Storage-class specifiers
Specify storage duration and linkage of objects and functions:
auto
- automatic duration and no linkageregister
- automatic duration and no linkage; address of this variable cannot be takenstatic
- static duration and internal linkage (unless at block scope)extern
- static duration and external linkage (unless already declared internal)
|
(since C11) |
Explanation
Storage-class specifiers appear in declarations. At most one specifier may be used, except that _Thread_local
(until C23)thread_local
(since C23) may be combined with static
or extern
to adjust linkage (since C11). The storage-class specifiers determine two independent properties of the names they declare: storage duration and linkage.
auto
specifier is only allowed for objects declared at block scope (except function parameter lists). It indicates automatic storage duration and no linkage, which are the defaults for these kinds of declarations.register
specifier is only allowed for objects declared at block scope, including function parameter lists. It indicates automatic storage duration and no linkage (which is the default for these kinds of declarations), but additionally hints the optimizer to store the value of this variable in a CPU register if possible. Regardless of whether this optimization takes place or not, variables declared register
cannot be used as arguments to the address-of operator, cannot use alignas (since C11), and register
arrays are not convertible to pointers.static
specifier specifies both static storage duration (unless combined with _Thread_local
) (since C11) and internal linkage (unless used at block scope). It can be used with functions at file scope and with variables at both file and block scope, but not in function parameter lists.extern
specifier specifies static storage duration (unless combined with _Thread_local
(until C23)thread_local
(since C23)) (since C11) and external linkage. It can be used with function and object declarations in both file and block scope (excluding function parameter lists). If extern
appears on a redeclaration of an identifier that was already declared with internal linkage, the linkage remains internal. Otherwise (if the prior declaration was external, no-linkage, or is not in scope), the linkage is external.
5) _Thread_local (until C23)thread_local (since C23) indicates thread storage duration. It cannot be used with function declarations. If it is used on a declaration of an object, it must be present on every declaration of the same object. If it is used on a block-scope declaration, it must be combined with either static or extern to decide linkage.
|
(since C11) |
If no storage-class specifier is provided, the defaults are:
-
extern
for all functions -
extern
for objects at file scope -
auto
for objects at block scope
For any struct or union declared with a storage-class specifier, the storage duration (but not linkage) applies to their members, recursively.
Function declarations at block scope can use extern
or none at all. Function declarations at file scope can use extern
or static
.
Function parameters cannot use any storage-class specifiers other than register
. Note that static
has special meaning in function parameters of array type.
Storage duration
Every object has a property called storage duration, which limits the object lifetime. There are four kinds of storage duration in C:
- automatic storage duration. The storage is allocated when the block in which the object was declared is entered and deallocated when it is exited by any means (goto, return, reaching the end). One exception is the VLAs; their storage is allocated when the declaration is executed, not on block entry, and deallocated when the declaration goes out of scope, not when the block is exited (since C99). If the block is entered recursively, a new allocation is performed for every recursion level. All function parameters and non-
static
block-scope objects have this storage duration, as well as compound literals used at block scope (until C23) - static storage duration. The storage duration is the entire execution of the program, and the value stored in the object is initialized only once, prior to main function. All objects declared
static
and all objects with either internal or external linkage that aren't declared_Thread_local
(until C23)thread_local
(since C23) (since C11) have this storage duration.
- automatic storage duration. The storage is allocated when the block in which the object was declared is entered and deallocated when it is exited by any means (goto, return, reaching the end). One exception is the VLAs; their storage is allocated when the declaration is executed, not on block entry, and deallocated when the declaration goes out of scope, not when the block is exited (since C99). If the block is entered recursively, a new allocation is performed for every recursion level. All function parameters and non-
|
(since C11) |
- allocated storage duration. The storage is allocated and deallocated on request, using dynamic memory allocation functions.
Linkage
Linkage refers to the ability of an identifier (variable or function) to be referred to in other scopes. If a variable or function with the same identifier is declared in several scopes, but cannot be referred to from all of them, then several instances of the variable are generated. The following linkages are recognized:
- no linkage. The identifier can be referred to only from the scope it is in. All function parameters and all non-
extern
block-scope variables (including the ones declaredstatic
) have this linkage.
- no linkage. The identifier can be referred to only from the scope it is in. All function parameters and all non-
- internal linkage. The identifier can be referred to from all scopes in the current translation unit. All
static
file-scope identifiers (both functions and variables) have this linkage.
- internal linkage. The identifier can be referred to from all scopes in the current translation unit. All
- external linkage. The identifier can be referred to from any other translation units in the entire program. All non-
static
functions, allextern
variables (unless earlier declaredstatic
), and all file-scope non-static
variables have this linkage.
- external linkage. The identifier can be referred to from any other translation units in the entire program. All non-
If the same identifier appears with both internal and external linkage in the same translation unit, the behavior is undefined. This is possible when tentative definitions are used.
Linkage and libraries
This section is incomplete Reason: should this be a separate top-level entry in c/language under Miscellaneous? |
Declarations with external linkage are commonly made available in header files so that all translation units that #include the file may refer to the same identifier that are defined elsewhere.
Any declaration with internal linkage that appears in a header file results in a separate and distinct object in each translation unit that includes that file.
Library interface:
// flib.h #ifndef FLIB_H #define FLIB_H void f(void); // function declaration with external linkage extern int state; // variable declaration with external linkage static const int size = 5; // definition of a read-only variable with internal linkage enum { MAX = 10 }; // constant definition inline int sum (int a, int b) { return a+b; } // inline function definition #endif // FLIB_H
Library implementation:
// flib.c #include "flib.h" static void local_f(int s) {} // definition with internal linkage (only used in this file) static int local_state; // definition with internal linkage (only used in this file) int state; // definition with external linkage (used by main.c) void f(void) {local_f(state);} // definition with external linkage (used by main.c)
Application code:
// main.c #include "flib.h" int main(void) { int x[MAX] = {size}; // uses the constant and the read-only variable state = 7; // modifies state in flib.c f(); // calls f() in flib.c }
Keywords
auto, register, static, extern, _Thread_local thread_local
Notes
The keyword |
(until C23) |
The typedef
and constexpr
(since C23) specifiers are formally listed as storage-class specifiers in the C language grammar, but do not specify storage.
The auto specifier is also used for type inference. |
(since C23) |
Names at file scope that are const
and not extern
have external linkage in C (as the default for all file-scope declarations), but internal linkage in C++.
Example
#include <stdio.h> #include <stdlib.h> /* static storage duration */ int A; int main(void) { printf("&A = %p\n", (void*)&A); /* automatic storage duration */ int A = 1; // hides global A printf("&A = %p\n", (void*)&A); /* allocated storage duration */ int *ptr_1 = malloc(sizeof(int)); /* start allocated storage duration */ printf("address of int in allocated memory = %p\n", (void*)ptr_1); free(ptr_1); /* stop allocated storage duration */ }
Possible output:
&A = 0x600ae4 &A = 0x7ffefb064f5c address of int in allocated memory = 0x1f28c30
References
- C23 standard (ISO/IEC 9899:2023):
- 6.2.2 Linkages of identifiers (p: 35-36)
- 6.2.4 Storage durations of objects (p: 36-37)
- 6.7.1 Storage-class specifiers (p: 97-100)
- C17 standard (ISO/IEC 9899:2018):
- 6.2.2 Linkages of identifiers (p: 29-30)
- 6.2.4 Storage durations of objects (p: 30)
- 6.7.1 Storage-class specifiers (p: 79)
- C11 standard (ISO/IEC 9899:2011):
- 6.2.2 Linkages of identifiers (p: 36-37)
- 6.2.4 Storage durations of objects (p: 38-39)
- 6.7.1 Storage-class specifiers (p: 109-110)
- C99 standard (ISO/IEC 9899:1999):
- 6.2.2 Linkages of identifiers (p: 30-31)
- 6.2.4 Storage durations of objects (p: 32)
- 6.7.1 Storage-class specifiers (p: 98-99)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.1.2.2 Linkages of identifiers
- 3.1.2.4 Storage durations of objects
- 3.5.1 Storage-class specifiers