Declares a member with explicit width, in bits. Adjacent bit-field members may be packed to share and straddle the individual bytes.

A bit-field declaration is astruct orunion member declaration which uses the followingdeclarator:

[edit]Explanation

Bit-fields can have only one of the following (possiblyconst orvolatile qualified) types:

• unsignedint, for unsigned bit-fields (e.g.unsignedint b:3; has the range[​0​, 7])
• signedint, for signed bit-fields (signedint b:3; has the range[-4, 3])
• int, for bit-fields with implementation-defined signedness (note that this differs from the meaning of the keywordint everywhere else, where it means "signed int"). For example,int b:3; may have the range of values[​0​, 7] or[-4, 3].

Additional implementation-defined types may be acceptable.It is also implementation-defined whether a bit-field may haveatomic type.(since C11)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...7unsignedint b:3;}; int main(void){struct S s={7};++s.b;// unsigned overflowprintf("%d\n", s.b);// output: 0}

Multiple adjacent bit-fields are permitted to be (and usually are) packed together:

#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: unusedunsigned b1:5,:11, b2:6, b3:2;}; int main(void){printf("%zu\n",sizeof(struct S));// usually prints 4}

The specialunnamed bit-field ofwidth 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: unusedunsigned b1:5;unsigned:0;// starts a new unsigned intunsigned b2:6;unsigned b3:15;}; int main(void){printf("%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 withsizeof and_Alignas(since C11)(until C23)alignas(since C23)(since C11).

[edit]Notes

The following usages of bit-fields causesundefined behavior:

• Callingoffsetof on a bit-field.

The following properties of bit-fields areunspecified:

• Alignment of the allocation unit that holds a bit-field.

The following properties of bit-fields areimplementation-defined:

• Whether bit-fields of typeint are treated as signed or unsigned.
• Whether types other thanint,signedint,unsignedint,_Bool(since C99), and (possiblyunsigned)_BitInt(N)(since C23) are permitted.

• 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).

In the C++ 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 typeint are always signed.

[edit]References

[edit]See also

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