Bit fields allow integer members of a structure to be stored in memory spaces smaller than normally allowed by the compiler. A bit field is declared by specifying the number of bits after the member name, separated by a colon. Bit fields are packed together efficiently in memory and accessed like regular structure members. They are interpreted as unsigned integers and only the declared number of lower bits can be assigned or accessed.

1. Bit Field

2. Bit Field
• 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.
• A structure declarator can also be a specified
number of bits, called a "bit field."
• Its length is set off from the declarator for the
field name by a colon.
• A bit field is interpreted as an positiveintegral
type.

3. Bit Field
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Example
struct mybitfields
{
unsigned short a : 4;
unsigned short b : 5;
unsigned short c : 7;
} test;
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main( )
{
clrscr();
test.a = 2;
test.b = 31;
test.c = 0;
printf("%d %d %d",test.a,test.b,test.c);
getch();
}
the bits would be arranged as follows:
00000001 11110010
Ccccccc bbbbb aaaa

4. Bit Field
• struct packed_struct {
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unsigned int f1:1;
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unsigned int f2:1;
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unsigned int f3:1;
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unsigned int f4:1;
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unsigned int type:4;
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unsigned int funny_int:9;
• } pack;
• Here the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4
bit type and a 9 bit funny_int.
• C automatically packs the above bit fields as compactly as
possible, provided that the maximum length of the field is less than or
equal to the integer word length of the computer.
• If this is not the case then some compilers may allow memory overlap
for the fields whilst other would store the next field in the next word
(see comments on bit fiels portability below).

5. Bit Field
• Access members as usual via:
• pack.type = 7;
• NOTE:
• Only n lower bits will be assigned to an n bit
number. So type cannot take values larger than
15 (4 bits long).
• Bit fields are always converted to integer type
for computation.
• You are allowed to mix ``normal'' types with bit
fields.
• The unsigned definition is important - ensures
that no bits are used as a flag.

6. Bit Field
struct DISK_REGISTER {
unsigned ready:1;
unsigned error_occured:1;
unsigned disk_spinning:1;
unsigned write_protect:1;
unsigned head_loaded:1;
unsigned error_code:8;
unsigned track:9;
unsigned sector:5;
unsigned command:5;
};