Set Specific Bits in a 32-bit Register

Code

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

// assumption: len cannot be 0
uint32_t set_bits(uint32_t reg, uint8_t pos, uint8_t len) {
    // create mask
    uint32_t mask = (((uint32_t) -1) >> (32 - len)) << pos;
    // apply mask
    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

This one tripped me up a bit. I had to cast -1 to an unsigned because otherwise the compiler did an arithmetic shift. This was also the case for ~0. The first shift (>> 32 - len) creates a mask of 1s on the RHS that is of length len, and the second shift (<< pos) moved the mask to start at pos.