| 
|-
| ''(none)''<sup>1</sup>
| Dynamic (heap)
|
| Uninitialized (initialized to {{code|0}} if using {{code|calloc()}})
|}

:<sup>1</sup> Allocated and deallocated using the {{code|malloc()}} and {{code|free()}} library functions.

Variables declared within a [[block (programming)|block]] by default have automatic storage, as do those explicitly declared with the [[Automatic variable|{{code|auto}}]]<ref group="note">The meaning of auto is a type specifier rather than a storage class specifier in C++0x</ref> or [[Register (keyword)|{{code|register}}]] storage class specifiers. The {{code|auto}} and {{code|register}} specifiers may only be used within functions and function argument declarations;{{cn|date=April 2025}} as such, the {{code|auto}} specifier is always redundant. Objects declared outside of all blocks and those explicitly declared with the [[Static variable|{{code|static}}]] storage class specifier have static storage duration. Static variables are initialized to zero by default by the [[compiler]].{{cn|date=April 2025}}

Objects with automatic storage are local to the block in which they were declared and are discarded when the block is exited. Additionally, objects declared with the {{code|register}} storage class may be given higher priority by the compiler for access to [[Register (computing)|registers]]; although the compiler may choose not to actually store any of them in a register. Objects with this storage class may not be used with the address-of ({{code|&}}) unary operator. Objects with static storage persist for the program's entire duration. In this way, the same object can be accessed by a function across multiple calls. Objects with allocated storage duration are created and destroyed explicitly with [[malloc|{{code|malloc}}]], {{code|free}}, and related functions.

The [[External variable|{{code|extern}}]] storage class specifier indicates that the storage for an object has been defined elsewhere. When used inside a block, it indicates that the storage has been defined by a declaration outside of that block. When used outside of all blocks, it indicates that the storage has been defined outside of the compilation unit. The {{code|extern}} storage class specifier is redundant when used on a function declaration. It indicates that the declared function has been defined outside of the compilation unit.

The [[Thread-local storage|{{code|_Thread_local}}]] (<code>thread_local</code> in [[C++]], and in C since [[C23 (C standard revision)|C23]],{{cn|date=April 2025}} and in earlier versions of C if the header <code><threads.h></code> is included) storage class specifier, introduced in [[C11 (C standard revision)|C11]], is used to declare a thread-local variable. It can be combined with {{code|static}} or {{code|extern}} to determine linkage.{{explain|reason=probably should link to, or otherwise explain, threads before discussing thread local storage|date=April 2025}}

Note that storage specifiers apply only to functions and objects; other things such as type and enum declarations are private to the compilation unit in which they appear.{{cn|date=April 2025}} Types, on the other hand, have qualifiers (see below).