How do bitfields in C structures work using the "a : b" syntax?

Bitfields in C Structures: Understanding the "a : b" Syntax

In C , the struct keyword allows the creation of user-defined data structures. One peculiar syntax you may encounter within structs is "a : b," where 'a' represents a member variable, and 'b' indicates the number of bits allocated to that member.

Consider the following example:

struct SMyDataWord
{
    int Name : 40;
    int Colour : 24;
};

In this struct, we have two member variables: Name and Colour. The "Name : 40" syntax specifies that the Name variable is allocated 40 bits, while the "Colour : 24" syntax allocates 24 bits to the Colour variable.

These bitfields effectively split the entire data word (64 bits in this case) into two distinct regions. Each bitfield occupies a contiguous block of bits within the data word.

In the provided example, the bits are distributed as follows:

• Name: Bits 0 to 39 (inclusive)
• Colour: Bits 40 to 63 (inclusive)

Bitfields are carried over from the C language and are useful for creating compact data structures or controlling the layout of data in memory.

Note that the total number of bits allocated to all bitfields may exceed the data type size of the struct. In such cases, the compiler will allocate additional memory to accommodate the entire struct.

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