Set Specific Bits in a 32-bit Register

Code

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

uint32_t set_bits(uint32_t reg, uint8_t pos, uint8_t len) {
    if(len==0||pos>31)return reg;
    if(len>32-pos)len=32-pos;
    uint32_t mask=((1u<<len)-1)<<pos;
    return reg|mask;
}

int main() {
    uint32_t reg;
    uint8_t pos, len;
    scanf("%u %hhu %hhu", &reg, &pos, &len);
    printf("%u", set_bits(reg, pos, len));
    return 0;
}

Solving Approach

pos-first led to turn on

len - how many leds to light

(1u<<len)-1 -moves mask of len leds on

mask<<pos- moves mask to the correct position

reg| mask-turns ON only leds others stay unchanged