Stage 2

Custom Types Deep-Dive

typedef, enum, struct, union — four tools and the tagged-union pattern used in real embedded firmware.

2.1 typedef — naming existing types

typedef.c

typedef uint32_t Timestamp_ms;
typedef uint8_t  NodeId;

Timestamp_ms now   = 1000u;
NodeId       sensor = 3u;

2.2 enum — named integer constants

enum.c

typedef enum {
    DIR_NORTH = 0,
    DIR_EAST,
    DIR_SOUTH,
    DIR_WEST
} Direction;

2.3 struct — named composite type

struct.c

typedef struct {
    float    x, y, z;
    uint32_t timestamp;
} AccelSample;

AccelSample s = { .x=0.1f, .y=9.8f, .z=0.0f, .timestamp=1234 };

2.4 union & tagged union

tagged-union.c

typedef enum { MSG_TEMP, MSG_ACCEL, MSG_STATUS } MsgType;
typedef struct {
    MsgType type;
    union {
        float    temperature;
        AccelSample accel;
        uint8_t  status_code;
    } data;
} SensorMsg;