What are Bit Fields in C and How Do :1, :7, :16, and :32 Specify Their Size?

Bit Fields in C : Understanding :1, :7, :16, and :32

In C , programmers use bit fields to create data structures with members that occupy a specified number of bits. This concept is often represented using colons (:) followed by a numeric value.

Consider the following example:

unsigned char a : 1;
unsigned char b : 7;

In this code, the colon and the following number (:1 and :7) indicate the bit sizes of the respective variables a and b. This means that a is a single bit, while b occupies 7 bits.

Bit fields are commonly used to create packed data structures, where the size of the structure is minimized by limiting the bit size of each member. For instance, in the following tOneAndSevenBits structure:

typedef struct {
    unsigned char a : 1;
    unsigned char b : 7;
} tOneAndSevenBits;

The structure occupies 8 bits, with 1 bit allocated for a and 7 bits for b.

Bit fields can also be used to access compressed values. Consider the following example:

typedef struct {
    unsigned char leftFour  : 4;
    unsigned char rightFour : 4;
} tTwoNybbles;

This structure creates a data type with two 4-bit nybbles (half-bytes) packed into a single 8-bit byte.

The C 11 standard defines bit fields in more detail:

"The optional attribute-specifier appertains to the entity being declared. The bit-field attribute is not part of the type of the class member. The constant-expression shall be an integral constant expression with a value greater than or equal to zero."

This specification allows for a flexible allocation of bit fields within class objects, with the alignment and packing of bit fields being implementation-defined.

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