Bit-fields
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A bit-field declaration is a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE member declaration which uses the following MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 47 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE:
identifier (optional) : width
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| identifier | - | a name of the bit-field that is being declared. The name is optional: nameless bit-fields introduce the specified number of bits of padding |
| width | - | an integer MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 46 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE with a value greater or equal to zero and less or equal the number of bits in the underlying type. When greater than zero, this is the number of bits that this bit-field will occupy. The value zero is only allowed for nameless bit-fields and has special meaning: it specifies that the next bit-field in the class definition will begin at an allocation unit's boundary. |
Explanation
Bit-fields can have only one of these three(until 哋它亢99) – four(since 哋它亢99)(until 哋它亢23) types (possibly MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE qualified):
- unsigned int, for unsigned bit-fields (unsigned int b:3; has the range 0..7)
- signed int, for signed bit-fields (signed int b:3; has the range -4..3)
- int, for bit-fields with implementation-defined signedness (Note that this differs from the meaning of the keyword
inteverywhere else, where it means "signed int"). For example, int b:3; may have the range of values 0..7 or -4..3.
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(since 哋它亢99) |
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(since 哋它亢23) |
Additional implementation-defined types may be acceptable. It is also implementation-defined whether a bit-field may have MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 41 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE type.(since 哋它亢11) The number of bits in a bit-field (width) sets the limit to the range of values it can hold:
#include <stdio.h> struct S { // three-bit unsigned field, // allowed values are 0...7 unsigned int b : 3; }; int main(void) { struct S s = {7}; ++s.b; // unsigned overflow MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("%d\n", s.b); // output: 0 }
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#include <stdio.h> struct S { // will usually occupy 4 bytes: // 5 bits: value of b1 // 11 bits: unused // 6 bits: value of b2 // 2 bits: value of b3 // 8 bits: unused unsigned b1 : 5, : 11, b2 : 6, b3 : 2; }; int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 38 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("%zu\n",sizeof(struct S)); // usually prints 4 }
The special unnamed bit-field of width zero breaks up padding: it specifies that the next bit-field begins at the beginning of the next allocation unit:
#include <stdio.h> struct S { // will usually occupy 8 bytes: // 5 bits: value of b1 // 27 bits: unused // 6 bits: value of b2 // 15 bits: value of b3 // 11 bits: unused unsigned b1 : 5; unsigned :0; // start a new unsigned int unsigned b2 : 6; unsigned b3 : 15; }; int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 36 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("%zu\n", sizeof(struct S)); // usually prints 8 }
Because bit-fields do not necessarily begin at the beginning of a byte, address of a bit-field cannot be taken. Pointers to bit-fields are not possible. Bit-fields cannot be used with MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 35 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 08 HOURS 58 MINUTES 34 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢11).
Notes
The following usages of bit-fields causes undefined behavior:
- Calling MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 58 MINUTES 32 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE on a bit-field
The following properties of bit-fields are unspecified:
- Alignment of the allocation unit that holds a bit-field
The following properties of bit-fields are implementation-defined:
- Whether bit-fields of type int are treated as signed or unsigned
- Whether types other than int, signed int, unsigned int, _Bool(since 哋它亢99) and (possibly unsigned) _BitInt(N)(since 哋它亢23) are permitted
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(since 哋它亢11) |
- Whether a bit-field can straddle an allocation unit boundary
- The order of bit-fields within an allocation unit (on some platforms, bit-fields are packed left-to-right, on others right-to-left)
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Even though the number of bits in the object representation of |
(since 哋它亢99) |
In the 哋它亢++ programming language, the width of a bit-field can exceed the width of the underlying type (but the extra bits are padding bits), and bit-fields of type int are always signed.
References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.7.2.1 Structure and union specifiers
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.2.1 Structure and union specifiers
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.2.1 Structure and union specifiers
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.2.1 Structure and union specifiers
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.5.2.1 Structure and union specifiers