Replace Bit Field in a 32-bit Register

Code

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

uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len) {
    // Your code here
    int mask = 0;
    for(int i=0;i<len;i++){
        mask = mask | (1<<i);
    }
    reg = reg & ~(mask << pos);
    return reg = reg | (val & mask) << pos;
}

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

Solving Approach

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

uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len) {
    // Your code here
    int mask = 0;
    for(int i=0;i<len;i++){
        mask = mask | (1<<i);
    }
    reg = reg & ~(mask << pos);
    return reg = reg | (val & mask) << pos;
}

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

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

uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len) {
    // Your code here
    int mask = 0;
    for(int i=0;i<len;i++){
        mask = mask | (1<<i);
    }
    reg = reg & ~(mask << pos);
    return reg = reg | (val & mask) << pos;
}

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

for(int i=0;i<len;i++){
        mask = mask | (1<<i);
    }

  1.after completion of for loop mask is 

mask = 1111

2.reg = reg & ~(mask << pos);

reg   = 1111 1111

(mask << pos) = 1111 0000

~(mask << pos)  =  0000 1111

now we are doing '&' and operation between this reg    &    ~(mask << pos)

                   reg     ->       1111 1111

~(mask << pos)  ->       0000 1111

                 result    -> &   0000 1111    

reg = result               

we are storing result in reg.

3.

reg = reg | (val & mask) << pos

                 val           ->       0000 0000

             mask          ->       0000 1111

             (val & mask)  =    0000 0000

now i am left shifting (val & mask) provided position

( (val & mask)  << pos )  =  0000 0000

4.

reg  value is  ->    0000 1111  

 (val & mask)  << pos value is -> 0000 0000

or '|' operation between  reg  |  (val & mask)  << pos

reg                                 ->           0000 1111 

 (val & mask)  << pos  ->           0000 0000

output                            -> (or)   0000 1111

this is the final output  -->  0000 1111