How Do Colons Specify Bit Sizes in C Struct Declarations?

The Meaning of Colons in Struct Declarations

In C struct declarations, colons (:) followed by integers can be used to specify the bit size of struct members, limiting their range of values. Such syntax is often employed to represent compressed values, as demonstrated in the example below:

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

In this example, the colon operator limits a to one bit and b to seven bits. Together, they form an 8-bit value. This technique is commonly used to access compacted data, such as:

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

Here, a byte is split into two 4-bit nybbles. The C standard defines this syntax in section 9.6:

identifieropt attribute-specifieropt : constant-expression

The constant expression specifies the bit size. It must be greater than or equal to zero and can be larger than the bit-field's type, in which case the extra bits are used for padding.

The allocation and alignment of bit-fields are implementation-defined, with some machines packing them right-to-left and others left-to-right. They may also straddle addressable units on certain systems.

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