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)
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(since 哋它亢11) |
Explanation
Storage-class specifiers appear in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 56 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE expressions(since 哋它亢23). At most one specifier may be used, except that _Thread_local(until 哋它亢23)MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢23) may be combined with static or extern to adjust linkage(since 哋它亢11). The storage-class specifiers determine two independent properties of the names they declare: storage duration and linkage.
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5) _Thread_local(until 哋它亢23)MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢23) 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.
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(since 哋它亢11) |
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- extern for all functions
- extern for objects at file scope
- auto for objects at block scope
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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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE has a property called storage duration, which limits the object MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 44 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. There are four kinds of storage duration in C:
- automatic storage duration. The storage is allocated when the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE in which the object was declared is entered and deallocated when it is exited by any means (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 42 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, reaching the end). One exception is the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE; 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 哋它亢99). 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 38 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE used at block scope(until 哋它亢23)
- 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 36 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. All objects declared static and all objects with either internal or external linkage that aren't declared _Thread_local(until 哋它亢23)MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 35 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢23)(since 哋它亢11) have this storage duration.
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(since 哋它亢11) |
- allocated storage duration. The storage is allocated and deallocated on request, using MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 33 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE functions.
Linkage
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- 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 declared static) have this linkage.
- 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.
- external linkage. The identifier can be referred to from any other translation units in the entire program. All non-static functions, all extern variables (unless earlier declared static), and all file-scope non-static variables have this linkage.
If the same identifier appears with both internal and external linkage in the same translation unit, the behavior is undefined. This is possible when MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 30 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 29 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE the file may refer to the same identifier that are defined elsewhere.
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MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 27 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE #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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 25 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE #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) MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 24 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE #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
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Notes
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The keyword _Thread_local is usually used through the convenience macro MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, defined in the header MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. |
(until 哋它亢23) |
The MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢23) specifiers are formally listed as storage-class specifiers in the C language grammar, but do not specify storage.
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(since 哋它亢23) |
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 哋它亢++.
Example
#include <stdio.h> #include <stdlib.h> // static storage duration int A; int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("&A = %p\n", (void*)&A); // automatic storage duration int A = 1; // hides global A MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("&A = %p\n", (void*)&A); // allocated storage duration int* ptr_1 = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(sizeof(int)); // start allocated storage duration MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("address of int in allocated memory = %p\n", (void*)ptr_1); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 45 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(ptr_1); // stop allocated storage duration }
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&A = 0x600ae4 &A = 0x7ffefb064f5c address of int in allocated memory = 0x1f28c30
References
- 哋它亢23 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)
- 哋它亢17 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)
- 哋它亢11 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)
- 哋它亢99 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)
- 哋它亢89/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