Bit field members

Both C and C++ allow integer members to be stored into memory spaces smaller than the compiler would ordinarily allow. These space-saving structure members are called bit fields, and their width in bits can be explicitly declared. Bit fields are used in programs that must force a data structure to correspond to a fixed hardware representation and are unlikely to be portable.

Bit field member declaration syntax
 
>>-type_specifier--+------------+--:--constant_expression--;---><
                   '-declarator-'
 

The constant_expression is a constant integer expression that indicates the field width in bits. A bit field declaration may not use either of the type qualifiers const or volatile.

A bit field can be any integral type or enumeration type.

The maximum bit-field length is 64 bits. To increase portability, do not use bit fields greater than 32 bits in size.

The following structure has three bit-field members kingdom, phylum, and genus, occupying 12, 6, and 2 bits respectively:

struct taxonomy {
     int kingdom : 12;
     int phylum : 6;
     int genus : 2;
     };

When you assign a value that is out of range to a bit field, the low-order bit pattern is preserved and the appropriate bits are assigned.

The following restrictions apply to bit fields. You cannot:

• Define an array of bit fields
• Take the address of a bit field
• Have a pointer to a bit field
• Have a reference to a bit field

If a series of bit fields does not add up to the size of an int, padding can take place. The amount of padding is determined by the alignment characteristics of the members of the structure. In some instances, bit fields can cross word boundaries.

Bit fields with a length of 0 must be unnamed. Unnamed bit fields cannot be referenced or initialized.

The following example demonstrates padding, and is valid for all implementations. Suppose that an int occupies 4 bytes. The example declares the identifier kitchen to be of type struct on_off:

struct on_off {
                  unsigned light : 1;
                  unsigned toaster : 1;
                  int count;            /* 4 bytes */
                  unsigned ac : 4;
                  unsigned : 4;
                  unsigned clock : 1;
                  unsigned : 0;
                  unsigned flag : 1;
                 } kitchen ;

The structure kitchen contains eight members totalling 16 bytes. The following table describes the storage that each member occupies:

Related information

• "Alignment of bit fields" in the ILE C/C++ Programmer’s Guide