15.6 Bit Fields

A structure field declaration with an integer type can specify the number of bits the field should occupy. We call that a bit field. These are useful because consecutive bit fields are packed into a larger storage unit. For instance,

unsigned char opcode: 4;

specifies that this field takes just 4 bits. Since it is unsigned, its possible values range from 0 to 15. A signed field with 4 bits, such as this,

signed char small: 4;

can hold values from -8 to 7.

You can subdivide a single byte into those two parts by writing

unsigned char opcode: 4;
signed char small: 4;

in the structure. With bit fields, these two numbers fit into a single char.

Here’s how to declare a one-bit field that can hold either 0 or 1:

unsigned char special_flag: 1;

You can also use the bool type for bit fields:

bool special_flag: 1;

Except when using bool (which is always unsigned, see Boolean Type), always specify signed or unsigned for a bit field. There is a default, if that’s not specified: the bit field is signed if plain char is signed, except that the option -funsigned-bitfields forces unsigned as the default. But it is cleaner not to depend on this default.

Bit fields are special in that you cannot take their address with ‘&’. They are not stored with the size and alignment appropriate for the specified type, so they cannot be addressed through pointers to that type.