25. Pack Multiple Fields into a 16-bit Control Register

#include <stdio.h>
#include <stdint.h>

typedef struct
{
    uint8_t mode     : 3;
    uint8_t speed    : 5;
    uint8_t reserved : 2;
    uint8_t status   : 6;
}config;

typedef union
{
    config c;
    uint16_t overall;
}config_reg;

uint16_t pack_register(uint8_t mode, uint8_t speed, uint8_t status) {

    config_reg r;

    r.c.mode = mode;
    r.c.speed = speed;
    r.c.status = status;
    r.c.reserved = 0;

    return r.overall;
}

int main() {
    uint8_t mode, speed, status;
    scanf("%hhu %hhu %hhu", &mode, &speed, &status);

    uint16_t reg = pack_register(mode, speed, status);
    printf("%u", reg);
    return 0;
}