Bit field

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A bit field is a common idiom used in computer programming to store a set of Boolean datatype flags compactly, as a series of bits. The bit field is stored in an integral type of known, fixed bit-width. Each Boolean flag is stored in a separate bit. Usually the source code will define a set of constants, each a power of two, that semantically associate each individual bit with its respective Boolean flag. The bitwise operators AND, OR, and NOT are used in combination to set, reset and test the flags.

A bit field is distinguished from a bit array in that the latter is used to store a large set of bits indexed by integers and is often wider than any integral type supported by the language. Bit fields, on the other hand, typically fit within a machine word, and the denotation of bits is independent of their numerical index. Nevertheless, a bit field can be safely and elegantly implemented using a bit array where the bit indices for each flag are values of an enumerated type (like the EnumSet class in Java); this avoids the dangers of direct bitwise manipulations.

[edit] Examples

Example implementation in C:

   #define PREFERENCE_LIKES_ICE_CREAM  ((unsigned char) (1 << 0)) // 0x01
   #define PREFERENCE_PLAYS_GOLF       ((unsigned char) (1 << 1)) // 0x02
   #define PREFERENCE_WATCHES_TV       ((unsigned char) (1 << 2)) // 0x04
   #define PREFERENCE_READS_BOOKS      ((unsigned char) (1 << 3)) // 0x08
    
   unsigned char preference;
    
   void setPreference(unsigned char flag)
   {
       preference |= flag;
   }
    
   void resetPreference(unsigned char flag)
   {
       preference &= ~flag;
   }
    
   unsigned char getPreference(unsigned char flag)
   {
       return (preference & flag) != 0;
   }

Instead of using hardcoded numerical representations for the powers of two (0x08), the use of the Bit Shift operator (1 << 3) with an incrementing shift operand is recommended for easier readability.

Kernighan and Ritchie's book, The C Programming Language, describes a method for defining and accessing fields directly. Using this method, the use of bitwise operators is not needed as bit members can be accessed the same as struct members. An example using a struct follows:

   struct preferences
   {
       unsigned int likesIceCream : 1;
       unsigned int playsGolf     : 1;
       unsigned int watchesTV     : 1;
       unsigned int readsBooks    : 1;
   }; 
    
   struct preferences fred;
    
   fred.likesIceCream = 1;
   fred.playsGolf = 1;
   fred.watchesTV = 1;
   fred.readsBooks = 0;
    
   if (fred.likesIceCream == 1)
   {
       ...

However, bit members in structs have practical drawbacks. First, the ordering of bits in memory varies from compiler to compiler. In addition, many popular compilers generate inefficient code for reading and writing bit members, and there are potentially severe thread safety issues relating to bit fields (especially on multiprocessor systems) due to the fact that most machines cannot manipulate arbitrary sets of bits in memory, but must instead load and store whole words. e.g the following would not be thread-safe, in spite of the use of a mutex:

   struct foo {
       int flag : 1;
       int counter : 15;
   };
    
   struct foo my_foo;
    
   ...
    
   pthread_mutex_lock(&my_mutex);
   flag = !flag;
   pthread_mutex_unlock(&my_mutex);

as on most machines it is impossible at the hardware level to load and store flag and counter separately. (In order for this to be thread safe, you would need to lock and unlock the mutex around all accesses to both flag and counter, even if counter doesn't itself need to be thread-safe.)

For these reasons, bitwise operators remain in wide use in programs in the C and C++ languages as of 2006.