Bit fields

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 a struct or union member declaration which uses the following declarator:

Explanation

Bit fields can have only one of four types (possibly const or volatile 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 int 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 have atomic 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...7
 unsigned int b : 3;
};
int main(void)
{
    struct S s = {7};
    ++s.b; // unsigned overflow
    printf("%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: unused
    unsigned b1 : 5, : 11, b2 : 6, b3 : 2;
};
int main(void)
{
    printf("%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)
{
    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 with sizeof and _Alignas (since C11).

Notes

The following properties of bit fields are undefined:

• The effect of calling offsetof 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, and _Bool (since C99) 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, and bit fields of type int are always signed.

References

• C17 standard (ISO/IEC 9899:2018):
• 6.7.2.1 Structure and union specifiers
• C11 standard (ISO/IEC 9899:2011):
• 6.7.2.1 Structure and union specifiers
• C99 standard (ISO/IEC 9899:1999):
• 6.7.2.1 Structure and union specifiers
• C89/C90 standard (ISO/IEC 9899:1990):
• 3.5.2.1 Structure and union specifiers

See also