Bit Fields, Unions, and Enums

Thu, 13 Aug 2026 10:00:00 +0300

Bismillah.

In this article we cover Bit Fields, Unions, and Enums.

1. Bit Fields

We established in the previous article that a struct distributes bytes — but what if we want to split a single byte into bits? That is the bit field: we specify the number of bits per member.

typedef struct {
 u8 a : 4; // 4 bits
 u8 b : 2; // 2 bits
 u8 c : 1; // 1 bit
} student; // 7 bits total ← inside one byte

+---+---+---+---+---+---+---+---+
| a (4 bits) | b (2) | c | (free) |
+---+---+---+---+---+---+---+---+

Note: the compiler packs bit fields into the byte, but does not guarantee identical behavior across all compilers (implementation-defined). During analysis, reading bit fields in assembly involves and/shift operations.

Union on P01NT

Bit Fields, Unions, and Enums

1. Bit Fields