Union declaration
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The MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for a union is identical to the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE type specifier except for the keyword used:
Syntax
union attr-spec-seq (optional) name (optional) { struct-declaration-list }
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(1) | ||||||||
union attr-spec-seq (optional) name
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(2) | ||||||||
| name | - | the name of the union that's being defined |
| struct-declaration-list | - | any number of variable declarations, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 51 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE declarations, and static assert declarations. Members of incomplete type and members of function type are not allowed. |
| attr-spec-seq | - | (哋它亢23)optional list of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, applied to the union type, not allowed for (2) if such form is not followed by a ; (i.e. not a forward declaration).
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Explanation
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If the member used to access the contents of a union is not the same as the member last used to store a value, the object representation of the value that was stored is reinterpreted as an object representation of the new type (this is known as type punning). If the size of the new type is larger than the size of the last-written type, the contents of the excess bytes are unspecified (and may be a trap representation). Before 哋它亢99 TC3 (DR 283) this behavior was undefined, but commonly implemented this way. |
(since 哋它亢99) |
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Similar to struct, an unnamed member of a union whose type is a union without name is known as anonymous union. Every member of an anonymous union is considered to be a member of the enclosing struct or union keeping their union layout. This applies recursively if the enclosing struct or union is also anonymous. struct v { union // anonymous union { struct { int i, j; }; // anonymous structure struct { long k, l; } w; }; int m; } v1; v1.i = 2; // valid v1.k = 3; // invalid: inner structure is not anonymous v1.w.k = 5; // valid MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 46 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE |
(since 哋它亢11) |
Keywords
Notes
See MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 44 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for the rules about initialization of structs and unions.
Example
#include <assert.h> #include <stdint.h> #include <stdio.h> int main(void) { union S { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE u32; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 42 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE u16[2]; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE u8; } s = {0x12345678}; // s.u32 is now the active member MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("Union S has size %zu and holds %x\n", sizeof s, s.u32); s.u16[0] = 0x0011; // s.u16 is now the active member // reading from s.u32 or from s.u8 reinterprets the object representation // printf("s.u8 is now %x\n", s.u8); // unspecified, typically 11 or 00 // printf("s.u32 is now %x\n", s.u32); // unspecified, typically 12340011 or 00115678 // pointers to all members of a union compare equal to themselves and the union MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 37 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 09 HOURS 31 MINUTES 36 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE*)&s == &s.u8); // this union has 3 bytes of trailing padding union pad { char c[5]; // occupies 5 bytes float f; // occupies 4 bytes, imposes alignment 4 } p = { .f = 1.23 }; // the size is 8 to satisfy float's alignment MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 31 MINUTES 35 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("size of union of char[5] and float is %zu\n", sizeof p); }
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Union S has size 4 and holds 12345678 size of union of char[5] and float is 8
Defect reports
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| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
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References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.7.2.1 Structure and union specifiers (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.2.1 Structure and union specifiers (p: 81-84)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.2.1 Structure and union specifiers (p: 112-117)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.2.1 Structure and union specifiers (p: 101-104)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.5.2.1 Structure and union specifiers